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.
In this study we have used 7 years (2011–2017) of quiet (Kp ≤ 2+) to moderately disturbed (Kp = 3) time nightside line‐of‐sight measurements from six midlatitude Super Dual Auroral Radar Network radars in the U.S. continent to characterize the subauroral convection in terms of magnetic latitude, magnetic local time, month, season, Kp, and the interplanetary magnetic field (IMF) clock angle. Our results show that (1) the quiet time (Kp ≤ 2+) subauroral flows are predominantly westward (20–90 m/s) in all months and become meridional (−20–20 m/s) near dawn and dusk, with the flows being the strongest and most structured in December and January. (2) The Kp dependency is prominent in all seasons such that for higher Kp the premidnight westward flow intensifies and the postmidnight eastward flow starts to emerge. (3) Sorting by IMF clock angle shows Bz+/Bz− features consistent with lower/higher Kp conditions, as expected, but also shows distinct differences that are associated with By sign. (4) There is a pronounced latitudinal variation in the zonal flow speed between 18 and 2 magnetic local time in winter (November to February) that exists under all IMF conditions but is most pronounced under IMF Bz− and higher Kp. Our analysis suggests that the quiet time subauroral flows are due to the combined effects of solar wind/magnetosphere coupling leading to penetration electric field and the neutral wind dynamo with the ionospheric conductivity modulating their relative dominance.
more » « less- Award ID(s):
- 1822056
- NSF-PAR ID:
- 10456661
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 124
- Issue:
- 6
- ISSN:
- 2169-9380
- Page Range / eLocation ID:
- p. 4608-4626
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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