The nature of the variability of the Total Electron Content (TEC) over Europe is investigated during the 2009 and 2019 Northern Hemisphere (NH) SSW events in this study. As the TEC variability is driven by geomagnetic and lower atmospheric forcing mechanisms, we investigate the dominant drivers and their respective contributions to TEC changes during both SSW events. We simulate the SSWs using the Whole Atmosphere Community Climate Model eXtended version (WACCM-X) and compare the semidiurnal solar and lunar tidal variabilities in the mesosphere-lower thermosphere (MLT) region. Further, in order to assess the mechanisms responsible for the TEC variability during both SSWs, we run numerical experiments using the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM). We constrain the TIE-GCM lower boundary with the WACCM-X fields and carry out simulations both with and without geomagnetic forcing for each of the SSWs. The TIE-GCM simulations allow us to isolate the geomagnetic and lower atmospheric forcing effects on the TEC. We find that there was a major enhancement in daytime TEC over Europe during the 2019 SSW event, which was predominantly geomagnetically forced (~80%), while for the 2009 SSW, the major variability in TEC was accounted for by lower atmospheric forcing.

In this study, a new high-latitude empirical model is introduced, named for Auroral energy Spectrum and High-Latitude Electric field variabilitY (ASHLEY). This model aims to improve specifications of soft electron precipitations and electric field variability that are not well represented in existing high-latitude empirical models. ASHLEY consists of three components, ASHLEY-A, ASHLEY-E and ASHLEY-Evar, which are developed based on the electron precipitation and bulk ion drift measurements from the Defense Meteorological Satellite Program (DMSP) satellites during the most recent solar cycle. On the one hand, unlike most existing high-latitude electron precipitation models, which have assumptions about the energy spectrum of incident electrons, the electron precipitation component of ASHLEY, ASHLEY-A, provides the differential energy fluxes in the 19 DMSP energy channels under different geophysical conditions without making any assumptions about the energy spectrum. It has been found that the relaxation of spectral assumptions significantly improves soft electron precipitation specifications with respect to a Maxwellian spectrum (up to several orders of magnitude). On the other hand, ASHLEY provides consistent mean electric field and electric field variability under different geophysical conditions by ASHLEY-E and ASHLEY-Evar components, respectively. This is different from most existing electric field models which only focus on the large-scale mean electric field and ignore the electric field variability. Furthermore, the consistency between the electric field and electron precipitation is better taken into account in ASHLEY.

Vertical shears of horizontal winds play an important role in the dynamics of the middle and upper atmosphere. Prior observations have indicated that these shears predominantly occur in the lower thermosphere. MIGHTI observations from the Ionospheric Connection Explorer indicate that strong wind shears are a common feature of the lower thermosphere and vary greatly between orbits. This work focuses on these strong shears, and examines their occurrences, horizontal scales and underlying organization. No preferred wind shear direction is found. The shears that persist for a short horizontal extent are slightly larger in amplitude and more numerous than those that persist across large horizontal scales. The altitude at which the strongest shears occur often shows a downward progression with local time, following the climatological winds. Consistent with prior observations, the strongest shears are often seen just below the wind maximum, which follows the downward phase propagation consistent with upward propagating tides.

The equatorial electrojet (EEJ) is an important manifestation of ionospheric electrodynamics. Day-to-day changes of the EEJ result from E-region dynamo processes that are primarily driven by highly variable atmospheric waves propagating up from the lower and middle atmosphere. Progress has been made in our understanding that upward propagating tides are one of the major contributors to the day-to-day variability in the EEJ, however current models are limited in their ability to capture the vertical propagation of tides from the lower and middle atmosphere to the upper atmosphere due to difficulties to adequately represent many processes that influence it. In this study, we thus propose a new data-driven approach to modeling day-to-day variability by taking advantage of widely available ground-based magnetic field measurements. The new approach based on an ensemble transform adjustment method is applied to the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) lower boundary conditions (LBCs) at about 97 km altitude in order to make the model’s tidal characteristics to be more consistent with observed magnetic perturbations associated with the EEJ. In this method, TIE-GCM ensemble simulations are driven by high-latitude ionospheric convection and auroral particle precipitation patterns specified by the AMGeO and by atmospheric waves and tides based on MERRA meteorological reanalysis. As part of forward modeling, the 3D Dynamo electrodynamic module is used to calculate magnetic perturbations on the ground and at low Earth orbit altitudes. A detailed analysis of the 21-day period from March 1 to 22, 2009 has shown that the modeled EEJ with the LBCs adjusted using ground-based magnetic perturbation data improves the agreement of the model to independent magnetic field observations from CHAMP. The use of routinely available ground-based magnetometer data to constrain the TIE-GCM LBCs could provide an opportunity to investigate how day-to-day tidal variability drives equatorial electrodynamics variability.

In this study, field-aligned currents (FACs) obtained from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) dataset have been used to specify high-latitude electric potential in the Global Ionosphere Thermosphere Model (GITM). The advantages and challenges of the FAC-driven simulation are investigated based on a series of numerical experiments and data-model comparisons for the 2013 St Patrick’s Day geomagnetic storm. It is found that the cross-track ion drift measured by the Defense Meteorological Satellite Program (DMSP) satellites can be well reproduced in the FAC-driven simulation when the electron precipitation pattern obtained from Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique is used in GITM. It is also found that properly including the neutral wind dynamo is very important when using FACs to derive the high-latitude electric field. Without the neutral wind dynamo, the cross-polar-cap potential and hemispheric integrated Joule heating could be underestimated by more than 20%. Moreover, the FAC-driven simulation is able to well reproduce the ionospheric response to the geomagnetic storm in the American sector. However, the FAC-driven simulation yields relatively larger data-model discrepancies compared to the AMIE-driven GITM simulation. This may result from inaccurate Joule heating estimations in the FAC-driven simulation caused by the inconsistency between the FAC and electron precipitation patterns. This study indicates that the FAC-driven technique could be a useful tool for studying the coupled ionosphere and thermosphere system provided that the FACs and electron precipitation patterns can be accurately specified.