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/N2data 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/N2enhancement for 12 h. This was followed by a considerable negative storm effect with 5–10 TECU and 20%–40% O/N2decrease. Results show that minor storm intervals can produce substantial mid‐latitude ionospheric and thermospheric dynamics in low solar flux conditions.
This study provides first storm time observations of the westward‐propagating medium‐scale traveling ionospheric disturbances (MSTIDs), particularly, associated with characteristic subauroral storm time features, storm‐enhanced density (SED), subauroral polarization stream (SAPS), and enhanced thermospheric westward winds over the continental US. In the four recent (2017–2019) geomagnetic storm cases examined in this study (i.e., 2018‐08‐25/26, 2017‐09‐07/08, 2017‐05‐27/28, and 2016‐02‐02/03 with minimum SYM‐H index −206, −146, −142, and −58 nT, respectively), MSTIDs were observed from dusk‐to‐midnight local times predominately during the intervals of interplanetary magnetic field (IMF) Bz stably southward. Multiple wavefronts of the TIDs were elongated NW‐SE, 2°–3° longitude apart, and southwestward propagated at a range of zonal phase speeds between 100 and 300 m/s. These TIDs initiated in the northeastern US and intensified or developed in the central US with either the coincident SED structure (especially the SED basis region) or concurrent small electron density patches adjacent to the SED. Observations also indicate coincident intense storm time electric fields associated with the magnetosphere–ionosphere–thermosphere coupling electrodynamics at subauroral latitudes (such as SAPS) as well as enhanced thermospheric westward winds. We speculate that these electric fields trigger plasma instability (with large growth rates) and MSTIDs. These electrified MSTIDs propagated westward along with the background westward ion flow which resulted from the disturbance westward wind dynamo and/or SAPS.more » « less
- NSF-PAR ID:
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- DOI PREFIX: 10.1029
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- Journal of Geophysical Research: Space Physics
- Medium: X
- Sponsoring Org:
- National Science Foundation
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