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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]

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 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]

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]

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]

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]

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]

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]

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]