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.

In atmospheric and space environment studies it is key to understand and to quantify the coupling of atmospheric regions and the solar impacts on the whole atmosphere system. There is thus a need for a numerical model that encompasses the whole atmosphere and can self‐consistently simulate the dynamic, physical, chemical, radiative, and electrodynamic processes that are important for the Sun‐Earth system. This is the goal for developing the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM). In this work, we report the development and preliminary validation of the thermospheric extension of WACCM (WACCM‐X), which extends from the Earth's surface to the upper thermosphere. The WACCM‐X uses the finite volume dynamical core from the NCAR Community Atmosphere Model and includes an interactive chemistry module resolving most known neutral chemistry and major ion chemistry in the middle and upper atmosphere, and photolysis and photoionization. Upper atmosphere processes, such as nonlocal thermodynamic equilibrium, radiative transfer, auroral processes, ion drag, and molecular diffusion of major and minor species, have been included in the model. We evaluate the model performance by examining the quantities essential for the climate and weather of the upper atmosphere: the mean compositional, thermal, and wind structures from the troposphere to the upper thermosphere and their variability on interannual, seasonal, and daily scales. These quantities are compared with observational and previous model results.

Solar activity during 2007–2009 was very low, and during this protracted solar minimum period, the terrestrial thermosphere was cooler and lower in density than expected. Measurements from instruments on the SOHO and TIMED spacecraft, and by suborbital rocket flights, indicate that solar extreme‐ultraviolet irradiance levels were lower than they were during the previous solar minimum. Analysis of atmospheric drag on satellite orbits indicate that the thermosphere was lower in density, and therefore cooler, and than at any time since the beginning of the space age. However, secular change due to increasing levels of carbon dioxide and other greenhouse gases, which cool the upper atmosphere, also plays a role in thermospheric climate. Simulations by the NCAR Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model are compared to thermospheric density measurements, yielding evidence that the primary cause of the low thermospheric density was the unusually low level of solar extreme‐ultraviolet irradiance.