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.

Model simulations of temperature and density trends in the upper thermosphere are generally consistent with satellite drag data, but some discrepancies remain. The most important of these is that satellite drag analyses under solar minimum conditions have measured density change of about −5% per decade near 400 km altitude, while model simulations of upper atmosphere cooling due to anthropogenic increases in carbon dioxide and other trace gases have predicted about half that rate. For solar moderate and maximum conditions, agreement is better. The rate of change is less during higher solar activity, because higher levels of nitric oxide cooling compete with the anthropogenic cooling. However, some past modeling studies used global mean models, and others attempted to scale to decadal rates from scenarios where carbon dioxide was doubled. Both of these approaches have shortcomings. Therefore, we have performed new, fully 3‐D simulations, using the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model, to better quantify secular change rates at various levels of solar activity. These simulations use a 12 year baseline (approximately one solar cycle) in order to more directly compare with measured rates. Our new findings are in better agreement with observations for solar minimum conditions, approximately −5% per decade at 400 km, and are also still in reasonable agreement at solar maximum, approximately −2% per decade. This confluence of observation and simulation strengthens the case that some of the best evidence of the impact of anthropogenic global change on the upper atmosphere is the continued systematic decrease of thermospheric density.

WINDII, the Wind Imaging Interferometer on the Upper Atmospheric Research Satellite measures winds and emission rates from selected excited metastable species. Here we report on measurements of the atmospheric Rayleigh scattering from the O(1S) background filter at 553 nm wavelength used to derive temperature profiles in the upper mesosphere from 70 km to 95 km, for solstice periods from December 1992/93 and January 1993/94. The data are first zonally averaged and then combined in local time over about one month. Based on these temperatures, an analysis of planetary wave structures and tidal perturbations employing least-mean-square (LMS) fits to the data has been conducted and the results are presented. The planetary wave structures observed were well described with a quasi two-day wave (QTDW). Amplitudes of 14 K and 10 K at 85 km height for downleg (descending) and upleg (ascending) sampling respectively at latitudes from 20°S to 40°S were found to be in good agreement with QTDW temperature results from the MLS/UARS experiment assuming a vertical amplitude structure of the type described by the HRDI/UARS mesospheric wind observations. It is shown that the diurnal tide amplitudes estimated from latitudes from 25°N to 35°S using the LMS fit maximize at the equator with an amplitude of about 6 K and decrease toward tropical latitudes, consistent with the classical tidal theory and predictions from the TIME-GCM model.

It is a great honor to receive the 2013 Bowie Medal knowing that it comes from such a prestigious and renowned geophysical society and from AGU colleagues whom I have interacted with for the past 45 years.

A set of numerical experiments have been conducted using the National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (NCAR TIME-GCM) to understand the effects of the quasi-two-day wave (QTDW) on the middle atmosphere horizontal wind and temperature fields. A zonal wavenumber three perturbation with a period of 48 hours and a latitudinal structure identical to the (3, 0) Rossby-gravity mode has been included at the lower-boundary of the model. A response in the middle atmosphere horizontal wind fields is observed with a structure qualitatively similar to observations and other model results. There is also some evidence to suggest an increase in the lower-thermosphere QTDW response due to the interaction with gravity waves. Changes are observed in the zonal mean wind and temperature fields that are clearly related to the QTDW, however it is unclear if these changes are the direct result of wave driving due to the QTDW or are from another source. Evidence for nonlinear interactions between the QTDW and the migrating tides is presented. This includes significant (40–50%) decreases in the amplitude of the migrating tides when the QTDW is present and the generation of wave components which can be tracked back to an interaction between the QTDW and the migrating tides. Clear evidence for the existence of a westward propagating zonal wavenumber six nonmigrating diurnal tidal component which results from the nonlinear interaction between the QTDW and the migrating tides is also presented.

The solar minimum period between solar cycles 23 and 24 was the longest since the beginning of space-based measurements, and many manifestations of solar activity were unusually low. Thermospheric neutral density was about 30% lower than during the previous solar minimum, but changes in the ionosphere between the two solar minima are more controversial. Solar radiation, geomagnetic activity, and anthropogenic increases in greenhouse gases can all play a role in these changes. In this paper, we address the latter of these potential contributions the degree to which secular change driven by greenhouse gases, primarily CO2, could be responsible for the observed changes. New 3-D model simulations find a global mean density decrease at 400 km of 5.8% between the two recent solar minima, which is larger than earlier 1-D model results and in better agreement with observations. From these model simulations and from other observational work, we estimate that the contribution of secular change to global mean neutral density decrease between the two recent solar minima is less than similar to 6%. The contribution of secular change to the global average decrease of F region ionosphere peak density (NmF2) and altitude (h(m)F(2)), near midday, is estimated to be 1.5% and 1.5 km, respectively. However, secular changes in the ionosphere exhibit large variations with local time, geographic location, and season. The midday change of NmF2 seen in the model simulations ranged between +6% and -9% and the change of h(m)F(2) ranged between +11 km and -11 km, depending on geographic location.

The CHAMP satellite has revealed new observations of the upper F region ionosphere and the thermosphere. This chapter reviews these new findings at low and equatorial latitude, from the point of view of coupling between the atmosphere and the magnetic field and between different atmospheric regions. The chapter deals with the gross features in the electron density and temperature and in the neutral density and wind. These features include the equatorial anomaly, the electron temperature morning overshoot, equatorial plasma irregularities, the zonal wind jet, terminator waves, or findings of the wave-4 structure in plasma and neutrals. Large-scale thermospheric features revealed by CHAMP are described from three perspectives. They are the thermosphere response to solar forcing, to ionospheric forcing, and to lower atmosphere forcing.

This modeling study quantifies the daytime low‐latitude vertical E×B drift changes in the longitudinal wave number 1 (wn1) to wn4 during the major extended January 2006 stratospheric sudden warming (SSW) period as simulated by the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM), and attributes the drift changes to specific tides and planetary waves (PWs). The largest drift amplitude change (approximately 5 m/s) is seen in wn1 with a strong temporal correlation to the SSW. The wn1 drift is primarily caused by the semidiurnal westward propagating tide with zonal wave number 1 (SW1), and secondarily by a stationary planetary wave with zonal wave number 1 (PW1). SW1 is generated by the nonlinear interaction of PW1 and the migrating semidiurnal tide (SW2) at high latitude around 90–100 km. The simulations suggest that the E region PW1 around 100–130 km at the different latitudes has different origins: at high latitudes, the PW1 is related to the original stratospheric PW1; at midlatitudes, the model indicates PW1 is due to the nonlinear interaction of SW1 and SW2 around 95–105 km; and at low latitudes, the PW1 might be caused by the nonlinear interaction between DE2 and DE3. The time evolution of the simulated wn4 in the vertical E×B drift amplitude shows no temporal correlation with the SSW. The wn4 in the low‐latitude vertical drift is attributed to the diurnal eastward propagating tide with zonal wave number 3 (DE3), and the contributions from SE2, TE1, and PW4 are negligible.

The daytime O(S-1) emission at 557.7nm observed at 250km with the Wind Imaging Interferometer (WINDII) is dominantly excited by photoelectron impact on atomic oxygen, so the volume emission rate is normally a measure of the atomic oxygen concentration. Daily polar maps of the 250km volume emission rate during geomagnetic disturbances display polar spirals extending out of the auroral region and down to the equator. Since the local time is fixed for a given latitude for a single day, the spiral maps form a spatio-temporal pattern in which the longitudinal variations cannot be distinguished from those in universal time. Simulations for 2 January 1993 implemented with the National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) using the equivalent satellite perspective show a remarkably similar spiral pattern. Hourly universal time simulations reveal a rapid equatorward expansion of the spirals during this modest geomagnetic event of Kp=4.7, which are mirrored in the meridional and vertical winds. Simulations of the electron density show that the emission within the spirals is caused by the recombination of O-2(+) ions with electrons, and not the atomic oxygen enhancement itself. All of this strongly suggests that the spirals are in fact large-scale Traveling Ionospheric Disturbances (TIDs), and comparisons of the WINDII data made with TID observations reported in the literature, including those made on the same day, support this conclusion. The TIME-GCM simulations suggest that a component of the spirals originates in the lower atmosphere and appears at thermospheric heights.

This chapter contains sections titled: Introduction Available Potential Energy in the Thermosphere Results Conclusion

A quasi-static model of global atmospheric electricity is used to examine the electrical coupling between magnetospheric processes in the upper atmosphere and atmospheric electricity in the lower atmosphere.

The thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM) is a community model developed and maintained at the National Center for Atmospheric Research. It also can be run at the NASA Community Coordinated Modeling Center, and is a component of the coupled magnetosphere-ionosphere-thermosphere Model (CMIT). This paper describes the TIE-GCM development history, model elements, model input and output, the equations solved, boundary conditions, and numerical techniques. Some model validation examples are shown, and future improvements and developments are discussed.

We conducted model simulations to examine how changes in concentration of radiatively active trace gases in the middle atmosphere affect long‐term changes in the upper atmosphere. We focused our model study on the impact of increases in carbon dioxide (CO2), methane (CH4), and water vapor (H2O), and decreases in ozone (O3) between 1983 and 2003. We used both the National Center for Atmospheric Research Whole Atmosphere Community Climate Model and the Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model, global mean version, in this study. The model simulations indicate that CO2 is the main forcing mechanism of long‐term changes in the thermsophere, with minor influences from O3, CH4, and H2O. At 400 km altitude, global mean thermospheric neutral density decreased by ~4.5% due to CO2 forcing alone, whereas it decreased by ~4.8% due to the combined forcing from all four gases. O3 depletion caused cooling in the stratosphere and mesosphere (maximum decrease of 0.5 K) due to reduced absorption of solar ultraviolet radiation, but had nearly no cooling effect in the thermosphere. However, due to thermal contraction in the stratosphere and mesosphere, O3 depletion caused a small decrease in thermospheric neutral density of ~0.25%. Increases in both CH4 and H2O may slightly warm the upper mesosphere and thermosphere due to increased chemical heating and absorption of solar ultraviolet radiation.

It is well known that solar EUV radiation and auroral heat and momentum sources have a significant effect on thermospheric and ionospheric structure and dynamics. Upper atmosphere general circulation models using these forcings have been reasonably successful in simulating the thermosphere and ionosphere responses for a number of geophysical event studies. These models can be used as forecast models of thermospheric and ionospheric structure and dynamics by using predicted inputs of solar EUV and UV radiation, auroral hemispheric power of precipitating particles, cross-polar cap potential drop and ion convection patterns. It is also necessary to have a satisfactory initial state to start the simulation. The NCAR TIE-GCM that simulates the thermosphere and ionosphere between 95 and 800 km altitude is used to show the sensitivity of the thermosphere and ionosphere to space weather events.

This chapter contains sections titled: Introduction Energetics of the mesosphere and thermosphere Solar Spectral Irradiance Global Mean Model Results for Solar Cycle Minimum Summary and Conclusions

Ionospheric day‐to‐day variability is a ubiquitous feature, even in the absence of appreciable geomagnetic activities. Although meteorological perturbations have been recognized as an important source of the variability, it is not well represented in previous modeling studies and the mechanism is not well understood. This study demonstrates that the thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) constrained in the stratosphere and mesosphere by the hourly whole atmosphere community climate model (WACCM) simulations is capable of reproducing observed features of day‐to‐day variability in the thermosphere‐ionosphere. Realistic weather patterns in the lower atmosphere in WACCM were specified by Modern Era Retrospective Reanalysis for Research and Application (MERRA). The day‐to‐day variations in mean zonal wind, migrating and nonmigrating tides in the thermosphere, vertical and zonal E × B drifts, and ionosphere F2 layer peak electron density (NmF2) are examined. The standard deviations of the drifts and NmF2 show local time and longitudinal dependence that compare favorably with observations. Their magnitudes are 50% or more of those from observations. The day‐to‐day thermosphere and ionosphere variability in the model is primarily caused by the perturbations originated in lower atmosphere, since the model simulation is under constant solar minimum and low geomagnetic conditions.

In this paper the impact of the field‐aligned ion drag on equatorial thermosphere temperature and density is quantitatively investigated on the basis of the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (NCAR TIEGCM) simulations under high solar activity (F107 = 180). The increase of upward vertical winds over the magnetic equator associated with the additional divergence of meridional winds, caused by the inclusion of field‐aligned ion drag, leads to a reduction in thermosphere temperature and density at the magnetic equator through enhanced adiabatic cooling. We found that the field‐aligned ion drag has an obvious impact on the thermosphere only over the magnetic equatorial region in the daytime and evening sectors, whereas it has less effect on the equatorial thermosphere anomaly (ETA) crests. The daytime neutral temperature over the magnetic equator is reduced by about 30 K, for altitudes above 250 km without significant altitudinal variations, when field‐aligned ion drag is included in the simulation. The thermosphere density in the magnetic equatorial region starts to change slightly at 300 km and depletes by about 5% at 400 km, while experiencing a greater decrease with altitude. Furthermore, the trough produced in the neutral temperature and density corresponds well with the magnetic dip equator. The ETA features during 12:00–18:00 LT become obvious as a result of the inclusion of the field‐aligned ion drag. Specifically, our results show that at 400 km the crest‐trough differences in neutral temperature are about 30–60 K, and the crest‐trough ratios in thermosphere density are 1.03–1.06, comparable with observations.

The complete mechanism of how upward propagating tropospheric tides connect to the upper atmosphere is not yet fully understood. One proposed mechanism is via ionospheric wind dynamo. However, other sources can potentially alter the vertical E × B drift: gravity and plasma pressure gradient driven current, the geomagnetic main field, and longitudinal variation in the conductivities. In this study we examine the contribution to the vertical drift from these sources, and compare them. We use March equinox results from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model. We found that the gravity and plasma pressure gradient driven current and the longitudinal variation of the conductivities excluding the variation due to the geomagnetic main field do not change the longitudinal variation of the vertical drift significantly. Modifying the geomagnetic main field will change the vertical drift at 5–6 LT, 18–19 LT and 23–24 LT at almost all longitudes. In general the influence of the geomagnetic main field on the vertical drift is larger, with respect to the maximum difference, at 18–19 LT and 23–24 LT, equal at 5–6 LT, and smaller at 14–15 LT than the influence due to nonmigrating tidal components in the neutral winds. Examination of the contribution from E‐ and F‐region neutral winds to the vertical drift shows that their importance depends on the local time and the solar activity. This implies that the vertical drift has to be analyzed at specific local times to examine the relation between the wave number in the vertical drift and in the neutral winds.