Variations in Thermosphere Composition and Ionosphere Total Electron Content Under “Geomagnetically Quiet” Conditions at Solar-Minimum

We conducted observational and modeling studies of thermospheric composition and ionospheric total electron content (TEC) variations during two geomagnetically quiet periods (maximum Kp = 1.7) at solar minimum. Daytime thermospheric O and N₂ column density ratio (∑O/N₂) observed by Global-scale Observations of the Limb and Disk and TEC from a network of ground-based Global Navigation Satellites System receivers both exhibited large (∼30% of reference values) and long-lived (5–11 h) day-to-day variations in roughly the same mid-latitude geographic regions. Numerical simulations replicated the observed variability, though not perfectly. Analysis of the simulations suggested that the variations were mainly generated in the high-latitudes and were subsequently advected equatorward and westward. When high-latitudes input was turned off in simulations, the variations were negligible. This suggested the potentially important role of high-latitude geomagnetic forcing in thermospheric composition and ionospheric density variations at mid-latitudes even during some “geomagnetically quiet” periods at solar-minimum.