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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]

Using 6 days cross-track wind data from CHAMP, we investigate the high-latitude westward wind gradient and reversal that occurs at fixed longitudes (-120° ~ -30° GLon in the Northern Hemisphere, 150° ~ 210° GLon in the Southern Hemisphere). The driver of the wind gradient at 60° ~ 70° GLat and in the noon sector is investigated via TIEGCM. The wind gradient is driven by the combined effects of the Earth's geomagnetic field configuration and the energy deposition from the solar wind. The temporal variations of the neutral wind cell in afternoon sector are critical to the formation of wind gradient. The development of wind gradient is due to the equatorward expansion of the afternoon westward wind cell from the geographic high-latitudes to 60° ~ 70° GLat. Whereas the disappearance is associated with the poleward contraction from 60° to ~70° GLat to higher latitudes. A further diagnostic analysis of model results shows that the primary internal driver of the wind gradient is a balance between ion drag and pressure gradient. In the acceleration phase, ion drag causes the formation of the strong gradient of westward wind, whereas pressure gradient decelerates it. A similar result is found in the deceleration phase. The ion drag is controlled predominantly by both the electron density and the relative motion between the ions and neutrals. [ABSTRACT FROM AUTHOR]

Fundamental understanding of the climate responses to solar variability is obscured by the large and complex climate variability. This long‐standing issue is addressed here by examining climate responses under an extreme quiet sun (EQS) scenario, obtained by making the sun void of all magnetic fields. It is used to drive a coupled climate model with whole atmosphere and ocean components. The simulations reveal significant responses, and elucidate aspects of the responses to changes of troposphere/surface forcing and stratospheric forcing that are similar and those that are different. Planetary waves (PWs) play a key role in both regional‐scale responses and the mean circulation changes. Intermediate scale stationary waves and regional climate respond to solar forcing changes in the troposphere and stratosphere in a similar way, due to similar subtropical wind changes in the upper troposphere. The patterns of these changes are similar to those found in a warming climate, but with opposite signs. Responses of the largest scale PW during northern hemisphere and southern hemisphere winters differ, leading to hemispheric differences in the interplay between dynamical and radiative processes. The analysis exposes remarkable general similarities between climate responses in EQS simulations and those under nominal solar minimum conditions, even though the latter may not always appear to be statistically significant. Plain Language Summary: Understanding how climate may change under different solar conditions is both interesting and important. However it is difficult to clearly identify solar signal from the very large climate variability on broad time scales. In this study, we tackle this problem by providing a lower bound of the solar minimum condition according to our current understanding of solar physics. By specifying this extremely low solar minimum condition in a climate model that takes into consideration of the effects of ocean and middle atmosphere, we are able to identify significant climate responses, which are very different between the northern and southern hemispheres. We gain an understanding of the processes driving these responses, including how the lower and upper atmospheric processes may enhance/offset each other. By comparing these climate responses to those under nominal solar minimum conditions, we expose climate patterns that are hidden under the large climate variability in the latter. Key Points: Climate simulations under extreme quiet sun conditions reveal significant responsesHemispheric differences in the interplay between dynamical and radiative processes identifiedQuantify tropospheric/surface and stratospheric responses that are similar or different [ABSTRACT FROM AUTHOR]

The Starlink satellites launched on 3 February 2022 were lost before they fully arrived in their designated orbits. The loss was attributed to two moderate geomagnetic storms that occurred consecutively on 3–4 February. We investigate the thermospheric neutral mass density variation during these storms with the Multiscale Atmosphere‐Geospace Environment (MAGE) model, a first‐principles, fully coupled geospace model. Simulated neutral density enhancements are validated by Swarm satellite measurements at the altitude of 400–500 km. Comparison with standalone TIEGCM and empirical NRLMSIS 2.0 and DTM‐2013 models suggests better performance by MAGE in predicting the maximum density enhancement and resolving the gradual recovery process. Along the Starlink satellite orbit in the middle thermosphere (∼200 km altitude), MAGE predicts up to 150% density enhancement near the second storm peak while standalone TIEGCM, NRLMSIS 2.0, and DTM‐2013 suggest only ∼50% increase. MAGE also suggests altitudinal, longitudinal, and latitudinal variability of storm‐time percentage density enhancement due to height dependent Joule heating deposition per unit mass, thermospheric circulation changes, and traveling atmospheric disturbances. This study demonstrates that a moderate storm can cause substantial density enhancement in the middle thermosphere. Thermospheric mass density strongly depends on the strength, timing, and location of high‐latitude energy input, which cannot be fully reproduced with empirical models. A physics‐based, fully coupled geospace model that can accurately resolve the high‐latitude energy input and its variability is critical to modeling the dynamic response of thermospheric neutral density during storm time. Plain Language Summary: On 3 February 2022, 40 Starlink satellites were launched by the SpaceX Corporation when a moderate geomagnetic storm occurred, followed by another storm on 4 February. The storm activities have been regarded as the culprit for the loss the Starlink satellites afterward. Although strong geomagnetic storms are well‐known to be able to increase the neutral atmospheric mass density so as to satellite drag in the thermosphere where many space vehicles are orbiting around the Earth, a not‐so‐strong storm was not expected to bring such huge impacts based on engineering design evaluation using empirical atmospheric density models. This study compares the performance of a state‐of‐the‐art physics‐based, fully coupled whole geospace model and empirical models in predicting the neutral mass density variation in the thermosphere. It turns out that the physics‐based model is more accurate in capturing the magnitude of storm enhancement of neutral density. It also resolves the gradual recovery process even though it is not reflected in some geomagnetic indices that are used to drive the empirical models. Using such a first‐principles whole geospace model is suggested as a necessary step in future space weather applications. Key Points: Our model predicts up to ∼150% enhancement in neutral density along Starlink orbit during a moderate geomagnetic storm on 3–4 February 2022Empirical models tend to underestimate storm effects in thermospheric density enhancementThe whole geospace model resolves the gradual recovery and wave perturbations missed by empirical models [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]

The Global-scale observation of limb and disk mission observed an extra electron density (Ne) peak after sunset at approximately 30°S near 40°W on 4 November 2019, which is poleward and immediately next to the southern equatorial ionization anomaly (EIA) crest. This Ne peak is different from previously reported mid-latitude peaks that occur at all local times. The Whole Atmosphere Community Climate Model-eXtended captures this phenomenon. Model diagnostic analysis reveals that the decrease of Ne and hmF2 between 15° and 25°S makes Ne near 30°S appear as an extra density peak relative to the southern EIA crest. Transport by poleward meridional wind decreases Ne between 15° and 25°S. Moreover, the upward E × B drifts due to pre-reversal enhancement lift the plasma between the dip equator and 15°S but do not affect Ne much between 15° and 25°S with a low drift speed. Comparison with days without the extra peak shows the importance of E × B drift latitudinal variations on the extra peak formation. This study provides new insights into the dynamic variability of the nighttime ionosphere. [ABSTRACT FROM AUTHOR]

Previous studies have established the importance of the increasing greenhouse gas (GHG) concentrations in causing trends in the thermosphere and ionosphere (T-I). Recent work indicates that the changing Earth's magnetic field is also important. We conduct whole atmosphere model simulations to examine T-I trends driven by these two drivers and their relative importance. We found that, (1) trends in the T-I, driven by either of the two drivers, exhibited significant latitudinal and longitudinal variability; (2) in the thermosphere, trends were predominantly driven by the GHG driver except that the magnetic field driver played a small role in the neutral temperature trend (∼25%) in some regions mainly in the longitude sector ∼120°W–20°E. The magnetic field driver played a more important role in the ionosphere in the longitude sector ∼120°W–20°E. In this longitude sector, the two drivers were comparable in driving the trends of hmF2, NmF2, and electron temperature; the relative importance of the two drivers to the ion temperature trend additionally depends on altitude, with the GHG driver being the dominant driver at lower altitudes (∼200–320 km), and the two drivers becoming comparable above; (3) although the magnetic field driver is important in the longitude sector ∼120°W–20°E, it drove both negative and positive trends in roughly equal amounts, consequently, its contributions to the global average trends in the T-I are negligible. [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]

We performed simulations of the atmosphere-ionosphere response to the solar eclipse of 21 August 2017 using the Whole Atmosphere Community Climate Model-eXtended (WACCM-X v. 2.0) with a fully interactive ionosphere and thermosphere. Eclipse simulations show temperature changes in the path of totality up to -3 K near the surface, -1 K at the stratopause, ±4 K in the mesosphere, and -40 K in the thermosphere. In the F region ionosphere, electron density is depleted by about 55%. Both the temperature and electron density exhibit global effects in the hours following the eclipse. There are also significant effects on stratosphere-mesosphere chemistry, including an increase in ozone by nearly a factor of 2 at 65 km. Dynamical impacts of the eclipse in the lower atmosphere appear to propagate to the upper atmosphere. This study provides insight into coupled eclipse effects through the entire atmosphere from the surface through the ionosphere. [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: 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]

We report a hitherto unknown latitudinal double‐peak structure in the amplitude of stationary planetary wave 1 (SPW1) geopotential height in the stratosphere and lower mesosphere during austral winter. The primary peak is located at 60–70°S and 30–40 km, and the secondary peak is at 30–50°S and 40–60 km. According to 36 years (1981–2016) of the Modern‐Era Retrospective analysis for Research and Applications, version 2, reanalysis data, the double‐peak structure occurs with frequencies of ~61%, 97%, 53%, and 25% in May, June, July and August, respectively, while it rarely exists in other seasons. Significant downward Eliassen‐Palm fluxes suggestive of downward‐propagating waves are often found above the secondary peak, and phase progressions show opposite directions on its two sides. From the free‐running Whole Atmosphere Community Climate Model and linear mechanistic model simulations, the secondary peak is likely generated by the interference of primary upward‐ and secondary downward‐propagating SPW1 excited in situ by gravity wave forcing in the upper mesosphere. The strengths of the primary and secondary waves need to be comparable to form an effective interference pattern, which may explain the missing double‐peak feature in boreal winter as the primary waves are too dominant. Summer‐to‐winter interhemispheric wave coupling is identified in the austral midwinter and appears to originate from the secondary SPW1 generated in the summer hemisphere. Since the double‐peak structure of SPW1 is sensitive to the mean wind, wave‐mean flow, and wave‐wave interactions, this study provides a reference for general circulation and mechanistic models to simulate the middle atmosphere wave dynamics in austral winter. Plain Language Summary: The winter‐hemisphere stationary planetary waves are the most dominant perturbations in the lower and middle atmosphere of the Earth. They make significant contribution to the dynamics, chemistry, transport, and transition of the stratosphere and lower mesosphere. It is therefore important to understand their characteristics, forming mechanisms, and impacts. In this paper, we newly identify an intriguing latitudinal double‐peak feature of stationary planetary waves in austral winter months. Its statistical significance has been proved by using long‐term satellite observations and reanalysis data. The underlying mechanisms suggested by modeling work not only rely on the structure of the polar vortex but also involve the secondary planetary wave generation and wave‐wave interactions. This work uses a combination of satellite measurements, reanalysis product, whole atmosphere model, and mechanistic model, which provides a powerful tool to digest the important atmospheric phenomenon. Key Points: We report a novel finding of double‐peak structure in geopotential field of stationary planetary wave 1 in austral winter middle atmosphereDownward SPW1 generated by in situ GW forcing interferes with upward SPW1 which generates the secondary peak near midlatitude stratopauseSummer‐to‐winter interhemispheric wave coupling is identified at 70‐80 km and opposite in direction from winter‐to‐summer ducting above it [ABSTRACT FROM AUTHOR]