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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 (SO/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. Plain Language Summary This study presents two cases when geomagnetic forcing can be a plausible source of mid-latitude thermospheric composition and ionosphere density variations even during what is typically considered as geomagnetically quiet times (magnetic activity index Kp < 2). The column density ratio of thermospheric O and N2 (SO/N2) plays a major role in the daytime ionospheric F-region plasma density at mid-latitudes. In this study, thermospheric and ionospheric variations during geomagnetically quiet times are investigated with the two-dimensional images of SO/N2 provided by a satellite located in geostationary orbit and the ground-based total electron content (TEC) maps. Both SO/N2 and TEC displayed similar strong, long-lived and localized depletions and enhancements at mid-latitudes. Numerical simulations driven by an empirical model of geomagnetic activity, but with a climatological tide in the lower boundary, qualitatively produced the patterns of observed variations. Analysis of simulations revealed that SO/N2 variations were initially formed at high-latitudes and then transported equatorward and westward. When geomagnetic forcing was turned off in simulations, the modeled SO/N2 and TEC variations were negligible. This study suggests the potentially important roles of high-latitude forcing in thermosphere and ionosphere variations at mid-latitudes even during some "geomagnetically quiet" periods at solar-minimum. [ABSTRACT FROM AUTHOR]

To investigate gravity wave (GW) perturbations in the midlatitude mesopause region during boreal equinox, 433 h of continuous Na lidar full diurnal cycle temperature measurements in September between 2011 and 2015 are utilized to derive the monthly profiles of GW-induced temperature variance, T′², and the potential energy density (PED). Operating at Utah State University (42° N, 112° W), these lidar measurements reveal severe GW dissipation near 90 km, where both parameters drop to their minima (~20 K² and ~50m² s-2, respectively). The study also shows that GWs with periods of 3-5 h dominate the midlatitude mesopause region during the summer-winter transition. To derive the precise temperature perturbations a new tide removal algorithm suitable for all ground-based observations is developed to de-trend the lidar temperature measurements and to isolate GW-induced perturbations. It removes the tidal perturbations completely and provides the most accurate GW perturbations for the ground-based observations. This algorithm is validated by comparing the true GW perturbations in the latest mesoscale-resolving Whole Atmosphere Community Climate Model (WACCM) with those derived from the WACCM local outputs by applying this newly developed tidal removal algorithm. [ABSTRACT FROM AUTHOR]