Statistical relationships between enhanced polar cap flows and PBIs

Abstract

Poleward boundary intensifications (PBIs) are auroral intensifications along the poleward boundary of the auroral oval and occur during all levels of geomagnetic activity. However, little is known about the triggering of PBIs. Recent case studies have indicated the existence of longitudinally localized flow channels in the polar cap near, and directed toward, the nightside open-closed field line boundary just before PBIs. Motivated by these studies, we analyze 115 events of coordinated observations by the Time History of Events and Macroscale Interactions during Substorms all-sky imager and Super Dual Auroral Radar Network HF radar at Rankin Inlet to determine if this polar cap flow-PBI relationship is commonly observed. We start with isolated and intense PBIs and examine the probability of them being associated with equatorward directed polar cap flows. Our results show the association to be frequent (90%), with one-to-one correlations occurring in ~50% of events. Considering the limitations of the radar observations, this result indicates that PBIs are commonly correlated with polar cap flow channels directed toward, and then traversing, the open-closed field line boundary. The flows statistically occur ~1-2 min before the PBI initiations, and the duration and width of the flows are comparable to those of the PBIs. We also perform a reverse study by starting with isolated polar cap flows and obtain similar results. The remarkably high occurrence of association between enhanced polar cap flows and PBIs indicates that enhanced mesoscale flows within the open field line region that traverse the open-closed field line boundary are an important driver of PBI formation. Key Points Fast, equatorward directed, and narrow polar cap flow channels are observed The occurrence of polar cap flows is highly associated with PBIs Time lag, duration, and width of polar cap flows and PBIs suggest association ©2013. American Geophysical Union. All Rights Reserved.