Using ICON and GOLD satellite observations, the response of the thermospheric daytime horizontal winds and neutral temperature to the 2020/2021 major sudden stratospheric warming (SSW) is studied at low- to middle latitudes (0° - 40°N). Comparison with observations during the non-SSW winter of 2019/2020 and the pre-SSW period (December 2020) clearly demonstrates the SSW-induced changes. The northward and westward thermospheric winds are enhanced during the warming event, while temperature around 150 km drops by up about 50 K compared to the pre-SSW phase. 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.

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; \citeA{strickland1999atmospheric}) 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, 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 $\sim96$ km to $140$ 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 $\sim105$ km 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.

Accurate photoionization rates are vital for the study and understanding of ionospheres and may account for the discrepancy in electron densities and mismatched altitude profiles of current E-region models. The underestimation of electron density profiles could be mitigated by high-resolution cross sections that preserve autoionization lines which allow solar photons to leak through to lower altitudes. We present new ionization rates calculated with high-resolution (0.001 nm) O and N2 photoionization and electron impact cross sections, and a high-resolution solar spectrum as inputs to CPI’s Atmospheric Ultraviolet Radiance Integrated Code [AURIC, Strickland et al., 1999]. The new electron impact cross sections show little structure and have minimal effect on calculations of ionization rates. Results from AURIC with updated O and N2 cross sections indicate increased production rates up to ~40% in the E-region, specifically between 100–115 km. Likewise, production rates determined using the ionospheric photoionization rate code from Meier et al. [2007] also illustrate an increase in the O and N2 production rates (typically of more than 10%) when using the newly calculated cross sections. Additionally, we find that O and N2 dominate the volume production rates above 130 km while O2 is expected to be the main contributor from 95–130 km. AURIC model results that use the default data and model results with the new O and N2 cross sections both track very well with electron density profiles determined from Arecibo ISR observations. AURIC model results using the new cross section calculations are in better agreement with Arecibo observations at higher altitudes. Our current findings indicate that O2 plays a dominant role in photoionization production rates in the E-region. Therefore it is crucial to update ab initio ionospheric models with high-resolution photoionization cross sections.