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This work reports the development of the first version of the E‐region Prompt Radio Occultation Based Electron Density (E‐PROBED) Model. This is an empirical model of E‐region electron density (Ne) between 90 and 120 km developed using radio occultation measurements from the COSMIC‐1 mission. This first version captures more than 80% of the observed variability in monthly‐mean latitude‐local time‐altitude E‐region Ne profiles but it does not account for longitudinal variability at constant local‐time. This work also reports a validation of E‐PROBED simulations through comparisons with ionosondes and incoherent scatter radar (ISR) E‐region Ne profiles. E‐PROBED generally agrees with these ground‐based observations during day‐time. During night‐time, there is a large disparity between E‐PROBED and ISR values. Finally, this work compares E‐PROBED with E‐region Ne simulated by the International Reference Ionosphere (IRI) and the Specified Dynamics—Whole Atmosphere Community Climate Model with Ionosphere/Thermosphere eXtension (SD‐WACCM‐X). One of the main differences amongst these models is on the simulation of variabilities that cannot be attributed to photoionization. IRI barely simulates any variability not driven by photoionization. Both E‐PROBED and SD‐WACCM‐X simulates variability not driven by photoionization. Another main difference is in the absolute magnitude of night‐time E‐region Ne values. Both IRI and SD‐WACCM‐X are substantially lower than E‐PROBED. This work first concludes that E‐PROBED can conveniently provide E‐region Ne latitude—local time variabilities and structures that COSMIC‐1 observes. This work also concludes that E‐region Ne have significant non‐photoionization driven variabilities. Plain Language Summary: Navigation and communication systems reliant on sending signals into space require accurate predictions from empirical models of ionospheric electron density (Ne) values because the frequency of the appropriate signals to use are a function of Ne. Empirical models currently struggle the most with the ionospheric E‐region (between 90 and 120 km) because most of these models were constructed using sparsely sampled E‐region Ne data from ground‐based observations. Recently, E‐region Ne profiles were finally retrieved from Global Navigation Satellite Systems Radio Occultation (GNSS RO) measurements. This work reports on a new empirical model of monthly‐mean E‐region Ne that uses global observations of E‐region Ne from GNSS RO. The model is called the E‐region Prompt Radio Occultation Based Electron Density (E‐PROBED) Model, and it captures more than 80% of the observed variability in monthly‐mean latitude‐local time‐altitude E‐region Ne profiles. It agrees well with both ground‐based observations and first principles Physics‐based model simulations. These findings suggest that E‐PROBED makes a viable alternative E‐region Ne empirical model for those interested in an E‐region Ne model constrained by satellite observations. Key Points: The first version of E‐region Prompt Radio Occultation Based Electron Density Model (E‐PROBED) is developedE‐PROBED agrees with ground‐based observations during day‐time but not during night‐timeE‐PROBED agrees with SD‐WACCM‐X that E‐region Ne have significant non‐photoionization‐driven variabilities [ABSTRACT FROM AUTHOR]

Over the past decades, various experimental and numerical model studies have indicated cooling trend in the mesosphere and lower thermosphere (MLT), while the magnitude of the trend varies noticeably. Previous studies using the lidar observations derived the temperature trends and solar responses solely from the traditional nocturnal measurements. While these archived results are more or less in agreement with modeling studies, one of the main uncertainties in these studies is the potential biases induced by the trends of the diurnal tide forced in the lower atmosphere, and that of the in situ exothermal reactions involving the photolysis. In the MLT, the diurnal tide has significant seasonal variations, considerable amplitude and is one of the dominant dynamic sources. However, its potential effects in the trend studies have rarely been discussed. In this paper, we present and compare the long‐term temperature trends in the upper mesosphere utilizing the daily mean and nightly mean temperature profiles measured by a Sodium (Na) Doppler lidar at midlatitude. The system was operating routinely in full diurnal cycles between 2002 and 2017, obtaining a unique multi‐year temperature data set. A customized multi‐linear regression (MLR) model is applied to determine the linear trends and the other fitting parameters, such as ENSO and solar F10.7 responses in the upper mesosphere. This study indicates the daily mean cooling trend between 84 and 98 km is larger than that of nightly mean trend by ∼−1 K/decade, while differences in the solar response are within the fitting uncertainties. Plain Language Summary: The upper atmosphere is sensitive to both the climate change in the lower atmosphere and the solar activity. Studies have demonstrated this region has been experiencing long‐term cooling trend over the past several decades. Most of the previous ground‐based investigations utilized nighttime temperature observations to derive the long‐term changes in the mesosphere and lower thermosphere. However, these nighttime trend results can be biased due to the lack of daytime temperature measurements, leading to the uncertainty in these achieved results. In this study, the daily mean temperatures measured by a lidar at middle latitude between 2002 and 2017 are used to determine the trend and the upper atmosphere response to solar activity. The customized algorithm includes new and important climate parameters. This investigation eliminates such potential bias in the temperature, and the results demonstrated noticeably larger cooling trend in this atmospheric region when compared with those derived from the nightly temperature observations. In addition, the solar response derived from these daily mean lidar temperatures are positive but less than those based on nightly mean observations. The revealed differences will lead to future investigations on the underlying mechanisms and, thus, uncover new aspect of the long‐term studies. Key Points: The temperature in the mesosphere and lower thermosphere is coolingThe cooling trend derived from the daily mean temperatures is noticeably larger than that revealed by the nightly average temperaturesThe WACCM‐X simulations also indicate the differences between the daily mean trend and the nightly mean trend with less magnitude [ABSTRACT FROM AUTHOR]

Using the latest coupled geospace model Multiscale Atmosphere‐Geospace Environment (MAGE) and observations from Jicamarca Incoherent scatter radar (ISR) and ICON ion velocity meter (IVM) instrument, we examine the pre‐reversal enhancement (PRE) during geomagnetic quiet time period. The MAGE shows comparable PRE to both the Jicamarca ISR and ICON observations. There appears to be a discrepancy between the Jicamarca ISR and ICON IVM with the later showed PRE about two times larger (∼40 m/s). This is the first time that MAGE is used to simulate the PRE. The results show that the MAGE can simulate the PRE well and are mostly consistent with observations. Key Points: The Multiscale Atmosphere‐Geospace Environment is able to simulate the pre‐reversal enhancement (PRE) very wellThe PRE was a persistent feature on different time scales (day, hour, and minute) during this fall equinox time periodJicamarca Incoherent scatter radar observed PRE is about half of size of that by the ICON ion velocity meter [ABSTRACT FROM AUTHOR]

Using the High attitude Interferometer WIND observation balloon and Antarctic Jang Bogo station high latitude conjugate observations of the thermospheric winds we investigate the seasonal and hemispheric differences between the northern and southern hemispheres in June 2018. We found that the summer (northern) hemisphere dayside meridional winds have a double‐hump feature, whereas in the winter (southern) hemisphere the dayside meridional winds have a single hump feature. We attribute that to stronger summer, perhaps, northern hemisphere cusp heating. We also compared the observation with NCAR Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) model. The TIEGCM reproduced the double‐hump feature because of added cusp heating. The summer hemisphere has stronger anti‐sunward winds. This is the first time we have very high latitude conjugate thermospheric wind observations. Plain Language Summary: Using balloon instrument in the northern hemisphere and ground based instrument in the southern hemisphere, we study the conjugacy of the thermospheric winds of high latitudes. We found that the more summer hemispheric heating alters the thermospheric winds and resulted in a double‐hump feature on the dayside meridional winds. We also used model with cusp heating to simulate the winds and were able to reproduce the double‐hump feature. Key Points: The stronger summer (northern) hemispheric power could be the cause of double hump in the dayside meridional windsThermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) was able to reproduce the double‐hump features seen by the High attitude Interferometer WIND observation, and single hump in the winter hemisphere observed at Jang Bogo stationTIEGCM overestimates the poleward wind on the dayside and equatorward wind on the nightside in the summer hemisphere [ABSTRACT FROM AUTHOR]

Motivated by numerous lower atmosphere climate model hindcast simulations, we performed simulations of the Earth's atmosphere from the surface up through the thermosphere‐ionosphere to reveal for the first time the century scale changes in the upper atmosphere from the 1920s through the 2010s using the Whole Atmosphere Community Climate Model—eXtended (WACCM‐X v. 2.1). We impose solar minimum conditions to get a clear indication of the effects of the long‐term forcing from greenhouse gas increases and changes of the Earth's magnetic field and to avoid the requirement for careful removal of the 11‐year solar cycle as in some previous studies using observations and models. These previous studies have shown greenhouse gas effects in the upper atmosphere but what has been missing is the time evolution with actual greenhouse gas increases throughout the last century, including the period of less than 5% increase prior to the space age and the transition to the over 25% increase in the latter half of the 20th century. Neutral temperature, density, and ionosphere changes are close to those reported in previous studies. Also, we find high correlation between the continuous carbon dioxide rate of change over this past century and that of temperature in the thermosphere and the ionosphere, attributed to the shorter adjustment time of the upper atmosphere to greenhouse gas changes relative to the longer time in the lower atmosphere. Consequently, WACCM‐X future scenario projections can provide valuable insight in the entire atmosphere of future greenhouse gas effects and mitigation efforts. Plain Language Summary: From seeing lower atmosphere computer climate models run for the past century, we decided to do the same for the upper atmosphere using the Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X) for the decades from the 1920s to 2010s. In this higher region, the atmosphere is affected strongly by the Sun and removing the Sun's effect is tricky in previous observation and model studies. We make the Sun's effect small to see only effects from the Earth's magnetic field and greenhouse gases. Earlier studies focused on recent decades show effects of greenhouse gas increases on the upper atmosphere but not for the early decades of the past century with greenhouse gas changes from less than 5% increase prior to the space age and the transition to the over 25% increase in the latter half of the 20th century. We cover this entire period and get results like those in studies before, with especially the temperature change matching very well with the greenhouse gas carbon dioxide change. Because WACCM‐X performs well over the past century, it will be useful to predict what will happen in the century ahead as greenhouse gases increase and humans make efforts to reverse the increase. Key Points: Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X) simulated continuous rate of climate change in the thermosphere/ionosphere over the past century follows closely that of CO2 and CH4Large CO2/thermosphere temperature correlation shows greenhouse gas impact on upper atmosphere climate is consistent over the past centuryConsistency with past century observations and model simulations gives confidence in results of WACCM‐X 21st century climate projections [ABSTRACT FROM AUTHOR]

Leveraging observations by two NASA missions—GOLD (Global‐scale Observations of the Limb and Disk) and ICON (Ionospheric Connection Explorer), we investigate concurrent responses of thermospheric composition, temperatures, and neutral winds to the geomagnetic storm on 3–4 November 2021, as well as their interplay at low and middle latitudes. The synergetic observations reveal remarkable depletions up to 60%–70% in GOLD O/N2, along with large enhancements in GOLD temperatures poleward of 30° in the middle thermosphere. Meridional winds from ICON observations are altered by ∼100 m/s equatorward of 25°N latitude and at 250 km, characterized by a reversal of prevailing northward winds to geomagnetic storm‐driven southward winds. This study fills a need, after a decade‐long gap, for observing concurrent and co‐located responses of composition, temperatures, and neutral winds in the thermosphere to geomagnetic storms. Plain Language Summary: Geomagnetic storms, arising from solar wind shocks emitted from the Sun, deposit a large portion of the absorbed solar energy into the Earth's high‐latitude atmosphere through Joule heating and high energy particle heating. Accurate prediction of the responses to geomagnetic storms in the thermosphere‐ionosphere is one of the core scientific objectives of space weather. The recently launched Global‐scale Observations of the Limb and Disk (GOLD) and Ionospheric Connection Explorer (ICON) missions provides an unparalleled opportunity to assess storm‐induced concurrent changes in multiple key parameters. During a G3 geomagnetic storm on 3 November 2021, GOLD observed the substantial changes of 60%–70% in thermosphere column density ratios of atomic oxygen to molecular nitrogen and temperatures. Exceptional meridional winds of 100–200 m/s were also seen by ICON over a broad altitude range. Such observations fill a decades‐long need for simultaneous observations of the key variables in the ionosphere‐thermosphere system. Key Points: Remarkable depletions in O/N2 and enhancements in temperature, up to 60%–70%, are seen by Global‐scale Observations of the Limb and Disk during the geomagnetic stormAveraged southward wind deviations of ∼100 m/s are observed by Ionospheric Connection Explorer, coincident with the largest gradient in O/N2 depletionsBoth O/N2 and temperatures recover rapidly from the disturbed states to pre‐storm states [ABSTRACT FROM AUTHOR]

During a minor geomagnetic storm occurring from Aug 2 (day‐of‐year (DOY) 214) to Aug 4 (DOY 216), 2021, the National Aeronautics and Space Administration Global‐scale Observations of the Limb and Disk (GOLD) mission observed different column density ratio of O to N2 (ΣO/N2) variations in storm main and recovery phases. The percentage difference of ΣO/N2 between DOY 215 (disturbed day, main phase) and DOY 213 (quiet reference day) exhibits a depletion on the east side of the GOLD field‐of‐view (FOV). However, that of ΣO/N2 between DOY 216 (recovery phase) and 213 shows depletions on the west side of GOLD FOV. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model qualitatively reproduced the observations. Analysis of the model output illustrates that the ΣO/N2 patterns in the two days are both formed due to the classical thermospheric composition theory and formed on DOY 214 and 215, respectively. Further investigation found that the ΣO/N2 depletion on DOY 214 and 215 both initially formed near 120–180°E, but the one on DOY 215 then quickly moved westward into the GOLD FOV, from local post‐midnight to pre‐midnight, near 19 UT. Then it moves equatorward and slowly westward. This results in the observed depletion structure on the west side of GOLD FOV. Model simulations show that the quick westward movement near 19 UT is due to the dominant positive Interplanetary Magnetic Field east‐west component (By) conditions. Key Points: GOLD observed two different ∑O/N2 variations during the main and recovery phase of a minor geomagnetic storm on August 3–4Numerical simulations illustrate that these two different patterns originated in almost the same longitude ranges and local midnightThe quick westward motion of ∑O/N2 depletion due to strong westward wind under dominant Interplanetary Magnetic Field By results in GOLD observed different patterns [ABSTRACT FROM AUTHOR]

We investigate in detail the occurrence and evolution of ionospheric equatorial plasma bubbles (EPBs) during a moderate storm on 17 September 2021, using Global‐scale Observations of the Limb and Disk (GOLD) observations and Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X) simulations. GOLD observations show that there were no EPBs on 16 September before the storm but EPBs occurred after the storm commencement on 17 September. The EPBs extended to ∼30° magnetic latitude. A diagnostic analysis of WACCM‐X simulations reveals that the rapid enhancement of prompt penetration electric fields (PPEFs) after the sudden storm commencement is the main reason that triggered the occurrence of the EPBs. Further quantitative analysis shows that vertical plasma drifts, which are enhanced by the PPEF, played a dominant role in strengthening the Rayleigh‐Taylor instability, leading to the occurrence of the EPBs and the large latitudinal extension of the EPBs to ∼30° magnetic latitude during the night of 17 September. Plain Language Summary: Equatorial plasma bubbles (EPBs) are frequently observed in the equatorial region after sunset and have been the focus of ionospheric research. They are an important space weather phenomenon that often significantly impact trans‐ionospheric radio wave signals. During geomagnetic storms, the occurrence and evolution of EPBs are more complicated, and their physical mechanisms are worthy of further investigation. In this study, we used Global‐scale Observations of the Limb and Disk (GOLD) observations and Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X) simulations to study the occurrence and evolution of EPBs during a moderate storm on 17 September 2021. GOLD observations show that EPBs occurred after the storm commencement on 17 September, whereas no EPBs were observed on 16 September. Additionally, these EPBs extend to ∼30° magnetic latitude. According to the WACCM‐X simulation analysis, the rapid enhancement of the prompt penetration electric fields (PPEFs) after the sudden storm commencement resulted in a significant Rayleigh‐Taylor instability (RTI) on 17 September, triggering the occurrence of EPBs. Furthermore, quantitative analysis revealed that PPEF induces enhanced vertical plasma drifts, which are the main reason for the strengthening RTI and the large latitudinal extension of the EPBs on 17 September. Key Points: Equatorial plasma bubbles (EPBs) extended poleward to ∼30° magnetic latitude during a moderate storm on 17 September 2021Diagnostic analysis reveals that occurrence of EPBs was due to rapidly enhanced eastward prompt penetration electric field (PPEF) after the sudden storm commencementPPEF‐driven upward drifts are the main factor causing large Rayleigh‐Taylor instability, and thus the formation and large latitudinal extension of the EPBs [ABSTRACT FROM AUTHOR]

As a companion study of the Part 1 (J. C. Wang et al., 2022, https://doi.org/10.1029/2022JA030948), the impact of the lower‐thermospheric circulation on atomic oxygen (O) in the mesosphere and lower thermosphere (MLT) region is investigated in this Part 2 using Specified Dynamics Configuration Runs of the Whole Atmosphere Community Climate Model eXtended (SD‐WACCMX) output. The asymmetry of the O profile in the summer and winter MLT region is mainly driven by local vertical advection, which is associated with the lower‐thermospheric winter‐to‐summer circulation and middle‐to‐upper thermospheric summer‐to‐winter circulation. It is found that meridional transport and eddy diffusion only weakly modulate the O budget within this altitude range. The globally and annually averaged transport effect due to the vertical advection is quantitatively estimated. It is shown that the vertical advection is the dominant mechanism in redistributing O at altitudes between 84 and 103 km, suggesting the vertical wind can efficiently transport O between its source and sink region within the vertical column. This study demonstrates that whole atmosphere coupling on seasonal time scales is a complex interaction involving multiple underlying mechanisms within the space‐atmosphere interaction region. Key Points: Vertical advection is the dominant mechanism acting to transport O in the mesosphere and lower thermosphere (MLT)A Hemispheric asymmetry in MLT O number density is driven by local vertical advectionInterhemispheric mass transport and eddy diffusion only play a minor role in controlling O at higher latitudes in the MLT [ABSTRACT FROM AUTHOR]

This work investigates mid‐ and low‐latitude ionospheric disturbances over the American sector during a moderate but geo‐effective geomagnetic storm on 13–14 March 2022 (π‐Day storm), using ground‐based Global Navigation Satellite System total electron content data, ionosonde observations, and space‐borne measurements from the Global‐scale Observations of Limb and Disk (GOLD), Swarm, the Defense Meteorological Satellite Program (DMSP), and the Ionospheric Connection Explorer (ICON) satellites. Our results show that this modest but geo‐effective storm created a number of large ionospheric disturbances, especially the dynamic multi‐scale electron density gradient features in the storm main phase as follows: (a) The low‐latitude equatorial ionization anomaly (EIA) exhibited a dramatic storm‐time deformation and reformation, where the EIA crests evolved into a bright equatorial band for 1–2 hr and then quickly separated back into the typical double‐crest structure with a broad crest width and deep equatorial trough. (b) Strong equatorial plasma bubbles (EPBs) occurred with an abnormally high latitude/altitude extension, reaching the geomagnetic latitude of ∼30°, corresponding to an Apex height of 2,600 km above the dip equator. (c) The midlatitude ionosphere experienced a conspicuous storm‐enhanced density (SED) plume structure associated with the subauroral polarization stream (SAPS). This SED/SAPS feature showed an unusual temporal variation that intensified and diminished twice. These distinct mid‐ and low‐latitude ionospheric disturbances could be attributed to the storm‐time electrodynamic effect of electric field perturbation, along with contributions from neutral dynamics and thermospheric composition change. Key Points: A common key driver of storm‐time electric field generated three dynamic ionospheric changes simultaneously from low to subauroral latitudesEquatorial ionization anomaly crests showed merging and separation during interplanetary magnetic field Bz oscillation, and strong equatorial plasma bubbles reached 30° MLAT corresponding to 2,600 km Apex heightSalient storm‐enhanced density associated with subauroral polarization stream occurred in midlatitudes, showing a dynamic temporal variation that intensified and diminished twice [ABSTRACT FROM AUTHOR]

During the storm on 3 and 4 November 2021, the Global‐scale Observations of the Limb and Disk (GOLD) mission observed well separated equatorial ionization anomaly (EIA) crests post sunset on 3 November, but merged EIA on 4 November. We used the Whole Atmosphere Community Climate Model‐eXtended to simulate the EIA structures during the two nights. The simulations show two separated post sunset EIA crests on 3 November but merged post sunset EIA crests on 4 November, which are qualitatively consistent with the GOLD observations. Numerical simulations and Ionospheric Connection Explorer neutral wind observations illustrate that the formation of merged EIA crests was due to several hours of downward E × ${\times} $ B drifts before and after sunset. Further diagnostic analysis revealed that it was mainly driven by westward electric fields caused by the disturbance dynamo electric field during the recovery phase of the storm. Plain Language Summary: The Ionospheric equatorial ionization anomaly (EIA) occurs almost every day and has been studied extensively. A large number of studies have shown that EIA has usually two separated electron density crests around ± $\pm $15° off the magnetic equator in the ionosphere. A structure of merged EIA with just one density crest located near the magnetic equator was observed by a satellite on 4 November 2021. We successfully simulate the merged EIA structure using a first‐principles model. Analysis of the model results reveals that thermospheric neutral winds were disturbed by a geomagnetic storm on 3 and 4 November 2021; the disturbance dynamo electric field due to the disturbed winds changed the usual drift pattern of the plasma, which eventually drive the ionospheric plasma to move toward the magnetic equator causing the merged EIA structure. Key Points: Merged equatorial ionization anomaly (EIA) structure was observed by Global‐scale Observations of the Limb and Disk during the storm of 3 and 4 November 2021This EIA structure was successfully reproduced by Whole Atmosphere Community Climate Model‐eXtended simulationsModel diagnostic analysis reveals that the merged EIA was mainly driven by disturbance dynamo electric fields during the storm [ABSTRACT FROM AUTHOR]

The Whole Heliosphere and Planetary Interactions initiative was established to leverage relatively quiet intervals during solar minimum to better understand the interconnectedness of the various domains in the heliosphere. This study provides an expansive mosaic of observations spanning from the Sun, through interplanetary space, to the magnetospheric response and subsequent effects on the ionosphere‐thermosphere‐mesosphere (ITM) system. To accomplish this, a diverse set of observational datasets are utilized from 2019 July 26 to October 16 (i.e., over three Carrington rotations, CR2220, CR2221, and CR2222) with connections of these observations to the more focused studies submitted to this special issue. Particularly, this study focuses on two long‐lived coronal holes and their varying impact in sculpting the heliosphere and driving of the magnetospheric system. As a result, the evolution of coronal holes, impacts on the inner heliosphere solar wind, glimpses at mesoscale solar wind variability, magnetospheric response to these evolving solar wind drivers, and resulting ITM phenomena are captured to reveal the interconnectedness of this system‐of‐systems. Plain Language Summary: The field of Heliophysics research spans a large range of plasma regimes and disciplines that are all linked through the complex interactions each system has with one another. To better explore the interconnected nature of the heliosphere, this study combines observations from across the sub‐domains of Heliophysics to construct snapshots of the Sun, solar wind, and the magnetospheric and ionospheric systems at Earth over three revolutions of the Sun. This allows for a unique vantage point to put these systems in perspective of one another and understand how they impact one another. Key Points: The evolution and impacts of two coronal holes are investigated over three Carrington rotationsThe evolution of the coronal hole led to changes in the solar wind throughout the inner solar systemSubsequent geospace response differed with each passage due to both changes in the solar wind and preconditioning within the systems [ABSTRACT FROM AUTHOR]

The Whole Heliosphere and Planetary Interactions (WHPI) is an international initiative to study the most recent solar minimum and its impact on the interconnected solar‐heliospheric‐planetary system by facilitating and encouraging interdisciplinary activities. Particular WHPI science foci include the global connected structure of the heliosphere and planetary space environments/atmospheres, the origins and impacts of high‐speed solar wind streams, coronal mass ejections from Sun‐to‐Heliopause, and comparative solar minima. This is achieved through a series of coordinated observing campaigns, including Parker Solar Probe perihelia, and scientific virtual interactions including a dedicated workshop where observers and modelers gathered to discuss, compare, and combine research results. This introduction sets the scene for the WHPI interval, placing it into the context of prior initiatives and describing the overall evolution of the system between 2018 and 2020. Along with the accompanying articles, it presents a selection of key scientific results on the interconnected solar‐heliospheric‐planetary system at solar minimum. Key Points: The Whole Heliosphere and Planetary Interactions initiative studies the solar‐heliospheric‐planetary system's response to solar minimumSolar minimum time periods provide an opportunity to characterize the baseline system and to trace events from "end to end"By comparing solar minima of multiple solar cycles, we gain insight into how the system changes over decadal times scales [ABSTRACT FROM AUTHOR]

We report a new ionosphere phenomenon: Equatorial ionization anomaly (EIA) discontinuity (EIAD), based on OI 135.6 nm radiance observations from the Global Observations of Limb and Disk (GOLD), ground‐based total electron content maps and in‐situ ion density data from Constellation Observing System for Meteorology, Ionosphere, and Climate‐2. The EIAD occurs when the OI radiance of the EIA crest has a local minimum, at a fixed UT, with the radiance in the local longitude region being weaker than that on the east and west sides. In the GOLD field‐of‐view, EIAD follows the seasonal variations of EIA. EIAD appears more often over the Atlantic Ocean and Africa than over South America. It occurs more in the southern crest during the December solstice, and more in the northern crest during both equinoxes. EIAD can occur under both quiet and disturbed times. Plain Language Summary: The equatorial ionization anomaly (EIA) is very dynamic and can exhibit various structures. Here we report a newly discovered EIA structure: EIA discontinuity, namely the EIA crest shows a lower electron density in the middle longitude range than in east and west longitude ranges. We first show the observation of EIA discontinuity observed concurrently by a geo‐stationary orbit satellite, a low‐earth‐orbit satellite and ground‐based global positioning system receiver. Then a statistical study illustrates that the EIA discontinuity is mostly captured in field‐of‐view of the geo‐stationary satellite in one hemisphere. It obeys the seasonal variation of EIA. The occurrence is higher in the spring equinox than in the fall equinox. Near the December solstices, it appears more in the southern crest. In both equinoxes, it appears more often in the northern crest. In August, its occurrence increases with the increase of solar irradiance. The EIA discontinuity can occur under both geomagnetically quiet and disturbed times. Key Points: Equatorial ionization anomaly (EIA) discontinuity is the EIA crest with a weaker electron density in a longitude region than longitudes to the east and westStatistical study shows that its occurrence has a preference in Atlantic Ocean and Africa than America within the Global Observations of Limb and Disk field‐of‐viewEIA discontinuity can occur under both geomagnetically quiet and disturbed times [ABSTRACT FROM AUTHOR]

We examine hemispheric asymmetry of the annual and semiannual variation of the ratio of O and N2 concentrations (O/N2) using observations by the Global Ultraviolet Imager (GUVI) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite and compare them with Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) model simulations. We found that in the equatorial region, the "equinox peaks" of the observed O/N2 are near the end of March and October, and the two annual lows are near the beginning of July and January. Compared to the equatorial region, in the northern hemisphere (NH) low latitudes, the first "equinox peak" clearly shifts toward the December solstice, whereas in the southern hemisphere (SH) low latitudes, the "equinox peaks" shift toward the June solstice (JS), forming the hemispheric asymmetry characteristics of the annual and semiannual variation. Seasonal variation of O/N2 shows no apparent phase variation with altitude, and the annual and semiannual pattern is consistent from year to year. WACCM‐X reproduces the observed annual and semiannual pattern in NH but in SH, it simulates an annual variation instead of the observed annual and semiannual variation. The largest discrepancy occurs near JS in the lower and middle thermosphere: the simulated O density has an annual high near JS in SH; the simulated N2 density has an annual high near JS in NH but a predominant annual low near JS in SH. These are not in the GUVI data. A weaker thermospheric meridional circulation in the winter hemisphere, or a reduced summer‐to‐winter latitudinal gradient of neutral temperature in WACCM‐X simulations would make model‐data comparisons more consistent. Plain Language Summary: We examine the difference of seasonal variation of the ratio of thermosphere O and N2 (O/N2) between the two hemispheres using data observed from a satellite from 2003 to 2007 and compare them with numerical model simulations. We found that in the equatorial region, the two equinox peaks of the observed O/N2 are somewhere in March/April and October, respectively, and the two annual lows are near the beginning of July and January. Compared to the equatorial region, in the northern low latitudes, the first "equinox peak" clearly shifts toward the December solstice, whereas in the southern low latitudes, the two "equinox peaks" shift toward the June solstice (JS). The seasonal variation of O/N2 is consistent from year to year and at different altitudes. The model reproduces the observed seasonal variation in the northern hemisphere but in the southern hemisphere, it only produces an annual variation. A weaker thermospheric meridional circulation in the winter hemisphere, or a decrease of the model simulated temperature in the summer (northern) hemisphere near the JS would make the simulated O and N2 densities, and thus O/N2, more consistent with the observed ones. Key Points: "Equinox peaks" of Global Ultraviolet Imager (GUVI) O/N2 shift toward the December and June solstices in the northern and southern low latitudes, respectivelyWhole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) reproduces GUVI observed annual and semiannual variation of O/N2 in northern hemisphere but misses semiannual variation in southern hemisphereMisrepresentation of gravity wave forcing in regular resolution WACCM‐X may be an important contributor to the discrepancy [ABSTRACT FROM AUTHOR]

This study develops a new Bubble Index to quantify the intensity of 2‐D postsunset equatorial plasma bubbles (EPBs) in the American/Atlantic sector, using Global‐scale Observations of the Limb and Disk (GOLD) nighttime data. A climatology and day‐to‐day variability analysis of EPBs is conducted based on the newly‐derived Bubble Index with the following results: (a) EPBs show considerable seasonal and solar activity dependence, with stronger (weaker) intensity around December (June) solstice and high (low) solar activity years. (b) EPBs exhibit opposite geomagnetic activity dependencies during different storm phases: EPBs are intensified concurrently with an increasing Kp, but are suppressed with high Kp occurring 3–6 hr earlier. (c) For the first time, we found that EPBs' day‐to‐day variation exhibited quasi‐3‐day and quasi‐6‐day periods. A coordinated analysis of Ionospheric Connection Explorer (ICON) winds and ionosonde data suggests that this multi‐day periodicity was related to the planetary wave modulation through the wind‐driven dynamo. Plain Language Summary: Equatorial plasma bubbles (EPBs) are plasma density depletions that frequently occur in the nighttime equatorial and low latitude ionosphere, which are well‐known for imposing significant space weather effects on navigation and communication systems. This study develops a novel Bubble Index to quantify EPBs' activity, using the Global‐scale Observations of the Limb and Disk (GOLD) nighttime data. Based on this new Bubble Index, a climatology study and day‐to‐day variability analysis of GOLD‐captured EPBs were conducted. The EPBs' seasonal variation and solar and geomagnetic activity dependence were revealed and examined. Moreover, for the first time, we found the day‐to‐day variation of EPBs' intensity showed quasi‐3‐day and quasi‐6‐day periods. A coordinated analysis of neutral wind and ionosonde measurements suggests that this multi‐day periodicity was likely caused by planetary wave modulation via the wind‐driven dynamo. These new results shed new light on the day‐to‐day variability of the ionosphere and its possible connection with atmospheric waves. Key Points: A new Bubble Index is developed to quantify 2‐D EPBs' intensity over the American/Atlantic sector using GOLD nighttime disk image dataA first‐time statistical analysis of GOLD data shows that equatorial plasma bubbles (EPBs) have opposite geomagnetic activity dependencies during different storm phasesEPBs' intensity exhibits ∼3‐ and ∼6‐day periodicity, coinciding with planetary waves feature observed in atmospheric winds and hmF2 [ABSTRACT FROM AUTHOR]

The geomagnetic storm on February 3, 2022 caused the loss of 38 Starlink satellites of Space‐X. The Global‐scale Observations of the Limb and Disk (GOLD) observations and Multi‐Scale Atmosphere Geospace Environment (MAGE) model simulations are utilized to investigate the thermospheric composition responses to the Space‐X storm. The percentage difference of the GOLD observed thermospheric O and N2 column density ratio (∑O/N2) between the storm time (February 3, Day‐of‐Year [DOY] 34) and quiet time (DOY 32) shows a depletion region in the local noon sector mid‐high latitudes in the southern hemisphere, which corresponds to the east side of GOLD field‐of‐view (FOV). This is different from the classic theory of thermospheric composition disturbance during geomagnetic storms, under which the ∑O/N2 depletion is usually generated at local midnight and high latitudes, and thus, appear on the west side of GOLD FOV. MAGE simulations reproduce the observations qualitatively and indicate that the ∑O/N2 depletion is formed due to strong upwelling in the local morning caused by strong Joule heating. Interestingly, enhanced equatorward winds appear near local midnight, but also in the local morning sector, which transports ∑O/N2 depletion equatorward. The depletion corotates toward the local afternoon and is observed in the GOLD FOV. The equatorward winds in the local morning are due to the ion‐neutral coupling under the conditions of a dominant positive interplanetary magnetic field east‐west component (By) during the storm. Key Points: Global‐scale Observations of the Limb and Disk (GOLD) observed a southern hemisphere ∑O/N2 depletion on the east side of its field‐of‐view (local afternoon)The GOLD observed southern ∑O/N2 is different from the classical theory of the thermosphere composition response to geomagnetic stormsSimulation shows that the observed ∑O/N2 depletion is generated in local morning, and transported equatorward, corotated to later local times [ABSTRACT FROM AUTHOR]

This study investigates thermospheric seasonal variations composed of annual and semiannual oscillations (AO and SAO) in the column density ratio of atomic oxygen to molecular nitrogen observed by Global‐scale Observations of Limb and Disk. On the basis of multiple analysis approaches, it is revealed that the AO is the predominant component at middle latitudes, while the SAO tends to be increasingly important toward the lower latitudes. The SAO is also evident in the southern mid‐latitudes. Quantitative analyses demonstrate that AO (SAO) peaks around winter solstices (equinoxes). Additionally, the seasonal variation at low‐latitudes exhibits greater amplitudes in the early morning and around noon, whereas it gradually weakens from early morning toward late afternoon at mid‐latitudes. A hypothesis is that seasonal variations are likely modulated by atmospheric tides, including those generated in the thermosphere and those propagating upward from the lower atmosphere. Plain Language Summary: Annual and semiannual variations are the most prominent phenomenon standing out on the seasonal scale in the thermosphere‐ionosphere system. Despite being recognized for decades, the characteristics of the AO and SAO are not yet well delineated, and relevant physics is not fully understood, due largely to limited observations and sampling constraints of low‐Earth‐orbit (LEO) satellites. For instance, distinguishing the local time versus longitude variations in the thermosphere is not feasible with LEO satellites alone. Leveraging the synoptic observations enabled by the Global‐scale Observations of Limb and Disk (GOLD) mission, we identified the AO and SAO in the column density ratios of atomic oxygen to molecular nitrogen at low and middle latitudes. More importantly, since GOLD provides full coverage of daytime local times on a daily basis, we quantified the local time dependence of seasonal variations in composition. Prevalent local time modulations could be driven by atmospheric tides. Key Points: Pronounced annual and semiannual oscillations (AOs and SAOs) are seen in Global‐scale Observations of Limb and Disk ∑O/N2AO is predominant at mid‐latitudes, and SAO is evident in both low‐latitudes and southern mid‐latitudesLocal time modulations of seasonal variations are prominent at low‐ and mid‐latitudes [ABSTRACT FROM AUTHOR]

This work uses the Specified Dynamics‐Whole Atmosphere Community Climate Model with Ionosphere/Thermosphere eXtension (SD‐WACCM‐X) to determine and explain the seasonality of the migrating semidiurnal tide (SW2) components of tropical upper mesosphere and lower thermosphere (UMLT) temperature, zonal wind, and meridional wind. This work also quantifies aliasing due to SW2 in satellite‐based tidal estimates. Results show that during equinox seasons, the vertical profiles of tropical UMLT temperature SW2 and zonal‐wind SW2's amplitudes have a double‐peak structure while they, along with meridional‐wind SW2, have a single‐peak structure in their amplitudes in June solstice. Hough mode reconstruction reveals that a linear combination of five SW2 Hough modes cannot fully reproduce these features. Tendency analysis reveals that for temperature, the adiabatic term, nonlinear advection term, and linear advection term are important. For the winds, the classical terms, nonlinear advection term, linear advection term, and gravity wave drag are important. Results of our alias analysis then indicate that SW2 can induce an ∼60% alias in zonal‐mean and DW1 components calculated from sampling like that of the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics satellite and the Aura satellite. This work concludes that in situ generation by wave–wave interaction and/or by gravity waves plays significant roles in the seasonality of tropical UMLT temperature SW2, zonal‐wind SW2, and meridional‐wind SW2. The alias analysis further adds that one cannot simply assume that SW2 in the tropical UMLT is negligible. Key Points: Thermodynamic and momentum budget of tropical migrating semidiurnal tide in the upper mesosphere and lower thermosphere calculatedHough modes cannot fully reconstruct tropical migrating semidiurnal tide component of temperature and windsWave–wave interaction and gravity waves significantly affect tropical migrating semidiurnal tide component of temperature and winds [ABSTRACT FROM AUTHOR]

Understanding the consequence of the complex interplay between solar flare and geomagnetic storm on the magnetosphere‐ionosphere (M‐I) coupling is a critical aspect to space weather research. This is the first attempt to simulate the concurrent solar flare and geomagnetic storm effects on M‐I coupling using the state‐of‐art geospace model which integrates Lyon‐Fedder‐Mobarry ‐Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model —the Rice Convection ring current Model (LTR) in a self‐consistent way. Our results indicate that dayside E‐region electron density enhancements peak nearly at the same time as the flare and these electron density enhancements at middle latitudes are not very sensitive to storm forcing. F‐region electron densities also have an immediate response to solar flares but take longer to recover compared to E‐region ionosphere, and dayside F‐region electron density enhancements are more prominent for more stormy conditions. These dayside middle‐latitude F‐region enhancements intensify tongue of ionization in the polar cap region. Both E‐ and F‐region electron density increases contribute to polar ionosphere conductance enhancements that have both local and global consequences. Flare‐induced conductance enhancements tend to reduce the amount of Joule dissipation into upper atmosphere and cross polar cap potential around solar flare peak (12:00 UT), and increase dayside field aligned currents and Joule dissipation afterward (12:20–13:30 UT). These effects are more appreciable for stronger solar wind driving conditions. These simulation results provide important references for interpreting observed solar activity/cycle dependence of magnetosphere‐ionosphere coupling phenomena. Plain Language Summary: The solar wind‐magnetosphere‐ionosphere coupling is analogous to a simple circuit in which solar wind‐magnetosphere interaction works as a generator and the ionosphere works as a load with its conductance. Thus, ionospheric conductance is an important factor controlling the solar wind energy dissipation in Earth's upper atmosphere and magnetosphere‐ionosphere electrodynamic process, and the polar ionospheric plasma dynamics. During solar flares, the dayside ionosphere conductance is significantly enhanced, thus providing a natural laboratory to test the hypothesis of how the ionospheric conductance regulates the Magnetosphere‐Ionosphere coupling. A self‐consistent magnetosphere‐ionosphere coupled model is used to understand how the solar‐flare induced conductance changes modify the dynamics of the magnetosphere‐ionosphere coupled system during both geomagnetic quiet and storm periods. Our results indicated that flare‐induced conductance enhancements tend to reduce the amount of Joule dissipation into the upper atmosphere and cross polar cap potential near the solar flare peak (∼12:00 UT), and increase dayside field aligned currents and Joule dissipation afterward (∼12:20–13:30 UT). These effects are more pronounced during more disturbed solar wind driving conditions. The stormtime polar ionosphere tongue‐like ionization structure is also intensified for the flare case as compared to the non‐flare case. Key Points: The concurrent solar flare and geomagnetic storm effects on M‐I coupling is simulated using whole geospace modelThe solar flare enhanced dayside F‐region electron densities intensify tongue of ionization in the polar capFlare‐enhanced ionosphere conductance reduces cross‐polar cap potential and hemispheric integrated Joule heating around flare peak [ABSTRACT FROM AUTHOR]

Salient depletions (30%–40% of the reference values) were observed in the vertical total electron content (VTEC) from the Global Navigation Satellite System (GNSS) network and the topside ionosphere electron density from Swarm satellites measurements over the Western Atlantic and Northeast American regions during a weak geomagnetic activity event (maximum Kp = 2.3) between Day‐of‐Year (DOY) 155 and 156 in 2019. These depletions coincided with thermospheric Oxygen (O) to Nitrogen (N2) column density ratio (∑O/N2) depletions observed by the Global‐scale Observations of the Limb and Disk (GOLD) mission. The National Center for Atmospheric Research (NCAR) Thermosphere–Ionosphere–Electrodynamics General Circulation Model (TIEGCM) reproduces VTEC depletions qualitatively, which is used to investigate the underlying mechanisms. Term analysis of the TIEGCM oxygen ion (O+) continuity equation showed that the VTEC depletion was dominated by chemical processes and neutral wind transport effects, which were partially offset by the transport due to E × B drifts and ambipolar diffusion. On the dayside, the contribution from the neutral wind transport was induced by a weak poleward meridional wind during the geomagnetic activity. With the decrease of electron density, the loss rate in the chemical term was reduced, which led to a large reduction of the contribution from chemical processes to electron density and TEC changes during the event, and the contribution of the neutral wind to VTEC depletion even exceeded that of chemical processes after 14:10 UT. Key Points: Thirty percentage VTEC depletions in mid‐latitude were observed during a weak geomagnetic activity event with a maximum Kp of 2.3Neutral wind transport played a more important role in generating the VTEC depletions than the chemical processesThe impact that the weak geomagnetic activity brings on mid‐latitude ionosphere cannot be simply ignored [ABSTRACT FROM AUTHOR]

We study the variations of the topside ionospheric ion density measured by Defense Meteorological Satellite Program satellites during the intense magnetic storm on 7–10 November 2004. It is found for the first time that quasi‐periodic enhancements in the ion density with a period of ∼6 hr occur nearly simultaneously at 0630, 0830, and 0930 local time in the dawn sector during the storm main phase with southward interplanetary magnetic field (IMF). The quasi‐periodic density enhancements extend from the southern subauroral latitudes to the northern subauroral latitudes. In the dusk sector, the topside ion density during the storm main phase is increased at middle latitudes for ∼12 hr but shows decrease or relatively small increase over the magnetic equator, indicating that penetration electric fields dominate the ion density redistribution. Similar quasi‐periodic enhancements in the topside ion density are also observed in the dawn sector during other intense magnetic storms. The solar wind and IMF do not have quasi‐periodic variations in this storm case. Periodic processes in geospace, such as periodic substorms in the magnetosphere, waves and tides in the atmosphere, and traveling ionospheric disturbances, cannot explain the observed periodic enhancements of the ionospheric ion density. We suggest that the magnetosphere‐ionospheric‐thermospheric system may have an intrinsic period of ∼6 hr and that oscillations of the magnetosphere‐ionospheric‐thermospheric system with this period can be excited during intense magnetic storms, although the mechanisms for the generation of the long‐periodic oscillations are not understood. Key Points: Quasi‐periodic ion density enhancements with a period of ∼6 hr near dawn are identified during intense magnetic storms for the first timeThe topside ionospheric density at middle and low latitudes near dusk is enhanced by penetration electric fields for ∼12 hrThe cause for the quasi‐periodic (6 hr) enhancements in the ionospheric ion density is unknown [ABSTRACT FROM AUTHOR]

The behaviors of the nitric oxide (NO) cooling in the lower thermosphere during the 14 December 2020 solar eclipse are studied using Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) measurements and WACCM‐X simulations. We found that NO cooling rate decreases during the solar eclipse in both SABER measurements and WACCM‐X simulations. The maximum decrease of the NO cooling is 40% in SABER measurements and 25% in WACCM‐X simulations. The NO cooling process is initiated almost entirely through the collisions with atomic oxygen (O) which depends linearly on NO and O densities and non‐linearly on the neutral temperature. During the eclipse, the NO concentration and temperature decreases are larger than that of O concentration. Consequently, the eclipse‐time NO concentration and temperature decreases are the major drivers of the NO cooling rate decrease. The decreases of the temperature and the NO concentration contribute comparably to the eclipse‐time NO cooling rate decrease. Plain Language Summary: Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) is an instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite satellite which was launched on 7 December 2001. Whole Atmosphere Community Climate Model with Thermosphere and Ionosphere eXtension (WACCM‐X) is the National Center for Atmospheric Research whole atmosphere global circulation model. In this work, we used SABER measurements and WACCM‐X simulations to investigate the behaviors of the NO cooling during the 14 December 2020 solar eclipse. The NO cooling decreases during the eclipse from both the SABER observations and WACCM‐X simulations. The decrease of the NO cooling is mostly caused by the decreases of the NO density and temperature during the solar eclipse. Key Points: Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observed a decrease in radiative cooling by NO in the thermosphere during the solar eclipse on 14 December 2020The eclipse‐induced NO cooling rate decrease observed by SABER is consistent with the WACCM‐X simulationThe eclipse‐time decreases of the NO concentration and temperature are the major drivers of the NO cooling rate decrease during the eclipse [ABSTRACT FROM AUTHOR]

In this study, the mechanism driving the narrow lower‐thermospheric winter‐to‐summer meridional circulation is thoroughly investigated for the first time using the Specified Dynamics configuration runs of the Whole Atmosphere Community Climate Model eXtended (SD‐WACCMX) simulations and the TIMED Doppler Interferometer (TIDI) observations. The mean meridional circulation in the SD‐WACCMX is qualitatively consistent with the TIDI measurements, though the magnitude in the SD‐WACCMX is about 50% weaker. The lower‐thermospheric winter‐to‐summer circulation is mainly driven by the resolved wave forcing, including the tides and internally generated inertia gravity waves (GWs). The momentum forcing from the parameterized sub‐grid scale GWs is not as significant as the resolved wave forcing in driving the lower‐thermospheric meridional circulation. The GW parameterization scheme in the SD‐WACCMX only includes GWs with phase velocities in the range of ±45 m/s, which might result in most of the parameterized sub‐grid GWs dissipating and breaking in the mesosphere and hardly impacting the lower thermosphere. Only including slow GWs in the SD‐WACCMX parameterization could potentially lead to the underestimation of the meridional wind in the model. Analysis also indicates the lower‐thermospheric meridional circulation is stronger in the summer hemisphere, which is attributed to the hemispheric asymmetry in the resolved wave momentum forcing. This study underlines the importance of the whole atmosphere coupling through wave propagation and dissipation. This understanding can guide the model development with an accurate representation of underlying physical processes in the mesosphere and lower thermosphere which drives the lower‐thermospheric circulation as well as the overall dynamics of this region. Plain Language Summary: Wave breaking and dissipation play a significant role in determining the large‐scale dynamics in the mesosphere and lower thermosphere (MLT). Wave forcing by such breaking and dissipation drives a meridional circulation from the mesospheric summer to the winter hemisphere, which maintains the temperature far from radiative equilibrium in the mesosphere. Above the mesospheric meridional circulation, observations of tracer gas distribution and numerical experiments suggest the existence of a reversed meridional circulation in the lower thermosphere. However, very little work has been done on thoroughly investigating the dynamics driving the lower‐thermospheric meridional circulation. In this study, the mechanism driving the lower‐thermospheric winter‐to‐summer meridional circulation is thoroughly investigated for the first time using the Specified Dynamics configuration runs of the Whole Atmosphere Community Climate Model eXtended simulations and the TIMED Doppler Interferometer observations. The lower‐thermospheric meridional circulation is mainly driven by the resolved wave forcing. The parameterized subgrid gravity wave forcing only plays a secondary role in driving the lower‐thermospheric meridional circulation. This study underlines the importance of the whole atmosphere coupling through wave propagation and dissipation. This understanding can guide the model development with an accurate representation of underlying physical processes in the MLT, which drives the lower‐thermospheric meridional circulation as well as the overall dynamics of this region. Key Points: Lower‐thermospheric winter‐to‐summer circulation in Specified Dynamics configuration runs of the Whole Atmosphere Community Climate Model eXtended qualitatively agrees with observation, though the magnitude is ∼50% weakerLower‐thermospheric circulation is mainly driven by the resolved wave forcing, including tides and inertia gravity waves (GWs)Hemispheric asymmetry in the resolved GW momentum forcing results in a stronger meridional circulation in the summer hemisphere [ABSTRACT FROM AUTHOR]

The Global‐scale Observation of Limb and Disk (GOLD) mission observed distinct post‐sunset, hemispherically asymmetric evolution of the equatorial ionization anomaly between 40° and 50°W on 19 November 2018, with the southern crest shifting poleward but the northern crest remaining in the same latitude region. The Whole Atmosphere Community Climate Model‐eXtended captured this phenomenon. Diagnostic analysis revealed that this asymmetric evolution was due to the hemispheric asymmetry in plasma transport by E × B drifts and neutral winds. In the Northern hemisphere, northward neutral wind transport dominated around sunset (20 UT near 45°W), causing the crest to be at lower altitudes and decay faster. After pre‐reversal enhancement (PRE) occurred at 20 UT in the northern hemisphere, E × B and neutral wind transport largely balanced each other, and the crest remained in the same latitude region. In the southern hemisphere EIA crest, PRE occurred around 1 hr later than the one in the north. The northward neutral wind transported the plasma to higher altitudes with less recombination. E × B transport augmented the neutral wind transport after 21 UT, lifting and pushing the crest poleward. The magnetic field configuration (large declination) and the resultant occurrence time difference in PRE between 40°W and 50°W not only play critical roles in the asymmetric evolution of the two crests, but also makes it possible for this phenomenon to be observed in the GOLD field‐of‐view. Plain Language Summary: A whole atmosphere model from the Earth's surface to the upper thermosphere (∼600 km) quantitively reproduced a hemispherically asymmetric movement of equatorial ionization anomaly (EIA) crests (between 40°W and 50°W; 22–24 UT on 19 November 2018) observed by the airglow measurements on a geo‐stationary satellite. Model results revealed that the neutral winds transported the plasma toward the magnetic equator and higher altitudes with less chemical loss in the southern hemisphere, whereas opposite condition occurred in the northern hemisphere between 19 and 21 UT. Consequently, ionospheric F‐peak height (hmF2) of the EIA in the southern hemisphere was much higher than that in the northern hemisphere. After 20 UT, pre‐reversal enhancement (PRE) appeared in the northern hemisphere. It offsets the transport due to field‐aligned wind in the vertical direction, which made the northern crest stationary. However, PRE appeared 1‐hr later in the southern hemisphere. The transport by E × B drifts in the southern EIA region augmented field‐aligned wind transport, lifting the southern EIA crest and shifting it poleward. The local time difference of the PRE occurrence in the two hemispheres drive this hemispherically asymmetric movement of EIA from 22 to 24 UT, which was observed by the geo‐stationary satellite. Key Points: Whole Atmosphere Community Climate Model‐eXtended simulation captures the Global‐scale Observation of Limb and Disk observed postsunset equatorial ionization anomaly (EIA) asymmetric evolution near 45°W longitudeThe offset of E × B drifts and neutral wind transport and strong recombination at lower altitude keep the northern crest at the same latitudePre‐reversal enhancement occurs 1 hr later in the south and augments neutral wind transport to lift the EIA crest and shift it poleward [ABSTRACT FROM AUTHOR]

We examine characteristics of the seasonal variation of thermospheric composition using column number density ratio ∑O/N2 observed by the NASA Global Observations of Limb and Disk (GOLD) mission from low‐mid to mid‐high latitudes. We also use ∑O/N2 derived from the Global Ultraviolet Imager (GUVI) limb measurements onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and estimated by the NRLMSISE‐00 empirical model to aid our investigation. We found that the ∑O/N2 seasonal variation is hemispherically asymmetric: in the southern hemisphere, it exhibits the well‐known annual and semiannual pattern, with highs near the equinoxes, and primary and secondary lows near the solstices. In the northern hemisphere, it is dominated by an annual variation, with a minor semiannual component with the highs shifting toward the wintertime. We also found that the durations of the December and June solstice seasons in terms of ∑O/N2 are highly variable with longitude. Our hypothesis is that ion‐neutral collisional heating in the equatorial ionization anomaly region, ion drag, and auroral Joule heating play substantial roles in this longitudinal dependency. Finally, the rate of change in ∑O/N2 from one solstice season to the other is dependent on latitude, with more dramatic changes at higher latitudes. Plain Language Summary: We study how the amount of atmosphere constituent in the Earth's thermosphere change over a year at low‐mid to mid‐high latitudes using the data measured by the We study how the amount of atmosphere constituent in the Earth's thermosphere change over a year at low‐mid to mid‐high latitudes using the data measured by the NASA Global Observations of Limb and Disk (GOLD) mission. The main constituents in the thermosphere are atomic oxygen (O) and molecular nitrogen (N2). The ratio of O and N2, O/N2, is a common way to measure the relative abundance of these two constituents. The GOLD mission uses instruments aboard on a geostationary satellite to observe O/N2. We found that how O/N2 changes over a year is different in the northern and southern hemispheres. In the southern hemisphere, it has an annual and seminal pattern, with high O/N2 near the equinoxes, and primary and secondary lows near the solstices. In the northern hemisphere, it is mainly an annual pattern, with a minor semiannual component. The semiannual component has high O/N2 values shifting away from the equinoxes toward the wintertime. We also found that whether O/N2 is larger in one hemisphere or the other highly depends on longitude. Key Points: GOLD ∑O/N2 is compared to ∑O/N2 from GUVI and NRLMSISE‐00 on a seasonal time scale, showing consistency∑O/N2 seasonal variation is hemispherically asymmetric with annual/semiannual in the SH but dominant annual in the NHThe equinox transition of ∑O/N2 highly depends on longitude and latitude [ABSTRACT FROM AUTHOR]

Following the 2022 Tonga Volcano eruption, dramatic suppression and deformation of the equatorial ionization anomaly (EIA) crests occurred in the American sector ∼14,000 km away from the epicenter. The EIA crests variations and associated ionosphere‐thermosphere disturbances were investigated using Global Navigation Satellite System total electron content data, Global‐scale Observations of the Limb and Disk ultraviolet images, Ionospheric Connection Explorer wind data, and ionosonde observations. The main results are as follows: (a) Following the eastward passage of expected eruption‐induced atmospheric disturbances, daytime EIA crests, especially the southern one, showed severe suppression of more than 10 TEC Unit and collapsed equatorward over 10° latitudes, forming a single band of enhanced density near the geomagnetic equator around 14–17 UT, (b) Evening EIA crests experienced a drastic deformation around 22 UT, forming a unique X‐pattern in a limited longitudinal area between 20 and 40°W. (c) Thermospheric horizontal winds, especially the zonal winds, showed long‐lasting quasi‐periodic fluctuations between ±200 m/s for 7–8 hr after the passage of volcano‐induced Lamb waves. The EIA suppression and X‐pattern merging was consistent with a westward equatorial zonal dynamo electric field induced by the strong zonal wind oscillation with a westward reversal. Plain Language Summary: The extreme Tonga volcano eruption on 15 January 2022 triggered profound and prolonged disturbances in the equatorial ionosphere. Using ground‐based Global Navigation Satellite System total electron content and ionosonde data, low‐Earth orbiting Ionospheric Connection Explorer wind observations, and geostationary Global‐scale Observations of the Limb and Disk ultraviolet measurements of thermospheric and ionospheric airglow emission, we found volcano‐induced atmospheric Lamb waves as a plausible source of the observed pronounced deformation of equatorial ionization anomaly (EIA) in the American sector. This deformation was triggered by volcano‐induced wave passage through the region, even after these waves had traveled 12,000–16,000 km distance for over 10 hr. The typical double‐crested EIA morphology was severely deformed during both the daytime and nighttime. Daytime EIA crests exhibited a severe suppression of more than 10 TEC Unit (TECu) (1TECu = 1016 m−2) and an equatorward collapse around 14–17 UT (10–13 LT) especially for the southern crest. Nighttime EIA crests exhibited a unique X‐pattern with crests drastically merging in a narrow longitude region between 20 and 40°W around 22 UT (20 LT). These large EIA deformations were coincident, sharing spatial scales with long‐lasting oscillations of thermospheric zonal wind following the arrival of volcano‐induced waves in the American sector. This study sheds new light on the potential far‐reaching and long‐lasting atmosphere‐ionosphere‐thermosphere impacts from a catastrophic geological event. Key Points: Daytime equatorial ionization anomaly (EIA) crests eroded severely by 10+ TEC Unit and collapsed equatorward over 10° latitude, following the arrival of volcano‐induced Lamb wavesEIA crests exhibited a unique deformation after sunset with an X‐pattern merging in a longitudinally‐confined area between 20° and 40°WThermospheric zonal wind showed post‐volcanic oscillations of ±200 m/s that may explain the EIA suppression and X‐pattern via dynamo effect [ABSTRACT FROM AUTHOR]

Numerical simulations using the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) are performed to elucidate the effects of the interplanetary magnetic field (IMF) on the middle thermosphere composition during a "geomagnetically quiet" period from the day of year (DOY) 110–111 in 2019 (when the Auroral electrojet (AE) index never exceeded 300 nT and the Kp never exceeded 2). In particular, this paper aims to examine how the Global‐scale Observations of the Limb and Disk (GOLD) mission observed daytime thermospheric O and N2 column density ratio (∑O/N2) depletion at mid‐latitudes originated under such a "geomagnetically quiet" condition. A comparison of electric potential, Joule heating rate per unit mass, ion velocity, neutral temperature and winds in the middle thermosphere (∼160 km) between real IMF and without the IMF east‐west component (By) indicates that a By dominant condition can enhance their strengths under this "geomagnetically quiet" condition. Consequently, ∑O/N2 depletion with a stronger magnitude (30% compared with ∼8% without By) and larger disturbed area was introduced in the post‐midnight sector at high‐latitudes due to strong and localized upwelling associated with the enhanced Joule heating rate per unit mass. The ∑O/N2 depletion was transported equatorward and corotated from local post‐midnight to early morning, and was observed by GOLD at middle latitudes during daytime. Plain Language Summary: This study utilizes numerical simulations with a climatological tide in the lower boundary to investigate how thermospheric composition disturbance is produced and transported to mid‐latitudes during a geomagnetically quiet period (DOY110‐112, 2019, maximum Kp = 1.7, AE < 300 nT). Through comparing parameters from simulations under real IMF and By = 0, it is found that the IMF By, which was dominant over this period, can significantly enhance the ionospheric potential, Joule heating rate per unit mass, ion drift and neutral winds compared with Bz only condition. As a result, the geomagnetic activity with AE < 300 nT can trigger stronger depletion (30%) of the column density ratio of thermospheric O and N2 (∑O/N2) in the polar region. Then the accelerated meridional wind transports the ∑O/N2 depletion equatorward to mid‐latitudes. The depletion is also transported from local post‐midnight to local early morning. This study shows the crucial role of dominant By during "quiet time". Additionally, it suggests that the response of ∑O/N2 to geomagnetic activity during the "quiet period" is similar to the one during geomagnetic storms. Key Points: IMF with dominant By enhances ion convection pattern, Joule heating rate and neutral windsStrong thermospheric composition disturbance can be generated and propagate into mid‐latitudes with dominant By during "quiet" timeResponse of thermospheric composition to geomagnetic activity with AE < 300 nT is similar to its response to geomagnetic storms [ABSTRACT FROM AUTHOR]

In the mesosphere and lower thermosphere (MLT) region, residual circulations driven by gravity wave breaking and dissipation significantly impact constituent distribution and the height and temperature of the mesopause. The distribution of CO2 can be used as a proxy for the residual circulations. Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) CO2 volume mixing ratio (VMR) and temperature measurements from 2003 to 2020 are used to study the monthly climatology of MLT residual circulations and the mesopause height. Our analyses show that (a) mesopause height strongly correlates with the CO2 VMR vertical gradient during solstices; (b) mesopause height has a discontinuity at midlatitude in the summer hemisphere, with a lower mesopause height at mid‐to‐high latitudes as a result of adiabatic cooling driven by strong adiabatic upwelling; (c) the residual circulations have strong seasonal variations at mid‐to‐high latitudes, but they are more uniform at low latitudes; and (d) the interannual variability of the residual circulations and mesopause height is larger in the Southern Hemisphere (SH; 4–5 km) than in the Northern Hemisphere (NH; 0.5–1 km). Key Points: Monthly climatology of the mesopause height, and MLT CO2 gradients as signatures of the residual circulations are derived from SABER dataBoth the regions of maximum vertical CO2 gradients and mesopause height show strong seasonal variations at mid‐to‐high latitudesInterannual variations of the height of maximum vertical CO2 gradients and mesopause height are larger in the SH than in the NH [ABSTRACT FROM AUTHOR]

In this work, the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model is used to investigate the responses of ionospheric electrodynamic processes to the solar flares at the flare peaks and the underlying physical mechanisms on September 6 and 10, 2017. Simulations show that solar flares increased global daytime currents and reduced the eastward electric fields during the daytime from the equator to middle latitudes. Furthermore, westward equatorial electric fields and equatorial counter electrojets occurred in the early morning. At the flare peak, these electrodynamic responses are predominantly related to the enhanced E‐region conductivity by flares, as the responses of neutral winds and F‐region conductivity to flares are negligible. Specifically, the Cowling conductance enhancement is not the major process causing the reduction of zonal electric fields. This electric field reduction is primarily associated with the decrease of the ratio between the field line‐integrated wind‐driven currents and the conductance. The flare‐induced conductivity enhancement is larger but the background wind speed is smaller in the E‐region than in the F‐region, as a result, the increase of total integrated wind‐driven currents is less than the conductance enhancement. Key Points: Flare‐induced conductivity enhancement, which is dominant over the decrease of eastward electric fields, increases global daytime currentsE‐region conductivity enhancement decreases the ratio between integrated wind‐driven currents and conductance and reduces electric fieldsThe conductance enhancement tends to induce westward electric fields and equatorial counter electrojets in the early morning [ABSTRACT FROM AUTHOR]

The Global‐scale Observation of Limb and Disk (GOLD) mission, for the first time, provides synoptic two‐dimensional (2D) maps of OI 135.6 nm observations. These maps describe the unambiguous and dynamic evolution of nighttime ionospheric F2‐peak electron densities (NmF2) as the 135.6 nm airglow emission radiance correlates well with NmF2 at night. On November 19, 2018, the 135.6 nm radiance measured by GOLD, NmF2 measured by a digisonde, and GPS total electron content (TEC) measurements at Cachoeira Paulista (CP) all showed a postsunset enhancement, with an increase near 22:30 universal time. The 135.6 nm radiance map showed that this enhancement was due to the southward movement of the southern equatorial ionization anomaly (EIA) crest. Therefore, the GOLD observation showed the linkage between postsunset enhancement of NmF2 and EIA movement. Furthermore, unlike the southward movement of the southern crest, the corresponding EIA northern crest, however, did not show northward motion. This is the first time that the EIA hemispheric asymmetry, which included both different densities and movement of two crests in a short time period (<2‐h), was captured. The cause of this asymmetric movement of the two crests is not clear and requires further investigation.Plain Language Summary: The postsunset enhancement of equatorial ionospheric NmF2 has been observed by low Earth orbit satellites and ground‐based equipment. However, these observations can only provide variations along satellite orbits or at a single location. In this paper, we provide, for the first time, two‐dimensional (2D) maps of nighttime OI 135.6 nm airglow observations by GOLD. These maps describe unambiguous and dynamic evolution of nighttime NmF2 as the 135.6 nm radiance is correlated well with nighttime NmF2. On the night of November 19, 2018, GOLD observed 135.6 nm radiance, NmF2 measured by digisonde, and total electron content at Cachoeira Paulista (CP) all exhibited a postsunset enhancement. This is the first time that the two dimensional (2D) measurements of the ionosphere by a geostationary satellite are compared with local observations in the same time range. The evolution of 135.6 nm radiance map suggests that this enhancement was mainly due to the southward movement of the southern equatorial ionization anomaly (EIA) crest. The corresponding northern EIA crest of CP is over the Ocean, which can be observed by GOLD and found to keep stationary, but not easily by other means of observations. This asymmetric movement of the two EIA crests is unknown and requires further investigation in the future.Key Points: The GOLD nighttime observations match well with digisonde and total electron content at both single location and large areaThe observed postsunset OI 135.6 nm radiance enhancement at Cachoeira Paulista is due to the southward motion of southern Equatorial ionization anomaly (EIA) crestAn event of EIA asymmetry motion occurred during the same time period (<2 h) [ABSTRACT FROM AUTHOR]

We conduct observational and modeling studies of thermospheric composition responses to weak geomagnetic activity (nongeomagnetic storms). We found that the thermospheric O and N2 column density ratio (∑O/N2) in part of the Northern Hemisphere measured by Global‐scale Observations of the Limb and Disk (GOLD) exhibited large and long‐lived depletions during weak geomagnetic activity in May and June 2019. The depletions reached 30% of quiet time values, extended equatorward to 10°N and lasted more than 10 hr. Furthermore, numerical simulation results are similar to these observations and indicate that the ∑O/N2 depletions were pushed westward by zonal winds. The ∑O/N2 evolution during weak geomagnetic activity suggests that the formation mechanism of the ∑O/N2 depletions is similar to that during a geomagnetic storm. The effects of weak geomagnetic activity are often ignored but, in fact, are important for understanding thermosphere neutral composition variability and hence the state of the thermosphere‐ionosphere system. Plain Language Summary: The column density ratio of O and N2 (∑O/N2) has been used to monitor geomagnetic storm effects in the thermosphere, as well as providing valuable information about the ionosphere. This triggers an important question: Can weak geomagnetic activities cause changes in thermospheric composition too? Here, we conduct studies based on geostationary orbit observations and numerical simulations. Model outputs replicate the general morphology of this variability for the cases examined. This made it possible to understand the cause of the composition response to weak geomagnetic forcing. We found that the ∑O/N2 depletion observed was pushed westward by the zonal wind. During weak geomagnetic activity, the ∑O/N2 response is similar to the response during a geomagnetic storm, albeit it is weaker. In summary, our study suggests that weak geomagnetic activity can also generate strong and long‐lived responses in thermosphere composition during solar minimum and that this response can be important to understanding the thermosphere and ionosphere variability during the so‐called quiet times. Key Points: The observed ∑O/N2 exhibits strong and long‐lived response to weak geomagnetic activityThe numerical simulation results resemble the observed ∑O/N2 responses during weak geomagnetic activityWeak geomagnetic activity may have important effects on thermosphere‐ionosphere variability that cannot be simply ignored [ABSTRACT FROM AUTHOR]

The recently launched Global‐scale Observations of the Limb and Disk (GOLD) Mission observed prominent quasi‐16‐day oscillations (Q16DOs) in the OI 135.6‐nm nightglow from the equatorial ionization anomaly (EIA) region during 26 November 2018 to 25 January 2019. Meanwhile, the enhanced and concurrent quasi‐16‐day wave (Q16DW) activity was observed in the middle atmosphere by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument. This implies that the F‐region ionosphere couples with the lower atmosphere through large‐scale waves. A noteworthy phenomenon was that the Q16DOs displayed an asymmetric behavior between the northern and southern EIA crests. This asymmetry indicates that the ionospheric Q16DOs might be driven by the Q16DW‐modulated tides, which not only modulated the E‐region dynamo winds but also modulated the F‐region field‐aligned neutral winds via vertical penetration, producing the oscillations in plasmas densities. Plain Language Summary: Driving of the short‐term variability in the ionosphere is a crucial scientific question of space physics, which connects the thermosphere and ionosphere to the solar and magnetospheric forcing, as well as terrestrial atmosphere weather. It is also the most challenge part of space weather forecast. Traveling planetary waves (PWs), including the eastward propagating Kelvin waves and westward propagating Rossby waves, are responsible for a large amount of day‐to‐day variability in ionospheric parameters. Our study showed an evidence of the Rossby waves coupling with the nighttime F‐region ionosphere observed by GOLD. More importantly, the observations suggested a physical pathway accounting for the PW coupling in the atmosphere‐ionosphere system; that is, PW‐modulated tides penetrate into the F‐region ionosphere and produce the field‐aligned motion of the plasmas, leading to the oscillations in electron density at the PW periods. Key Points: Significant quasi‐16‐day oscillations (Q16DOs) were observed in the OI 135.6‐nm nightglow by GOLDThe Q16DOs in nightglow are closely correlated with quasi‐16‐day wave (Q16DW) activities in the middle atmosphereThe EIA crests showed antisymmetric Q16DOs between the two hemispheres likely driven by Q16DW‐modulated tides [ABSTRACT FROM AUTHOR]

This work reports the analysis of the local‐time variations of daytime CO2 in the Upper Mesosphere and Lower Thermosphere region as observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. Results show that daytime SABER CO2 in the upper mesosphere between latitudes 30°S and 50°N is lower in the morning than in the afternoon. The same was found in the lower thermosphere but between latitudes 50°S and 50°N. The opposite was found in the upper mesosphere between 50°S and 30°S. These results are compared to simulations of CO2 by the Whole Atmosphere Community Climate Model–eXtended. It was found that the local‐time variations of Whole Atmosphere Community Climate Model–eXtended CO2 are weaker than SABER CO2. Model diagnostics indicated that these local‐time variations in the model are driven primarily (secondarily) by strong vertical (meridional) gradients in Upper Mesosphere and Lower Thermosphere CO2 volume mixing ratios and strong local‐time variations in vertical (meridional) winds. This work concludes that SABER CO2 shows significant local‐time variations that are not well simulated by Whole Atmosphere Community Climate Model–eXtended, and we suggest that this is likely because of weaker tidal forcing in the model. Key Points: SABER CO2 in the UMLT region has stronger local‐time variation than WACCM‐X CO2WACCM‐X suggests that tidal perturbations in meridional and vertical advection drive the local‐time variations of CO2SABER CO2 indicates that WACCM‐X simulates weaker tidal perturbations in meridional and vertical advection [ABSTRACT FROM AUTHOR]

We conducted numerical simulations to examine dayside thermosphere and ionosphere responses to concurrent solar flares and a geomagnetic storm during 6–11 September 2017, as well as the interplay of flare and storm effects. We found that E‐region electron density response to the flares was much smaller inside the auroral oval than it was outside the auroral oval due to an extra source of ionization by auroral particle precipitation. During quiet times in this period, flare enhanced ionization, which decreased daytime eastward electric fields, caused an initial weakening of the vertical E × B drifts. This weakening of the vertical E × B drifts was on the order of ~20% for an X8.2 flare in this period. During storm times, flare‐induced initial changes of the vertical plasma drifts varied between being positive and being negative, depending on latitudes, due to storm‐time disturbance dynamo effects. Storm effects also overall weakened flare effects on geopotential height, ionospheric F2 peak height hmF2, and ionospheric F2 peak electron density NmF2. In addition, storm effects changed the latitudinal distributions of flare effects on hmF2, NmF2, vertical plasma drifts, and O/N2. On the other hand, no major X‐class flare occurred shortly before or during the storm. Model simulations indicate that the weaker C‐ and M‐class flares that occurred before and during the storm had a minor effect on the amplitude of the large‐scale traveling atmosphere disturbances generated by the storm. Key Points: Flare effects were smaller inside auroral oval than outside the oval due to an extra source of ionization by particle precipitationStorm effects changed the latitudinal distributions of flare effects on hmF2, NmF2, vertical plasma drifts, and O/N2C‐ and M‐ class flares had a minor effect on how the upper atmosphere responded to storm [ABSTRACT FROM AUTHOR]

The Global‐scale Observations of the Limb and Disk (GOLD) mission provides, for the first time, a synoptic view of the thermospheric response to a moderate geomagnetic storm at a 30‐min cadence. The observations showed that the OI 135.6‐nm emission substantially decreased at middle and high latitudes, but it increased at low latitudes. The depletion pattern between 4 and 6 November 2018 was likely due to the strong upwelling of the high‐latitude thermosphere. The deepest depletion in brightness that occurred near local noon was of ~300–400 R (~35% of the quiet time background level) and displayed a striking westward movement. First‐principles modeling reasonably reproduced the overall morphology of brightness response to the geomagnetic storm despite the depletion magnitude in brightness being underestimated. The postprocessing of the simulation results revealed that the westward movement of the deepest depletion was attributable to the horizontal wind changes driven by the geomagnetic storm. Plain Language Summary: Geomagnetic storms are triggered by enhanced solar wind emitted from the Sun, and a large portion of the absorbed solar energy dissipates into the high latitudes of the Earth's atmosphere through Joule heating and ion drag. Geomagnetic storms usually bring tremendous disturbances into the thermosphere‐ionosphere (e.g., neutral and plasma density), which significantly impact human activities (e.g., satellite orbit determination, navigation, and telecommunications). Accurate prediction of the response to geomagnetic storms in the thermosphere‐ionosphere is one of the core scientific objectives of space weather. The newly launched Global‐scale Observations of the Limb and Disk mission provides unprecedented synoptic measurements of ionosphere‐thermosphere properties. This study reports a large response of the OI 135.6‐nm emission to a moderate geomagnetic storm that occurred on 8–9 November 2018 observed by Global‐scale Observations of the Limb and Disk, which demonstrates a huge potential to advance our knowledge of the thermosphere‐ionosphere system and ability to simulate and forecast space weather. Key Points: GOLD provided, for the first time, synoptic observations of the thermospheric response to a geomagnetic storm with a 30‐min cadenceDayglow brightness depletions occurred at high latitudes and were extended into midlatitudes with evident interhemispheric asymmetryThe deepest depletion occurred near local noon and moved westward during and after the geomagnetic storm [ABSTRACT FROM AUTHOR]

This special issue reports the latest results on long‐term variations and trends in the middle and upper atmosphere, in particular, in the mesosphere and thermosphere. The issue is based on selected papers from the 10th IAGA/ICMA/SCOSTEP workshop on Long‐term changes and trends in the atmosphere, held in Hefei, China, 14–18 May 2018. This introduction highlights recent progress in key science areas of long‐term changes in the upper atmosphere. Key Points: Long‐term change in the middle and upper atmosphere is caused by increasing carbon dioxide and ozone changeLong‐term decrease in thermosphere density increases risks of space trafficThis introduction highlights of recent progress on long‐term changes in the middle and upper atmosphere [ABSTRACT FROM AUTHOR]

In recent years, the middle atmosphere has evoked great scientific interest as long‐term changes can be clearly captured owing to the large perturbation amplitudes at these altitudes. In the present study, more than 25 years of the data are used to investigate the long‐term trends in the middle atmosphere by suitably combining the observations from different techniques (Rocketsonde, High‐Resolution Doppler Imager (HRDI)/Upper Atmosphere Research Satellite (UARS), Halogen Occultation Experiment (HALOE)/UARS, Sounding of the Atmosphere using Broadband Emission Radiometry (SABER)/Thermosphere Ionosphere Mesosphere Energetic Dynamics (TIMED), and Mesosphere‐stratosphere‐troposphere (MST) radar) from Indian region. As different instruments/techniques are used and the time periods are not the same, extreme care has been taken while merging various data sets to obtain meaningful long‐term trends. To understand the observed long‐term trends, Whole Atmosphere Community Climate Model–eXtended model simulations for the Indian low‐latitude regions are also used. A significant cooling trend of ~−1.7 ± 0.5 K/decade between 30 and 80 km is noticed. Large decreasing trend (~5 m/s/decade) in the eastward winds is noticed which is significant between 70 and 80 km only changing from strong eastward wind in 1970s to weak westward wind in recent decade. No significant trends are observed in the meridional wind. These observations are well captured by the Whole Atmosphere Community Climate Model–eXtended model simulations while considering changes in concentrations of greenhouse gases including carbon dioxide (CO2), methane (CH4), water vapor (H2O), and chlorofluorocarbon species that cause depletion of stratospheric ozone (O3). Thus, it is prudent to conclude that long‐term decreasing trends in the zonal winds and cooling trends in the temperature in the middle atmosphere are caused to a large extent by greenhouse gases suggesting the role of anthropogenic changes in the dynamics of the middle atmosphere. Plain Language Summary: In the changing climate, several studies have shown long‐term trends in troposphere, stratosphere, and mesosphere. In general, long‐term trends in the middle atmosphere (10–100 km) can be clearly captured due to increasing amplitudes with the altitude. However, studies on long‐term trends in the temperature and winds over the low latitudes in the middle atmosphere are sparse. In the present study, long‐term trends observed in the temperature and winds obtained from the observations from multiinstrumental data covering more than 25 years is provided. To explain the observed trends we also used Whole Atmosphere Community Climate Model–eXtended model simulations. Long‐term decreasing trends in the zonal winds and cooling trends in the temperature in the middle atmosphere are observed. These observations are clearly captured in the Whole Atmosphere Community Climate Model–eXtended model simulations that examine the long‐term changes in the atmosphere due to the long‐term changes in concentrations of greenhouse gases, suggesting the role of anthropogenic changes in the dynamics of the middle atmosphere. Key Points: Long‐term trends in the middle atmosphere temperature and winds over Indian region are presented using multiinstrumental dataTo explain the observed trends, WACCM‐X model simulations are used for the Indian region while considering major GHGsWACCM‐X model is able to simulate these observed trends suggesting the role of anthropogenic change on the middle atmosphere [ABSTRACT FROM AUTHOR]

We conducted global simulations of temperature change due to anthropogenic trace gas emissions, which extended from the surface, through the thermosphere and ionosphere, to the exobase. These simulations were done under solar maximum conditions, in order to compare the effect of the solar cycle on global change to previous work using solar minimum conditions. The Whole Atmosphere Community Climate Model‐eXtended was employed in this study. As in previous work, lower atmosphere warming, due to increasing anthropogenic gases, is accompanied by upper atmosphere cooling, starting in the lower stratosphere, and becoming dramatic, almost 2 K per decade for the global mean annual mean, in the thermosphere. This thermospheric cooling, and consequent reduction in density, is less than the almost 3 K per decade for solar minimum conditions calculated in previous simulations. This dependence of global change on solar activity conditions is due to solar‐driven increases in radiationally active gases other than carbon dioxide, such as nitric oxide. An ancillary result of these and previous simulations is an estimate of the solar cycle effect on temperatures as a function of altitude. These simulations used modest, five‐member, ensembles, and measured sea surface temperatures rather than a fully coupled ocean model, so any solar cycle effects were not statistically significant in the lower troposphere. Temperature change from solar minimum to maximum increased from near zero at the tropopause to about 1 K at the stratopause, to approximately 500 K in the upper thermosphere, commensurate with the empirical evidence, and previous numerical models. Plain Language Summary: We conducted global simulations of temperature change due to emissions of trace gases due to human activity, which extended from the surface, throughout the atmosphere, to space. These simulations were done under conditions of high solar activity, in order to compare the effect of the solar cycle to previous work using low solar activity. The Whole Atmosphere Community Climate Model‐eXtended was employed. As in previous work, lower atmosphere warming, due to increasing emissions of greenhouse gases, is accompanied by upper atmosphere cooling, starting in the lower stratosphere, and becoming dramatic, almost 2 K per decade on average, above 100‐km altitude. This upper atmosphere cooling, and consequent reduction in density, is less than the almost 3 K per decade for low solar activity conditions calculated in previous simulations. The dependence of global change on solar activity is due to solar‐driven increases in other gases that cool the thermosphere, so greenhouse gases such as carbon dioxide have less effect. An ancillary result is an estimate of the solar cycle effect on temperatures as a function of altitude, which increased from near zero at about 15 km to approximately 500 K at about 400 km, commensurate with previous work. Key Points: We performed whole‐atmosphere climate change simulations, for solar maximum conditions, and compared them to previous work for solar minimumIn the thermosphere and ionosphere, anthropogenic climate change at solar maximum is smaller than at solar minimum, in these simulationsThe solar cycle effect on temperature ranges from 500 K in the thermosphere to near zero at the tropopause [ABSTRACT FROM AUTHOR]

The Global UltraViolet Imager (GUVI) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite provides a data set of vertical thermospheric composition (O, N2, and O2 densities) and temperature profiles from 2002–2007. Even though GUVI sampling is limited by orbital constraint, we demonstrated that the GUVI data set can be used to derive the altitude profiles of the amplitudes and phases of annual oscillation (AO) and semiannual oscillation (SAO), thereby providing important constraints on models seeking to explain these features. We performed a seasonal and interannual analysis of GUVI limb O, O2, and N2 densities and volume number density ratio O/N2 at constant pressure levels. These daytime observations of O and O/N2 in the lower thermosphere show a strong AO at midlatitudes and a clear SAO at lower latitudes. The global mean GUVI O/N2 number density ratio shows the AO, with slightly larger values in January than in July and a SAO with O/N2 greater during equinoxes than at the solstices. O and N2 densities on fixed pressure levels in the upper thermosphere are anticorrelated with solar extreme ultraviolet flux. On the other hand, O/N2 is smaller during solar minimum and larger during solar maximum. The thermospheric AO and SAO in composition have a constant phase with altitude throughout the thermosphere. Key Points: The global mean GUVI O/N2 shows both an annual variation and a semiannual variation with O/N2 greater during equinoxes than at the solsticesO and N2 densities on fixed pressure levels in the upper thermosphere are anticorrelated with solar EUV flux, but O/N2 positively correlates with solar cycleGUVI reveals that phases of AO and SAO in thermospheric O and N2 densities at different heights are consistent [ABSTRACT FROM AUTHOR]

The period of 6–11 September 2017 was an active period in which multiple solar flares and a major geomagnetic storm occurred. The two largest flares, an X9.3 and an X8.2, were a disk flare and a limb flare, respectively. We conducted model simulations and data analysis to examine solar flare effects on the coupled thermosphere and ionosphere (TI) system in connection with flare location effects and to investigate the occurrences of large‐scale traveling atmosphere disturbances (TADs) due to flares and storms. Soft X‐ray enhancement, which dominates E region ionization, is essentially not affected by the location of a flare on the Sun; solar extreme ultraviolet enhancement, which dominates ionization above ~150 km, is much weaker for a limb flare compared to a disk flare with the same magnitude. Consequently, flare responses in the lower thermosphere, and ionosphere E region are not affected by flare locations, but above ~150 km, the TI system responds more strongly to disk flares than to limb flares. However, our studies show that during the space weather events in September 2017, these flare location effects were masked by other factors including local time and longitude. Large‐scale TADs occurred when there were both flares and storms. The presence of the flares changed the magnitudes and propagation speeds of the large‐scale TADs. However, there was no evidence that large‐scale TADs occurred when there were only flares and not storms, indicating that solar flares alone were not sufficient to excite large‐scale TADs. Key Points: Flare location effects were masked by geophysical conditions including local time and longitudeSolar flares changed the amplitudes and propagation speeds of TADs excited by stormsSolar flares alone were not the sufficient conditions to excite large‐scale TADs [ABSTRACT FROM AUTHOR]

We investigate trends and solar irradiance effects in the mesosphere using the Whole Atmosphere Community Climate Model with eXtended thermosphere and ionosphere (WACCM‐X) and radar measurements of winds at Collm (51°N, 13°E), for the period of 1980–2014. We found that in the mesosphere, dynamics significantly impact temperature and wind trends, as well as how solar irradiance affects the temperature and winds. The global average temperature trends are negative, with a maximum of ~−1 K per decade in the middle to lower mesosphere. Solar irradiance effects on the global average temperature are positive and decrease monotonically with decreasing altitude, changing from ~3 K/100 solar flux units (sfu) near the mesopause to ~1 K per 100 sfu in the lower mesosphere. In the summer upper mesosphere, temperature trends can become near 0 or positive, likely due to dynamical effects. Both wind trends and solar effects on the winds show dynamical patterns with negative and positive values, indicating that they are predominantly controlled by dynamics. The wind trends and solar effects on the winds are on the orders of ~±5 m/s per decade and ~±5 m/s per 100 sfu, respectively, and they are not as statistically significant as their temperature counterparts. At Collm (51°N, 13°E), the observed zonal winds at 90 km have a larger trend of 1.98 m/s per decade compared to the simulated zonal winds and it is statistically significant, but both the simulated and observed meridional winds do not have statistically significant trends. Key Points: In the mesosphere, temperature trends are mostly negative but can be positive at middle and high latitudes in the upper mesosphereWind trends/solar effects on winds vary greatly with location/season but are less statistically significant than temperature counterpartsDynamics significantly impact temperature and wind trends, as well as how solar irradiance affects the temperature and winds [ABSTRACT FROM AUTHOR]

We investigate the latitudinal and longitudinal variations of thermospheric nitric oxide (NO) cooling rate from 2005 to 2016. We compare the maximum of the NO cooling rate and its altitude from measurements by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on National Aeronautics and Space Administration (NASA)'s TIMED (Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics) satellite and from simulations by the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). In both data and model, the maximum NO cooling rate varies with ~11‐year solar cycle, while the altitude of the peak value does not. At local noon, the maximum NO cooling rate from TIEGCM is significantly larger than that derived from SABER, especially in solar minimum. There are differences in the altitudes of the maximum NO cooling rates between the observations and simulations. The largest difference occurs at noon at low latitudes, with SABER values exceeding TIEGCM values by more than 10 km with a maximum difference of 16 km near solar maximum. Model sensitivity tests suggest that the biases are caused by a model temperature overestimation in the low thermosphere, and tides in the TIEGCM lower boundary play an important role in inducing the low‐latitude temperature overestimation by the model. In addition, SABER's height‐integrated cooling rate (NO flux) exceeds that from TIEGCM, primarily due to the lower cooling rates at 140–200 km in the model. Key Points: Compare maximum of NO cooling rate and its altitude from SABER measurements and TIEGCM simulationsThe SABER median peak cooling altitude usually exceeds the model median peak cooling altitude, sometimes by as much as 16 kmStrong solar cycle and seasonal modulations of the altitude of the maximum NO cooling rate are most evident at midlatitude locations [ABSTRACT FROM AUTHOR]

This work uses Sounding of the Atmosphere using Broadband Emission Radiometry CO2 data from 2002 to 2015 and Specified Dynamics‐Whole Atmosphere Community Climate Model (SD‐WACCM) outputs from 1979 to 2014 to show, for the first time, the solar cycle response of CO2 in the Austral winter mesosphere and lower thermosphere region. Both Sounding of the Atmosphere using Broadband Emission Radiometry and SD‐WACCM show that CO2 experiences a decrease during solar maximum throughout the Austral winter mesosphere and lower thermosphere region. This work highlights the regions where CO2 experiences its strongest and weakest solar cycle responses as modeled by SD‐WACCM. The region with the strongest solar cycle response experiences around 5% reduction in CO2 between solar maximum and solar minimum. The region with weakest solar cycle response experiences less than 1% reduction in CO2 between solar maximum and solar minimum. It is shown that the region of the strongest CO2 response is driven by photodissociation, downwelling, and reduced eddy diffusion. On the other hand, the region of the weakest CO2 response is driven by the opposing effects of photodissociation and enhanced eddy diffusion. This is the first work to show that the solar cycle could affect the Austral winter lower thermosphere circulation and eddy diffusion processes. These anomalies in the lower thermospheric circulation and eddy diffusion are found to be related to the solar cycle response in the Austral winter mesosphere wave‐mean flow dynamics. This work therefore concludes that the solar cycle affects lower thermospheric CO2 via modulations of the lower thermospheric circulation and eddy diffusion processes. Key Points: Response of CO2 in the Austral winter MLT to the 11‐year solar cycle is presentedSolar cycle response of CO2 in the Austral winter MLT is driven by photochemistry and transportThe residual circulation and eddy diffusion in the Austral winter lower thermosphere respond to the 11‐year solar cycle [ABSTRACT FROM AUTHOR]

The solar minimum period between solar cycles 23 and 24 has generated extensive study, in part because the changes in the upper atmosphere and ionosphere were so dramatic. Thermospheric density during 2008–2009, at the benchmark 400‐km altitude, was shown to be approximately 30% lower than the previous solar minimum during 1996, in observations and in model simulations. Ionospheric densities were also estimated to be lower, by 10% to 20%. However, earlier analyses using data from the global network of Global Navigation Satellite System receivers did not find any significant reduction in global mean total electron content. Recent work reexamining those data using a limited set of receivers, but with consistent processing, identified a 19% reduction (Emmert et al., 2017, https://doi.org/10.1002/2016JA023680). In this study, we compare model simulations of the thermosphere‐ionosphere system during the 2008–2009 solar minimum to its predecessor and estimate a 16% reduction in global mean total electron content. This is consistent with the neutral thermosphere density change seen in model simulations and in satellite drag observations. In the model simulations, the primary driver is a 10% reduction in solar extreme ultraviolet irradiance, with a smaller contribution from lower geomagnetic activity, and a very small decrement due to increase in atmospheric carbon dioxide. Plain Language Summary: The Sun goes through 11‐year activity cycles, best known with regard to the number of sunspots. At solar minimum, when sunspots mostly vanish, the extreme ultraviolet region of the solar spectrum also diminishes. This causes lower temperature and density in the ionosphere and upper atmosphere above 100 km, where the extreme ultraviolet radiation is absorbed. Past solar minimum periods have seemed basically similar with regard to extreme ultraviolet radiation, and other manifestations of solar activity. However, the solar minimum during 2008–2009 was longer and had fewer sunspots than its predecessor during 1996, or than any minimum period in the past century. The thermosphere was also lower in density, and the ionosphere also appeared to be lower in density, but there were some contradictory measurements. Recent reanalysis of ionospheric measurements found better agreement with the thermospheric measurements. Modeling studies attempting to explain the changes in the ionosphere and thermosphere have suggested that they were caused by a decrease of about 10% in solar extreme ultraviolet radiation. Here we show that the model results and the ionospheric measurements are now in good agreement. This demonstrates that solar minima are not all the same and may have implications for understanding the Sun during extended periods of very low activity. Key Points: Reanalysis of ionospheric electron content by Emmert et al. () found it lower during the 2008‐2009 solar minimum than the 1996 minimumModel simulations showed that both ionospheric electron content and thermospheric neutral density were lower during 2008‐2009 than in 1996Thermosphere and ionosphere reductions during the 2008‐2009 solar minimum were consistent in observations and in model simulations [ABSTRACT FROM AUTHOR]

Abstract: We simulated anthropogenic global change through the entire atmosphere, including the thermosphere and ionosphere, using the Whole Atmosphere Community Climate Model‐eXtended. The basic result was that even as the lower atmosphere gradually warms, the upper atmosphere rapidly cools. The simulations employed constant low solar activity conditions, to remove the effects of variable solar and geomagnetic activity. Global mean annual mean temperature increased at a rate of +0.2 K/decade at the surface and +0.4 K/decade in the upper troposphere but decreased by about −1 K/decade in the stratosphere‐mesosphere and −2.8 K/decade in the thermosphere. Near the mesopause, temperature decreases were small compared to the interannual variation, so trends in that region are uncertain. Results were similar to previous modeling confined to specific atmospheric levels and compared favorably with available measurements. These simulations demonstrate the ability of a single comprehensive numerical model to characterize global change throughout the atmosphere. [ABSTRACT FROM AUTHOR]

Abstract: New ionosphere and electrodynamics modules have been incorporated in the thermosphere and ionosphere eXtension of the Whole Atmosphere Community Climate Model (WACCM‐X), in order to self‐consistently simulate the coupled atmosphere‐ionosphere system. The first specified dynamics WACCM‐X v.2.0 results are compared with several data sets, and with the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM), during the deep solar minimum year. Comparisons with Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite of temperature and zonal wind in the lower thermosphere show that WACCM‐X reproduces the seasonal variability of tides remarkably well, including the migrating diurnal and semidiurnal components and the nonmigrating diurnal eastward propagating zonal wavenumber 3 component. There is overall agreement between WACCM‐X, TIE‐GCM, and vertical drifts observed by the Communication/Navigation Outage Forecast System (C/NOFS) satellite over the magnetic equator, but apparent discrepancies also exist. Both model results are dominated by diurnal variations, while C/NOFS observed vertical plasma drifts exhibit strong temporal variations. The climatological features of ionospheric peak densities and heights (NmF2 and hmF2) from WACCM‐X are in general agreement with the results derived from Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) data, although the WACCM‐X predicted NmF2 values are smaller, and the equatorial ionization anomaly crests are closer to the magnetic equator compared to COSMIC and ionosonde observations. This may result from the excessive mixing in the lower thermosphere due to the gravity wave parameterization. These data‐model comparisons demonstrate that WACCM‐X can capture the dynamic behavior of the coupled atmosphere and ionosphere in a climatological sense. [ABSTRACT FROM AUTHOR]

Abstract: Key developments have been made to the NCAR Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X). Among them, the most important are the self‐consistent solution of global electrodynamics, and transport of O+ in the F‐region. Other ionosphere developments include time‐dependent solution of electron/ion temperatures, metastable O+ chemistry, and high‐cadence solar EUV capability. Additional developments of the thermospheric components are improvements to the momentum and energy equation solvers to account for variable mean molecular mass and specific heat, a new divergence damping scheme, and cooling by O(3P) fine structure. Simulations using this new version of WACCM‐X (2.0) have been carried out for solar maximum and minimum conditions. Thermospheric composition, density, and temperatures are in general agreement with measurements and empirical models, including the equatorial mass density anomaly and the midnight density maximum. The amplitudes and seasonal variations of atmospheric tides in the mesosphere and lower thermosphere are in good agreement with observations. Although global mean thermospheric densities are comparable with observations of the annual variation, they lack a clear semiannual variation. In the ionosphere, the low‐latitude E × B drifts agree well with observations in their magnitudes, local time dependence, seasonal, and solar activity variations. The prereversal enhancement in the equatorial region, which is associated with ionospheric irregularities, displays patterns of longitudinal and seasonal variation that are similar to observations. Ionospheric density from the model simulations reproduces the equatorial ionosphere anomaly structures and is in general agreement with observations. The model simulations also capture important ionospheric features during storms. [ABSTRACT FROM AUTHOR]

Abstract: We investigate atomic hydrogen (H) variability from the mesopause to the upper thermosphere, on time scales of solar cycle, seasonal, and diurnal, using measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite, and simulations by the National Center for Atmospheric Research Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X). In the mesopause region (85 to 95 km), the seasonal and solar cycle variations of H simulated by WACCM‐X are consistent with those from SABER observations: H density is higher in summer than in winter, and slightly higher at solar minimum than at solar maximum. However, mesopause region H density from the Mass‐Spectrometer‐Incoherent‐Scatter (National Research Laboratory Mass‐Spectrometer‐Incoherent‐Scatter 00 (NRLMSISE‐00)) empirical model has reversed seasonal variation compared to WACCM‐X and SABER. From the mesopause to the upper thermosphere, H density simulated by WACCM‐X switches its solar cycle variation twice, and seasonal dependence once, and these changes of solar cycle and seasonal variability occur in the lower thermosphere (~95 to 130 km), whereas H from NRLMSISE‐00 does not change solar cycle and seasonal dependence from the mesopause through the thermosphere. In the upper thermosphere (above 150 km), H density simulated by WACCM‐X is higher at solar minimum than at solar maximum, higher in winter than in summer, and also higher during nighttime than daytime. The amplitudes of these variations are on the order of factors of ~10, ~2, and ~2, respectively. This is consistent with NRLMSISE‐00. [ABSTRACT FROM AUTHOR]

Numerical studies have shown that there is a lower thermospheric winter-to-summer circulation that is driven by wave dissipation and that it plays a significant role in trace gas distributions in the mesosphere and lower thermosphere, and in the composition of the thermosphere. However, the characteristics of this circulation are poorly known. Direct observations of it are difficult, but it leaves clear signatures in tracer distributions. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite has obtained CO2 concentration from 2002 to present. This data set, combined with simulations by the Whole Atmosphere Community Climate Model, provides an unprecedented opportunity to infer the morphology of this circulation in both the summer and winter hemispheres. Our study show that there exists a maximum vertical gradient of CO2 at summer high latitudes, driven by the convergence of the upwelling of the mesospheric circulation and the downwelling of the lower thermospheric circulation; in the winter hemisphere, the maximum vertical gradient of CO2 is located at a higher altitude, driven by the convergence of the upwelling of the lower thermospheric circulation and the downwelling of the solar-driven thermospheric circulation; the bottom of the lower thermospheric circulation is located between ~ 95 km and 100 km, and it has a vertical extent of ~10 km. Analysis of the SABER CO2 and temperature at summer high latitudes showed that the bottom of this circulation is consistently higher than the mesopause height by ~10 km. [ABSTRACT FROM AUTHOR]