Traveling Ionospheric Disturbances in the Vicinity of Storm‐Enhanced Density at Midlatitudes.

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. Plain Language Summary: Traveling ionospheric disturbances (TIDs) are an important means of storm time global dispersion of the solar wind and magnetosphere energy and momentum deposited at high latitudes. While the meridional propagation of large‐scale TIDs is very efficient, zonal propagation is also very important. We provide the first observations of dramatic storm time zonal propagation of TIDs associated with strong subauroral dynamics and structures storm‐enhanced density, subauroral polarization stream, and enhanced thermospheric westward winds. These are documented in a collection of Global Navigation Satellite System (GNSS) total electron content observations in the continental US during several recent geospace storms between 2016 and 2018. Medium‐scale TIDs (MSTIDs) occurred predominately in the dusk‐evening sector, propagating westward at up to 200 m/s phase speed. Thermospheric westward winds showed simultaneous enhancements, reaching up to 200 m/s. We suggest several magnetosphere–ionosphere–thermosphere coupling electrodynamical processes at subauroral latitudes trigger plasma instability and MSTIDs, and poleward electric fields induced by the disturbance wind dynamo facilitate the westward propagation of these electrified MSTIDs. These observations provide new scenarios for subauroral dynamic processes at the mesoscale level. Key Points: Storm time medium‐scale traveling ionospheric disturbances occur regularly near the base of storm‐enhanced density in the continental USDisturbance wavefronts developed primarily during storm main phase at evening–midnight local times, elongated NW‐SE, and traveled southwestwardDisturbance onset coincided with intense electric fields and zonal propagation was synchronized with enhanced westward thermospheric winds [ABSTRACT FROM AUTHOR]

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