Understanding the total electron content variability over Europe during 2009 and 2019 SSWs

The nature of the variability of the Total Electron Content (TEC) over Europe is investigated during 2009 and 2019 Northern Hemisphere (NH) SSW events in this study by using a combination of Global Navigation Satellite System (GNSS) based TEC observations and Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) simulations. To simulate the SSW effects in TIE-GCM, the dynamical fields from the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) simulations of 2009 and 2019 SSWs are specified at the TIE-GCM lower boundary. The observed and simulated TEC are in overall good agreement and therefore the simulations are used to understand the sources of mid-latitude TEC variability during both SSWs. Through comparison of TIE-GCM simulations with and without geomagnetic forcing, we find that the TEC variability during the 2019 SSW event, was predominantly geomagnetically forced, while for the 2009 SSW, the major variability in TEC was accounted for by the changes in vertically propagating migrating semidiurnal solar (SW2) and lunar (M2) tides. By comparing the TIE-GCM simulations with and without the SW2 and M2 tides, we find that these semidiurnal tides contribute to similar to 20%-25% increase in the quiet background TEC.