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This study investigates the day-to-day variability of equatorial plasma bubbles (EPBs) over the Atlantic–American region and their connections to atmospheric planetary waves during the sudden stratospheric warming (SSW) event of 2021. The investigation is conducted on the basis of the GOLD (Global Observations of the Limb and Disk) observations, the ICON (Ionospheric Connection Explorer) neutral wind dataset, ionosonde measurements, and simulations from the WACCM-X (Whole Atmosphere Community Climate Model with thermosphere–ionosphere eXtension). We found that the intensity of EPBs was notably reduced by 35% during the SSW compared with the non-SSW period. Furthermore, GOLD observations and ionosonde data show that significant quasi-6-day oscillation (Q6DO) was observed in both the intensity of EPBs and the localized growth rate of Rayleigh–Taylor (R-T) instability during the 2021 SSW event. The analysis of WACCM-X simulations and ICON neutral winds reveals that the Q6DO pattern coincided with an amplification of the quasi-6-day wave (Q6DW) in WACCM-X simulations and noticeable ∼6-day periodicity in ICON zonal winds. The combination of these multi-instrument observations and numerical simulations demonstrates that certain planetary waves like the Q6DW can significantly influence the day-to-day variability of EPBs, especially during the SSW period, through modulating the strength of prereversal enhancement and the growth rate of R-T instability via the wind-driven dynamo. These findings provide novel insights into the connection between atmospheric planetary waves and ionospheric EPBs. [ABSTRACT FROM AUTHOR]

Variability in the ionosphere during the 2020–2021 sudden stratospheric warming (SSW) is investigated using a combination of Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) observations and the Whole Atmosphere Community Climate Model with thermosphere–ionosphere eXtension (WACCM-X) simulations. The unprecedented spatial–temporal sampling of the low latitude ionosphere afforded by COSMIC-2 enables investigating the short-term (<5 days) variability in the ionosphere during the SSW event. The COSMIC-2 observations reveal a reduction in the diurnal and zonal mean ionosphere total electron content (ITEC) and reduced amplitude of the diurnal variation in the ionosphere during the SSW. Enhanced ITEC amplitudes of the semidiurnal solar and lunar migrating tides and the westward propagating semidiurnal tide with zonal wavenumber 3 are also observed. The WACCM-X simulations demonstrate that these variations are driven by variability in the stratosphere–mesosphere during the 2020–2021 SSW event. The results show the impact of the 2020–2021 SSW on the mean state, diurnal, and semidiurnal variations in the ionosphere, as well as the capabilities of the COSMIC-2 mission to observe short-term variability in the ionosphere that is driven by meteorological variability in the lower atmosphere. [ABSTRACT FROM AUTHOR]

The NCAR Whole Atmosphere Community Climate Model (WACCM) is used to investigate the dynamical influence of the lower and middle atmosphere on the upper mesosphere and lower thermosphere. In simulations using a methodology adapted from the "specified dynamics" (nudged) version of the model, horizontal winds and temperature over part of the vertical range of the atmosphere are relaxed toward results from a previous simulation that serves as the true simulation, equivalent to meteorological analysis. In the upper mesosphere, the magnitude of the divergence of the constrained simulations from the true simulation depends on the vertical extent and frequency of the data used for nudging the model and grows with altitude. The simulations quantify the error growth of the model dynamical fields when data and forcing terms are known exactly and there are no model biases. The error growth rate and the ultimate discrepancy between the nudged and true fields depend strongly on the method used for representing gravity wave drag. The largest error growth occurs when the gravity wave parameterization uses interactive wave sources that depend on convective activity or fronts. Errors are reduced when the same parameterization is used with smoothly varying specified wave sources. The smallest errors are seen when the parameterized gravity wave drag is replaced by linear Rayleigh friction damping on the wind speed. These comparisons demonstrate the role of gravity waves in transporting the variability of the troposphere into the mesosphere and lower thermosphere. [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]

Using the high-rate phase and amplitude scintillation data from FORMOSA7/COSMIC two mission and back-propagation method, we geolocate plasma irregularities that cause scintillations. The results of geolocation are compared with the NASA GOLD UV image data of plasma bubbles. The root mean square of the zonal difference between estimated locations of plasma irregularities and plasma bubbles are about 1.5° and for single intersection cases 0.5° in the magnetic longitude. The geolocation data provide more accurate scintillation location around the globe compared to assigning to the tangent point and is valuable space weather product, which will be routinely available for public use. [ABSTRACT FROM AUTHOR]

We analyse reprocessed electron density profiles and total electron content (TEC) profiles of the ionosphere in September 2008 (around solar minimum) and September 2013 (around solar maximum) obtained by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC/FORMOSAT-3). The TEC profiles describe the total electron content along the ray path from the GPS satellite to the low Earth orbit as function of the tangent point of the ray. Some of the profiles in the magnetic polar regions show small-scale fluctuations on spatial scales <50 km. Possibly the trajectory of the tangent point intersects spatial electron density irregularities in the magnetic polar region. For derivation of the morphology of the electron density and TEC fluctuations, a 50 km high-pass filter is applied in the s domain, where s is the distance between a reference point (bottom tangent point) and the tangent point. For each profile, the mean of the fluctuations is calculated for tangent point altitudes between 400 and 500 km. At first glance, the global maps of ΔNe and Δ TEC are quite similar. However, Δ TEC might be more reliable since it is based on fewer retrieval assumptions. We find a significant difference if the arithmetic mean or the median is applied to the global map of September 2013. In agreement with literature, Δ TEC is enhanced during the post-sunset rise of the equatorial ionosphere in September 2013, which is associated with spread F and equatorial plasma bubbles. The global map of Δ TEC at solar maximum (September 2013) has stronger fluctuations than those at solar minimum (September 2008). We obtained new results when we compare the global maps of the quiet phase and the storm phase of the geomagnetic storm of 15 July 2012. It is evident that the TEC fluctuations are increased and extended over the southern magnetic polar region at the day of the geomagnetic storm. The north–south asymmetry of the storm response is more pronounced in the upper ionosphere (ray tangent points h = 400–500 km) than in the lower ionosphere (ray tangent points h = 200–300 km). [ABSTRACT FROM AUTHOR]

The impact of strong and weak stratospheric polar vortices on geomagnetic semidiurnal solar and lunar tides is investigated during Northern Hemisphere (NH) winters using ground-based magnetic field observations at the Huancayo (12.05° S, 284.67° E; magnetic latitude: 0.6° S) equatorial observatory. We analyze the periods between December 15 and March 1 for 34 NH winters between 1980 and 2020 and find that the response of semidiurnal solar and lunar tides as seen in geomagnetic field depends on the strength of the stratospheric polar vortex. During weak polar vortex events, geomagnetic semidiurnal solar and lunar tidal amplitudes show an average enhancement by ~ 25% and ~ 50%, respectively, which is consistent with the known results during sudden stratospheric warmings. When the stratospheric polar vortex is strong, geomagnetic semidiurnal solar and lunar tidal amplitudes decline on an average by ~ 15% and ~ 25%, respectively, during weak polar vortex events. Our results also reveal that the response of the geomagnetic semidiurnal solar tidal variations to strong and weak polar vortex conditions is delayed by approximately 10 days while the response of geomagnetic semidiurnal lunar tidal variations do not show a time delay. These results provide observational evidence that along with weak polar vortices in the Northern Hemisphere, the strong stratospheric polar vortices also have pronounced effects on the equatorial ionosphere. [ABSTRACT FROM AUTHOR]

We present simultaneous, independent measurements of the atmospheric semidiurnal lunar tide in neutral winds and plasma velocities from NASA's Ionospheric Connection Explorer, and in atomic oxygen 135.6 nm airglow measured by the Global‐scale Observations of the Limb and Disk. Westward tidal winds near 115 km at the magnetic equator occur during part of the upward phase of the in‐situ plasma drift. Vertical motions associated with the field‐aligned plasma velocity occur away from the magnetic equator. The morphology of the lunar tide, and the phasing between the airglow and plasma velocities are consistent with E × B drift as a mechanism for linking neutral wind and plasma perturbations. This work provides the first observational quantification of global‐scale E‐ and F‐region coupling through E × B and field‐aligned vertical drifts. Plain Language Summary: Tidal winds propagating upward from the neutral atmosphere influence ionospheric dynamo electric fields and vertical plasma drifts. However, these mechanisms have never been fully identified in comprehensive and simultaneous measurements spanning the ionosphere‐thermosphere‐mesosphere (ITM) system. The atmospheric lunar tide is an effective marker of ITM coupling because it originates at Earth's surface, propagates vertically, and is fully sampled in daytime‐only and post‐sunset satellite measurements. We utilize measurements from two new NASA missions to track the lunar tide in neutral and plasma velocities, and in an electron density proxy. Our work provides observational confirmation, and quantitative estimates of F‐region vertical plasma motions driven by neutral winds and electrodynamical forces. Key Points: The semidiurnal lunar tide is identified in simultaneous, independent measurements of F‐region airglow, and neutral and plasma velocitiesTidal patterns observed in airglow, plasma and neutral motions are consistent with both field‐aligned and E × B mechanisms of plasma upliftThe semidiurnal lunar tide is an effective marker of atmosphere‐ionosphere coupling [ABSTRACT FROM AUTHOR]

Prediction systems to enable Earth system predictability research on the subseasonal time scale have been developed with the Community Earth System Model, version 2 (CESM2) using two configurations that differ in their atmospheric components. One system uses the Community Atmosphere Model, version 6 (CAM6) with its top near 40 km, referred to as CESM2(CAM6). The other employs the Whole Atmosphere Community Climate Model, version 6 (WACCM6) whose top extends to ∼140 km, and it includes fully interactive tropospheric and stratospheric chemistry [CESM2(WACCM6)]. Both systems are utilized to carry out subseasonal reforecasts for the 1999–2020 period following the Subseasonal Experiment's (SubX) protocol. Subseasonal prediction skill from both systems is compared to those of the National Oceanic and Atmospheric Administration CFSv2 and European Centre for Medium-Range Weather Forecasts (ECMWF) operational models. CESM2(CAM6) and CESM2(WACCM6) show very similar subseasonal prediction skill of 2-m temperature, precipitation, the Madden–Julian oscillation, and North Atlantic Oscillation to its previous version and to the NOAA CFSv2 model. Overall, skill of CESM2(CAM6) and CESM2(WACCM6) is a little lower than that of the ECMWF system. In addition to typical output provided by subseasonal prediction systems, CESM2 reforecasts provide comprehensive datasets for predictability research of multiple Earth system components, including three-dimensional output for many variables, and output specific to the mesosphere and lower-thermosphere (MLT) region from CESM2(WACCM6). It is shown that sudden stratosphere warming events, and the associated variability in the MLT, can be predicted ∼10 days in advance. Weekly real-time forecasts and reforecasts with CESM2(CAM6) and CESM2(WACCM6) are freely available. Significance Statement: We describe here the design and prediction skill of two subseasonal prediction systems based on two configurations of the Community Earth System Model, version 2 (CESM2): CESM2 with the Community Atmosphere Model, version 6 [CESM2(CAM6)] and CESM 2 with Whole Atmosphere Community Climate Model, version 6 [CESM2(WACCM6)] as its atmospheric component. These two systems provide a foundation for community-model based subseasonal prediction research. The CESM2(WACCM6) system provides a novel capability to explore the predictability of the stratosphere, mesosphere, and lower thermosphere. Both CESM2(CAM6) and CESM2(WACCM6) demonstrate subseasonal surface prediction skill comparable to that of the NOAA CFSv2 model, and a little lower than that of the ECMWF forecasting system. CESM2 reforecasts provide a comprehensive dataset for predictability research of multiple aspects of the Earth system, including the whole atmosphere up to 140 km, land, and sea ice. Weekly real-time forecasts, reforecasts, and models are publicly available. [ABSTRACT FROM AUTHOR]

In this study, the comparison between the orbit electron density and total ion density measured by payloads of Tri-GNSS Radio-Occultation System (TGRS) and Ion Velocity Meter (IVM) onboard the FORMOSAT-7/COMSIC2 (F7/C2) satellites is presented. The collocated TGRS and IVM observations for each of the F7/C2 satellites at ~715 and ~540 km are evaluated during the whole year of 2020. Comparative analysis reveals that the TGRS and IVM density observations have high correlation coefficients of 0.92 - 0.96 for each of the F7/C2 satellites. The mean differences are around -0.03 × 104 to 0.02 × 104 cm-3 with standard deviations ranging from ~0.91 × 104 to 2.18 × 104 cm-3, demonstrating a good agreement between the independent TGRS and IVM observations. Furthermore, the collocated observations are utilized to examine the global spatial and temporal variations of the topside ionosphere. The results show that the morphologies of the topside ionosphere in TGRS orbit electron density are nearly identical to the IVM ion density, suggesting that both F7/C2 payloads reliably produce accurate topside ionosphere observations. We found that the TGRS orbit electron density tends to be 10 - 30% smaller than the IVM ion density in the daytime equatorial ionosphere region at both high and low orbits, but the density differences become smaller of approximately -30 - 0% during the nighttime. These density differences could be due to the error caused by the spherical symmetry assumption in TGRS orbit electron density estimation. Observing system simulation experiments are further performed to evaluate the relative errors between the TGRS and IVM density observations at high and low orbits. [ABSTRACT FROM AUTHOR]

We report for the first time the day‐to‐day variation of the longitudinal structure in height of the F2 layer (hmF2) in the equatorial ionosphere using multi‐satellite observations of electron density profiles by the Constellation Observing System for Meteorology, Ionosphere and Climate‐2 (COSMIC‐2). These observations reveal a ∼3‐day modulation of the hmF2 wavenumber‐4 structure viewed in a fixed local time frame during January 30–February 14, 2021. Simultaneously, ∼3‐day planetary wave activity is discerned from zonal wind observations at ∼100 km by the Ionospheric Connection Explorer (ICON) Michelson Interferometer for Global High‐Resolution Thermospheric Imaging (MIGHTI). This signature is not observed at ∼180–250 km altitudes, suggesting the dissipation of this wave below the F‐region. We propose that the 3‐day variation identified in hmF2 is likely caused by the planetary wave‐tide interaction through the E‐region dynamo. Plain Language Summary: The F‐region ionosphere at ∼200–400 km altitudes often shows global‐scale structures and variations that are attributed to neutral atmospheric tides and planetary waves propagating from the lower atmosphere. Previous observations have identified 3‐day planetary waves at E‐region altitudes (∼100 km), but the vertical extent of these waves has not yet been determined due to lack of high altitude observations. For this study, we use newly obtained concurrent atmospheric and ionospheric observations that provide the necessary coverage. Our study provides the first observational evidence of the vertical propagation of a 3‐day planetary wave across both E‐ and F‐region altitudes. The results suggest that this wave causes the day‐to‐day variation of the four‐peaked longitudinal structure in the equatorial ionosphere through modulating the E‐region dynamo rather than direct propagation into the F‐region. Key Points: First examination of 3‐day wave activity across ∼100–250 km altitudes and the ionospheric peak height change using coordinated satellitesIonospheric longitudinal structure shows a ∼3‐day variation coinciding the planetary wave observed in atmospheric winds at ∼100 km altitudeThe neutral atmospheric 3‐day wave signature is not observed at ∼180–250 km altitudes, suggesting the modulation of tides in the E‐region [ABSTRACT FROM AUTHOR]

We conducted observational and modeling studies of thermospheric composition and ionospheric total electron content (TEC) variations during two geomagnetically quiet periods (maximum Kp = 1.7) at solar minimum. Daytime thermospheric O and N2 column density ratio (SO/N2) observed by Global-scale Observations of the Limb and Disk and TEC from a network of ground-based Global Navigation Satellites System receivers both exhibited large (~30% of reference values) and long-lived (5-11 h) day-to-day variations in roughly the same mid-latitude geographic regions. Numerical simulations replicated the observed variability, though not perfectly. Analysis of the simulations suggested that the variations were mainly generated in the high-latitudes and were subsequently advected equatorward and westward. When high-latitudes input was turned off in simulations, the variations were negligible. This suggested the potentially important role of high-latitude geomagnetic forcing in thermospheric composition and ionospheric density variations at mid-latitudes even during some "geomagnetically quiet" periods at solar-minimum. Plain Language Summary This study presents two cases when geomagnetic forcing can be a plausible source of mid-latitude thermospheric composition and ionosphere density variations even during what is typically considered as geomagnetically quiet times (magnetic activity index Kp < 2). The column density ratio of thermospheric O and N2 (SO/N2) plays a major role in the daytime ionospheric F-region plasma density at mid-latitudes. In this study, thermospheric and ionospheric variations during geomagnetically quiet times are investigated with the two-dimensional images of SO/N2 provided by a satellite located in geostationary orbit and the ground-based total electron content (TEC) maps. Both SO/N2 and TEC displayed similar strong, long-lived and localized depletions and enhancements at mid-latitudes. Numerical simulations driven by an empirical model of geomagnetic activity, but with a climatological tide in the lower boundary, qualitatively produced the patterns of observed variations. Analysis of simulations revealed that SO/N2 variations were initially formed at high-latitudes and then transported equatorward and westward. When geomagnetic forcing was turned off in simulations, the modeled SO/N2 and TEC variations were negligible. This study suggests the potentially important roles of high-latitude forcing in thermosphere and ionosphere variations at mid-latitudes even during some "geomagnetically quiet" periods at solar-minimum. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (∼10–50 km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are caused by the breaking of planetary‐scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid descent and warming of air in polar latitudes, mirrored by ascent and cooling above the warming. The rapid warming and descent of the polar air column affect tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode, making cold air outbreaks over North America and Eurasia more likely. SSWs affect the atmosphere above the stratosphere, producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (nonionized) particles and electron densities, and electric fields. These effects span both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal prediction. This work reviews the current knowledge on the most important aspects of SSWs, from the historical background to dynamical processes, modeling, chemistry, and impact on other atmospheric layers.Plain Language Summary: The stratosphere is the layer of the atmosphere from ∼10 to 50 km, with pressures decreasing to ∼1 hPa (0.1% of surface pressure) at the top. The polar stratosphere during winter is normally very cold, with strong westerly winds. Roughly every 2 years in the Northern Hemisphere, the quiescent vortex suddenly warms over a week or two, and the winds slow dramatically, resulting in easterly winds that are more similar to the summer. These events, known as sudden stratospheric warmings (SSWs), were discovered in the early 1950s, and today, they are observed in detail by satellites. After several decades researching SSWs, considerable progress has been made in dynamical aspects of SSWs, but our understanding of how they affect both surface weather and the upper atmosphere is incomplete. We observe that variability of the stratospheric circulation (SSWs being an extreme event) is associated with shifts in the jet stream and the paths of storms, with associated effects on rainfall and temperatures. The likelihood of cold weather spells and damaging wind storms is also affected. Almost all SSWs have occurred in the Northern Hemisphere, but there was one spectacular major SSW in 2002 in the Southern Hemisphere.Key Points: Sudden stratospheric warmings are dramatic events of the polar stratosphere that affect the atmosphere from the surface to the thermosphereOur understanding of sudden stratospheric warmings has accelerated recently, particularly the predictability of surface weather effectsMore observations, improved climate models, and big data methods will address uncertainties in key aspects of sudden stratospheric warmings [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (~10-50 km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are caused by the breaking of planetary-scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid descent and warming of air in polar latitudes, mirrored by ascent and cooling above the warming. The rapid warming and descent of the polar air column affect tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode, making cold air outbreaks over North America and Eurasia more likely. SSWs affect the atmosphere above the stratosphere, producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (nonionized) particles and electron densities, and electric fields. These effects span both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal prediction. This work reviews the current knowledge on the most important aspects of SSWs, from the historical background to dynamical processes, modeling, chemistry, and impact on other atmospheric layers. [ABSTRACT FROM AUTHOR]

Launched on 25 June 2019, the Formosa Satellite Mission 7 and Constellation Observing System for Meteorology, Ionosphere, and Climate and Formosa Satellite Mission 2 (FORMOSAT-7/COSMIC-2, F7/C2) is the first operational global navigation satellite system (GNSS) radio occultation (RO) mission for the weather and space weather forecast that is focusing on mid-latitude and tropical regions. F7/C2 is a bi-lateral satellite constellation mission between Taiwan and the United States. Launched to the low inclination orbit of 24 degrees, the constellation consists of six satellites equipped with GNSS receivers, the advanced Tri-GNSS Radio-occultation System (TGRS), capable of receiving GPS, GLONASS, and Galileo signals. Alongside TGRS, there are two additional space weather-related instruments, the Ion Velocity Meter (or IVM) and the Radio Frequency Beacon (or RF Beacon) provided by U. S. Air Force. IVM observes in-situ ion species, densities, temperatures, and velocities; RF Beacon transmits beacon signals with three frequencies (400, 965, and 2200 MHz) to ground-based receivers for the derivation of the ionosphere electron content (IEC) and SNR of the radio signals. Both IVM and RF Beacon observations provide information on plasma irregularities of the ionosphere. [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]

Launched in 2006, the Formosa Satellite Mission 3-Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) was the first constellation of microsatellites carrying global positioning system (GPS) radio occultation (RO) receivers. Radio occultation is an active remote sensing technique that provides valuable information on the vertical variations of electron density in the ionosphere, and temperature, pressure, and water vapor in the stratosphere and troposphere. COSMIC has demonstrated the great value of RO data in ionosphere, climate, and meteorological research and operational weather forecasting. However, there are still challenges using RO data, particularly in the moist lower troposphere and upper stratosphere. A COSMIC follow-on constellation, COSMIC-2, was launched into equatorial orbit in 2019. With increased signal-to-noise ratio (SNR) from improved receivers and digital beam steering antennas, COSMIC-2 will produce at least 5,000 high-quality RO profiles daily in the tropics and subtropics. In this paper, we summarize 1) recent (since 2011 when the last review was published) contributions of COSMIC and other RO observations to weather, climate, and space weather science; 2) the remaining challenges in RO applications; and 3) potential contributions to research and operations of COSMIC-2. [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]