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The ionosphere equatorial ionization anomaly (EIA) is usually characterized by two plasma density maxima in the Earth's equatorial region. Merged EIA (MEIA) is a unique phenomenon in the evolution of the EIA. Currently, the occurrence characteristics of MEIA are still not well understood. In this study, we investigate the occurrence characteristics of nighttime MEIA using NASA Global-scale Observations of the Limb and Disk (GOLD) observations between October 2018 and the end of 2023. We found that the occurrence of nighttime MEIA exhibits solar cycle, seasonal, and local time variations. The occurrence rate of the MEIA is inversely dependent on solar activity. Occurrence of the MEIA maximizes near the equinoxes, with a primary (secondary) low occurrence rate near the June (December) solstice. In addition, occurrences of the MEIA are suppressed during the pre-reversal enhancement (PRE), resulting in relatively fewer events. Furthermore, it was found that the occurrence of the MEIA is not significantly dependent on the strength of geomagnetic activity. As far as we know, this study represents the first instance of utilizing observations from GOLD observations to investigate the characteristics of MEIA occurrences and their correlations with solar activity, season, and local time. [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]

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]

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]

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 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]

Neutral density shows complicated temporal and spatial variations driven by external forcing of the thermosphere/ionosphere system, internal dynamics, and thermosphere and ionosphere coupling. Temporal variations include abrupt changes with a time scale of minutes to hours, diurnal variation, multi-day variation, solar-rotational variation, annual/semiannual variation, solar-cycle variation, and long-term trends with a time scale of decades. Spatial variations include latitudinal and longitudinal variations, as well as variation with altitude. Atmospheric drag on satellites varies strongly as a function of thermospheric mass density. Errors in estimating density cause orbit prediction error, and impact satellite operations including accurate catalog maintenance, collision avoidance for manned and unmanned space flight, and re-entry prediction. In this paper, we summarize and discuss these density variations, their magnitudes, and their forcing mechanisms, using neutral density data sets and modeling results. The neutral density data sets include neutral density observed by the accelerometers onboard the Challenging Mini-satellite Payload (CHAMP), neutral density at satellite perigees, and global-mean neutral density derived from thousands of orbiting objects. Modeling results are from the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM), and from the NRLMSISE-00 empirical model. [ABSTRACT FROM AUTHOR]

We present an overview of the data and models collected for the Whole Heliosphere Interval, an international campaign to study the three-dimensional solar-heliospheric-planetary connected system near solar minimum. The data and models correspond to solar Carrington Rotation 2068 (20 March - 16 April 2008) extending from below the solar photosphere, through interplanetary space, and down to Earth's mesosphere. Nearly 200 people participated in aspects of WHI studies, analyzing and interpreting data from nearly 100 instruments and models in order to elucidate the physics of fundamental heliophysical processes. The solar and inner heliospheric data showed structure consistent with the declining phase of the solar cycle. A closely spaced cluster of low-latitude active regions was responsible for an increased level of magnetic activity, while a highly warped current sheet dominated heliospheric structure. The geospace data revealed an unusually high level of activity, driven primarily by the periodic impingement of high-speed streams. The WHI studies traced the solar activity and structure into the heliosphere and geospace, and provided new insight into the nature of the interconnected heliophysical system near solar minimum. [ABSTRACT FROM AUTHOR]

Abstract: The effect of carbon dioxide (CO2) cooling on trends of hmF 2 and NmF 2 are investigated using a coupled thermosphere and ionosphere general circulation model. Model simulations indicate that CO2 cooling not only causes contraction of the upper atmosphere and changes of neutral and ion composition but also changes dynamics and electrodynamics in the thermosphere/ionosphere. These changes determine the altitude dependence of ionospheric trends and complex latitudinal, longitudinal, diurnal, seasonal, and solar cycle variations of trends of hmF 2 and NmF 2. Under the CO2 cooling effect, trends of NmF 2 are negative with magnitude from 0% to ∼−40% for doubled CO2, depending on location, local time, season, and solar activity. The corresponding trends of hmF 2 are mostly negative with a magnitude from 0 to −40km, but can be positive with a magnitude from 0 to ∼10km at night, with maximum positive trends occurring after midnight under solar minimum conditions. [Copyright &y& Elsevier]