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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]

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]