A set of Observing System Simulation Experiments (OSSEs) are performed to assess the impact of assimilating Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) electron density profiles and ground‐based Global Navigation Satellite System (GNSS) total electron content (TEC) observations in a whole atmosphere data assimilation system. The OSSEs are performed using the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCMX) with data assimilation provided by the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter. Results from the OSSEs demonstrate that the assimilation of ionosphere observations improves the short‐term (1 hr) forecasts and analyses. The OSSEs show that the short‐term forecasts and analyses are further improved when the ionosphere observations adjust the thermosphere neutral composition and temperature in addition to the ionosphere electron density. Based on an initialized forecast experiment, we find that adjusting the thermosphere neutral composition and temperature also leads to improved forecast skill in the ionosphere on longer time scales (i.e., beyond 1 hr). Additionally, it is shown that using a 1 hr data assimilation cycle, and removal of second‐order divergence damping in WACCMX+DART significantly improves tidal amplitudes, which were previously found to be too small. These initial results represent the first whole atmosphere data assimilation system with capabilities to assimilate observations from the troposphere to the ionosphere‐thermosphere.

Reduced daytime upward E×B drifts and weakened fountain effects in equatorial ionosphere have been frequently observed during the initial stage of solar flares. The cause of this phenomenon, however, remains unresolved. The latest state‐of‐art whole geospace model provides an unprecedented opportunity to explore the origin of this response. We show that both prompt penetration electric fields (PPEFs) and internal changes in the wind dynamo process are responsible for the reduced upward ion drifts. Solar‐flare‐induced PPEFs are caused by a reduced high‐latitude potential as a result of flare‐enhanced ionospheric conductances which are distinct from traditional PPEFs that respond to changes in solar wind conditions or magnetosphere dynamics. The neutral wind dynamo source is mainly a reduction in the background low‐latitude eastward electric field. This reduction occurs to maintain current continuity in response to the flare enhancement of low‐latitude Cowling conductance that is relatively greater than the enhancement of the dynamo current source.

The three‐dimensional computerized ionospheric tomography (3DCIT) technique is used to reconstruct the spatial distribution of storm‐enhanced density (SED) based on the global positioning system total electron content measurements over the North American area during the 17 March 2013 storm. The reconstruction results are carefully validated with observations from three ionosonde stations, the constellation observing system for meteorology, ionosphere, and climate (COSMIC) radio occultations, and the Millstone Hill incoherent scatter radar. The electron density profiles from the 3DCIT reconstruction show a good agreement with the ionosonde and COSMIC electron density profiles. The 3DCIT‐derived electron density difference between the storm day of 17 March and the quiet day of 16 March also captures the similar SED plume signature that was observed by the Millstone Hill incoherent scatter radar. The 3DCIT reconstruction allows us for the first time to unveil the 3‐D configuration of the SED plume and its spatiotemporal evolution. It was found that the SED plume first appeared around 400 km and then expanded downward to ~300 km as well as upward to ~500 km over the course of a 3‐hr period from 19 to 22 UT on 17 March. Our study also showed that the density enhancement within the SED plume occurred mostly above the storm time F layer peak height.

S: In this work, the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model is used to investigate the responses of ionospheric electrodynamic processes to the solar flares at the flare peaks and the underlying physical mechanisms on September 6 and 10, 2017. Simulations show that solar flares increased global daytime currents and reduced the eastward electric fields during the daytime from the equator to middle latitudes. Furthermore, westward equatorial electric fields and equatorial counter electrojets occurred in the early morning. At the flare peak, these electrodynamic responses are predominantly related to the enhanced E‐region conductivity by flares, as the responses of neutral winds and F‐region conductivity to flares are negligible. Specifically, the Cowling conductance enhancement is not the major process causing the reduction of zonal electric fields. This electric field reduction is primarily associated with the decrease of the ratio between the field line‐integrated wind‐driven currents and the conductance. The flare‐induced conductivity enhancement is larger but the background wind speed is smaller in the E‐region than in the F‐region, as a result, the increase of total integrated wind‐driven currents is less than the conductance enhancement.