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The response of the thermospheric daytime longitudinally averaged zonal and meridional winds and neutral temperature to the 2020/2021 major sudden stratospheric warming (SSW) is studied at low-to middle latitudes (0^◦- 40^◦N) using observations by NASA’s ICON and GOLD satellites. The major SSW commenced on 1 January 2021 and lasted for several days. Results are compared with the non-SSW winter of 2019/2020 and pre-SSW period of December 2020. Major changes in winds and temperature are observed during the SSW. The northward and westward winds are enhanced in the thermosphere especially above ∼140 km during the warming event, while temperature around 150 km drops up to 50 K compared to the pre-SSW phase. Changes in the zonal and meridional winds are likely caused by the SSW-induced changes in the propagation and dissipation conditions of internal atmospheric waves. Changes in the horizontal circulation during the SSW can generate upwelling at low-latitudes, which can contribute to the adiabatic cooling of the low-latitude thermosphere. The observed changes during the major SSW are a manifestation of long-range vertical coupling in the atmosphere. 

Abstract Most ionospheric models cannot sufficiently reproduce the observed electron density profiles in the E‐region ionosphere, since they usually underestimate electron densities and do not match the profile shape. Mitigation of these issues is often addressed by increasing the solar soft X‐ray flux which is ineffective for resolving data‐model discrepancies. We show that low‐resolution cross sections and solar spectral irradiances fail to preserve structure within the data, which considerably impacts radiative processes in the E‐region, and are largely responsible for the discrepancies between observations and simulations. To resolve data‐model inconsistencies, we utilize new high‐resolution (0.001 nm) atomic oxygen (O) and molecular nitrogen (N₂) cross sections and solar spectral irradiances, which contain autoionization and narrow rotational lines that allow solar photons to reach lower altitudes and increase the photoelectron flux. This work improves upon Meier et al. (2007,https://doi.org/10.1029/2006gl028484) by additionally incorporating high‐resolution N₂photoionization and photoabsorption cross sections in model calculations. Model results with the new inputs show increased O⁺production rates of over 500%, larger than those of Meier et al. (2007,https://doi.org/10.1029/2006gl028484) and total ion production rates of over 125%, while production rates decrease by ∼15% in the E‐region in comparison to the results obtained using the cross section compilation from Conway (1988,https://apps.dtic.mil/sti/pdfs/ADA193866.pdf). Low‐resolution molecular oxygen (O₂) cross sections from the Conway compilation are utilized for all input cases and indicate that is a dominant contributor to the total ion production rate in the E‐region. Specifically, the photoionization contributed from longer wavelengths is a main contributor at ∼120 km. 

Abstract E‐region models have traditionally underestimated the ionospheric electron density. We believe that this deficiency can be remedied by using high‐resolution photoabsorption and photoionization cross sections in the models. Deep dips in the cross sections allow solar radiation to penetrate deeper into the E‐region producing additional ionization. To validate our concept, we perform a study of model electron density profiles (EDPs) calculated using the Atmospheric Ultraviolet Radiance Integrated Code (AURIC; D. Strickland et al., 1999,https://doi.org/10.1016/s0022-4073(98)00098-3) in the E‐region of the terrestrial ionosphere. We compare AURIC model outputs using new high‐resolution photoionization and photoabsorption cross sections, and solar spectral irradiances during low solar activity with incoherent scatter radar (ISR) measurements from the Arecibo and Millstone Hills observatories, Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC‐1) observations, and outputs from empirical models (IRI‐2016 and FIRI‐2018). AURIC results utilizing the new high‐resolution cross sections reveal a significant difference to model outputs calculated with the low‐resolution cross sections currently used. Analysis of AURIC EDPs using the new high‐resolution data indicate fair agreement with ISR measurements obtained at various times at Arecibo but very good agreement with Millstone Hills ISR observations from ∼96–140 km. However, discrepancies in the altitude of the E‐region peak persist. High‐resolution AURIC calculations are in agreement with COSMIC‐1 observations and IRI‐2016 model outputs between ∼105 and 140 km while FIRI‐2018 outputs underestimate the EDP in this region. Overall, AURIC modeling shows increased E‐region electron densities when utilizing high‐resolution cross sections and high‐resolution solar irradiances, and are likely to be the key to resolving the long standing data‐model discrepancies.