Salient Midlatitude Ionosphere-Thermosphere Disturbances Associated With SAPS During a Minor but Geo-Effective Storm at Deep Solar Minimum

This work conducts a focused study of subauroral ion-neutral coupling processes and midlatitude ionospheric/thermospheric responses in North America during a minor but quite geo-effective storm on September 27-28, 2019 under deep solar minimum conditions. Several prominent storm-time disturbances and associated electrodynamics/dynamics were identified and comprehensively analyzed using Millstone Hill and Poker Flat incoherent scatter radar measurements, Fabry-Perot interferometer data, total electron content data from Global Navigation Satellite System observations, and thermospheric composition O/N-2 data from the Global-scale Observations of Limb and Disk mission. Despite solar minimum conditions, this minor storm produced several prominent dynamic features, in particular (a) Intense subauroral polarization stream (SAPS) of 1,000 m/s, overlapping with a deepened main trough structure. (b) An enhanced westward wind of 230 m/s and a significant poleward wind surge of 85 m/s occurred in the post-SAPS period. (c) Large-scale traveling ionospheric disturbances (TIDs) were generated and propagated equatorward across mid-latitudes in the storm main phase. TID characteristics were significantly affected by SAPS, evolving into divergent propagation patterns. (d) SAPS was situated on the poleward edge of a considerable storm-enhanced density structure. (e) The midlatitude ionosphere and thermosphere exhibited a prolonged positive storm effect in the main phase and beginning of recovery phase, with 5-10 TECU increase and 10%-30% O/N-2 enhancement for 12 h. This was followed by a considerable negative storm effect with 5-10 TECU and 20%-40% O/N-2 decrease. Results show that minor storm intervals can produce substantial mid-latitude ionospheric and thermospheric dynamics in low solar flux conditions.