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Abstract This study focuses on understanding what drives the previously observed deep nighttime ionospheric hole in the American sector during the January 2013 sudden stratospheric warming (SSW). Performing a set of numerical experiments with the thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) constrained by a high‐altitude version of the Navy Global Environmental Model, we demonstrate that this nighttime ionospheric hole was the result of increased poleward and down magnetic field line plasma motion at low and midlatitudes in response to alteredF‐region neutral meridional winds. Thermospheric meridional wind modifications that produced this nighttime depletion resulted from the well‐known enhancements in semidiurnal tidal amplitudes associated with stratospheric warming (SSWs) in the upper mesosphere and thermosphere. Investigations into other deep nighttime ionospheric depletions and their cause were also considered. Measurements of total electron content from Global Navigation Satellite System receivers and additional constrained TIME‐GCM simulations showed that nighttime ionospheric depletions were also observed on several nights during the January‐February 2010 SSW, which resulted from the same forcing mechanisms as those observed in January 2013. Lastly, the recent January 2021 SSW was examined using Modern‐Era Retrospective Analysis for Research and Applications, Version 2, COSMIC‐2 Global Ionospheric Specification electron density, and ICON Michelson Interferometer for Global High‐Resolution Thermospheric Imaging horizontal wind data and revealed a deep nighttime ionospheric depletion in the American sector was likely driven by modified meridional winds in the thermosphere. The results shown herein highlight the importance of thermospheric winds in driving nighttime ionospheric variability over a wide latitude range. 

Abstract Over‐the‐Horizon communication is strongly dependent on the state of the ionosphere, which is susceptible to solar flares. Trans‐ionospheric high frequency (HF, 3–30 MHz) signals can experience strong attenuation following a solar flare that lasts typically for an hour, commonly referred to as shortwave fadeout (SWF). In this study, we examine the role of dispersion relation and collision frequency formulations on the estimation of SWF in riometer observations using a new physics‐based model framework. The new framework first uses modified solar irradiance models incorporating high‐resolution solar flux data from the GOES satellite X‐ray sensors as input to compute the enhanced ionization produced during a flare event. The framework then uses different dispersion relation and collision frequency formulations to estimate the enhanced HF absorption. The modeled HF absorption is compared with riometer data to determine which formulation best reproduces the observations. We find the Appleton‐Hartree dispersion relation in combination with the averaged collision frequency profile reproduces riometer observations with an average skill score of 0.4, representing 40% better forecast ability than the existing D‐region Absorption Prediction model. Our modeling results also indicate that electron temperature plays an important role in controlling HF absorption. We suggest that adoption of the Appleton‐Hartree dispersion relation in combination with the averaged collision frequency be considered for improved forecasting of ionospheric absorption following solar flares.