Abstract
We investigate ionospheric flow patterns from 28th January 2002 associated with the development of the nightside distorted end of a “J”-shaped Transpolar Arc (nightside distorted TPA). Based on the nightside ionospheric flows near to the TPA, detected by the SuperDARN radars, we discuss how the distortion of the nightside end toward the pre-midnight sector is produced. The “J”-shaped TPA was seen under southward Interplanetary Magnetic Field (IMF) conditions, in the presence of a dominant dawnward IMF-By component. At the onset time of the nightside distorted TPA, particular equatorward plasma flows at the TPA growth point were observed in the post-midnight sector, flowing out of the polar cap and then turning toward the pre-midnight sector of the main auroral oval along the distorted nightside part of the TPA. We suggest that these plasma flows play a key role in causing the nightside distortion of the TPA. SuperDARN also found ionospheric flows typically associated with “Tail Reconnection during IMF Northward Non-substorm Intervals” (TRINNIs) on the nightside main auroral oval before and during the TPA interval, indicating that nightside magnetic reconnection is an integral process to the formation of the nightside distorted TPA. During the TPA growth, SuperDARN also detected anti-sunward flows across the open-closed field line boundary on the dayside that indicate the occurrence of low-latitude dayside reconnection and ongoing Dungey cycle driving. This suggests that nightside distorted TPA can grow even in Dungey-cycle-driven plasma flow patterns.
Highlights
Transpolar arcs (TPA) are the bar-shaped part of “theta aurora”, connecting the nightside and dayside auroral ovals within 25 the polar cap (Frank et al 1982)
Since theta auroras were discovered in the beginning of 1980’s, TPAs have been the focus of much research, and various formation mechanisms have been proposed based on investigations of the ionospheric flow patterns and the relationship to the orientation of Interplanetary Magnetic Field (IMF)
A migration of the equatorward plasma flows, which rotated to align with the main auroral oval, at the point where the TPA starts to protrude into the polar cap toward the dayside, plays a significant role in the formation of the distorted nightside end of the TPA
Summary
Transpolar arcs (TPA) are the bar-shaped part of “theta aurora”, connecting the nightside and dayside auroral ovals within 25 the polar cap (Frank et al 1982). The time of this image corresponds to the gold solid vertical line on the solar wind data in panel (a).
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