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.

Observations of the Global Positioning System (GPS) total electron content (TEC) are used to study the coupling between the 2009 sudden stratospheric warming (SSW) and ionospheric perturbations. The observations reveal both migrating and nonmigrating perturbations to the semidiurnal tide in the equatorial ionization anomaly crest region that are associated with changes in electric fields induced by the tidal dynamo. In particular, a significant enhancement is observed in the nonmigrating semidiurnal westward propagating tide with zonal wavenumber 1 (SW1) in GPS TEC during the SSW. The SW1 perturbations in the low‐latitude ionosphere are found to oscillate with a similar period as planetary wave‐1 activity in the Northern Hemisphere stratosphere. This connection is attributed to the nonlinear interaction between tides and planetary waves and strongly supports the theory that planetary wave‐tide interaction is the primary mechanism coupling SSWs to ionospheric variability. Enhancements are also observed in the nonmigrating semidiurnal tide with zonal wavenumber 0 (S0) during this time period and may be related to the nonlinear interaction between the migrating semidiurnal tide and planetary wave‐2. The connection between planetary wave‐2 and S0 is, however, less clear which may be attributed to differences in the zonal mean zonal winds in the mesosphere and lower thermosphere during the times of peak planetary wave‐1 and planetary wave‐2 activity. We conclude that the changing zonal winds during SSWs play an important role in the coupling between ionospheric variability and the forcing from planetary waves of lower atmospheric origin during SSWs.

For the first time a mesoscale‐resolving whole atmosphere general circulation model has been developed, using the National Center for Atmospheric Research Whole Atmosphere Community Climate Model with ∼0.25° horizontal resolution and 0.1 scale height vertical resolution above the middle stratosphere (higher resolution below). This is made possible by the high accuracy and high scalability of the spectral element dynamical core from the High‐Order Method Modeling Environment. For the simulated January–February period, the latitude‐height structure and the magnitudes of the temperature variance compare well with those deduced from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations. The simulation reveals the increasing dominance of gravity waves (GWs) at higher altitudes through both the height dependence of the kinetic energy spectra, which display a steeper slope (∼−3) in the stratosphere and an increasingly shallower slope above, and the increasing spatial extent of GWs (including a planetary‐scale extent of a concentric GW excited by a tropical cyclone) at higher altitudes. GW impacts on the large‐scale flow are evaluated in terms of zonal mean zonal wind and tides: with no GW drag parameterized in the simulations, forcing by resolved GWs does reverse the summer mesospheric wind, albeit at an altitude higher than climatology, and only slows down the winter mesospheric wind without closing it. The hemispheric structures and magnitudes of diurnal and semidiurnal migrating tides compare favorably with observations.

Numerical simulations are performed for a sudden stratosphere warming (SSW) under different atmospheric tide and planetary wave forcing conditions to investigate the tidal variability in the mesosphere and lower thermosphere (MLT). The influence of variability of different tides in the MLT on generating perturbations to the low latitude ionosphere is also investigated. Significant changes are found to occur in the migrating semidiurnal solar (SW2) and lunar (M2) tides as well as in the westward propagating nonmigrating semidiurnal tide with zonal wave number 1 (SW1). The changes in the zonal mean atmosphere that occur during SSWs lead to an enhancement in the SW2 and M2 tides. The vertical wavelength of the SW2 is also changed, resulting in phase variability in the SW2 at a constant altitude. Significant enhancements in the SW1 are found to occur only in the presence of additional planetary wave forcing, and this demonstrates that nonlinear planetary wave‒tide interactions lead to the enhanced SW1 during SSWs. The amplitude and phase variability of the SW2 is found to be capable of producing temporal variability in the vertical plasma drift velocity that is similar to the observed variability. Changes in the M2 during SSWs can contribute up to an additional ∼30% of the total ionosphere variability; however, the overall influence of the lunar tide is found to be dependent upon the phase of the moon relative to the timing of the SSW. Although the influence is relatively minor, the SW1 also contributes to the low latitude ionosphere variability during SSWs. The simulation results for the vertical plasma drift velocity and total electron content (TEC) further illustrate that significant longitude variability occurs in the ionosphere response to SSWs.

The joint Taiwan‐United States FORMOSAT‐3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) mission, hereafter called COSMIC, is the first satellite constellation dedicated to remotely sense Earth's atmosphere and ionosphere using a technique called Global Positioning System (GPS) radio occultation (RO). The occultations yield abundant information about neutral atmospheric temperature and moisture as well as space weather estimates of slant total electron content, electron density profiles, and an amplitude scintillation index, S4. With the success of COSMIC, the United States and Taiwan are moving forward with a follow‐on RO mission named FORMOSAT‐7/COSMIC‐2 (COSMIC‐2), which will ultimately place 12 satellites in orbit with two launches in 2016 and 2019. COSMIC‐2 satellites will carry an advanced Global Navigation Satellite System (GNSS) RO receiver that will track both GPS and Russian Global Navigation Satellite System signals, with capability for eventually tracking other GNSS signals from the Chinese BeiDou and European Galileo system, as well as secondary space weather payloads to measure low‐latitude plasma drifts and scintillation at multiple frequencies. COSMIC‐2 will provide 4–6 times (10–15X in the low latitudes) the number of atmospheric and ionospheric observations that were tracked with COSMIC and will also improve the quality of the observations. In this article we focus on COSMIC/COSMIC‐2 measurements of key ionospheric parameters.