Solar eruptions cause geomagnetic storms in the near‐Earth environment, creating spectacular aurorae visible to the human eye and invisible dynamic changes permeating all of geospace. Just equatorward of the aurora, radars and satellites often observe intense westward plasma flows called subauroral polarization streams (SAPS) in the dusk‐to‐midnight ionosphere. SAPS occur across a narrow latitudinal range and lead to intense frictional heating of the ionospheric plasma and atmospheric neutral gas. SAPS also generate small‐scale plasma waves and density irregularities that interfere with radio communications. As opposed to the commonly observed duskside SAPS, intense eastward subauroral plasma flows in the morning sector were recently discovered to have occurred during a super storm on 20 November 2003. However, the origin of these flows termed “dawnside SAPS” could not be explained by the same mechanism that causes SAPS on the duskside and has remained a mystery. Through real‐event global geospace simulations, here we demonstrate that dawnside SAPS can only occur during major storm conditions. During these times, the magnetospheric plasma convection is so strong as to effectively transport ions to the dawnside, whereas they are typically deflected to the dusk by the energy‐dependent drifts. Ring current pressure then builds up on the dawnside and drives field‐aligned currents that connect to the subauroral ionosphere, where eastward SAPS are generated. The origin of dawnside SAPS explicated in this study advances our understanding of how the geospace system responds to strongly disturbed solar wind driving conditions that can have severe detrimental impacts on human society and infrastructure.
We analyze horizontal plasma drifts measured by the Defense Meteorological Satellite Program satellites during two intense magnetic storms. It is found, for the first time, that westward plasma flows associated with subauroral polarization streams (SAPS) in the dusk‐evening sector penetrate continuously to equatorial latitudes. The westward ion drifts between subauroral and equatorial latitudes occur nearly simultaneously. The latitudinal profile of the westward ion drifts at low latitudes (approximately within ±30° magnetic latitude [MLat]) is relatively flat, and the westward ion drifts at the magnetic equator reach 200–300 m s−1. In the dawn‐morning sector, eastward ion drifts at subauroral latitudes are also SAPS. The storm‐time dawnside auroral boundary moves to ∼±55° MLat, and the dawnside SAPS penetrate to ∼±20° MLat at 0930 local time. A dawnside SAPS flow channel appears to exist, although it is not as well defined as the duskside SAPS flow channel. Thermospheric wind data measured by the Challenging Minisatellite Payload satellite are analyzed, and zonal disturbance winds are derived. Disturbance winds can reach equatorial latitudes rapidly near midnight but are limited to ±40° geographic latitude or higher near noon. The effects of disturbance winds on the zonal ion drifts at middle and low latitudes are discussed. It is suggested that both the westward ion drifts at middle and low latitudes in the dusk‐evening sector and the eastward ion drifts at middle and lower latitudes in the dawn‐morning sector are caused primarily by penetration of the SAPS and auroral electric fields.
more » « less- NSF-PAR ID:
- 10375077
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 126
- Issue:
- 12
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We identified a few new storm‐time ionospheric phenomena by analyzing disturbances in topside ion density, electron temperature, and ion temperature at ∼840 km altitude measured by the
Defense Meteorological Satellite Program satellites during the 20 November 2003 magnetic storm. The storm‐time ion density enhancements showed different features at different local times. Longitudinal structures in the enhanced ion density occurred in the morning sector and extended from equatorial regions to middle latitudes. Ion density increase due to enhanced fountain effect was observed in the evening sector and lasted for ∼18 hr. A positive ionospheric storm occurred during the late recovery phase of the storm and was associated with increased atomic oxygen to molecular nitrogen column density ratio. Electron temperature at subauroral latitudes reached 8000 K during the storm, ∼4000 K higher than the quiet‐time temperature. The subauroral temperature enhancement lasted for 2–3 days. Simultaneous enhancements in the ion density, electron temperature, and ion temperature from subauroral to equatorial latitudes occurred in the night‐time ionosphere and lasted for ∼18 hr. A negative correlation between ion density and electron/ion temperature variations occurred in the dusk sector for ∼12 hr. An enhanced ion temperature crest in the winter hemisphere during the magnetic storm lasted for 2 days. A decrease in the ion temperature crest was also observed with an increase of the ion density. These new features in the ionospheric density and temperature, together with the results from previous studies, provide a more comprehensive scenario of the ionospheric response to the superstorm. -
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