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Title: The IMF dependence of the local time of transpolar arcs: Implications for formation mechanism
Authors: Fear, R. C.
Milan, S. E.
First Published: 13-Mar-2012
Publisher: American Geophysical Union (AGU); Wiley
Citation: Journal of Geophysical Research-SPACE PHYSICS, 2012, 117
Abstract: [1] Transpolar arcs are auroral features that extend from the nightside auroral oval into the polar cap. It is well established that they occur predominantly when the interplanetary magnetic field (IMF) has a northward component (Bz > 0). Results concerning how the magnetic local time at which transpolar arcs form might depend upon the IMF dawn-dusk component (BY) are more mixed. Some studies have found a correlation between these two variables, with Northern Hemisphere arcs forming predominantly premidnight when BY > 0 and postmidnight when BY < 0 and vice versa in the Southern Hemisphere. However, a more recent statistical study found that there was no significant correlation, and other studies find that the formation of moving arcs is triggered by a change in the sign of the IMF BY component. In this paper, we investigate the relationship between the magnetic local time at which transpolar arcs form and the IMF BY component. It is found that there is indeed a correlation between the magnetic local time at which transpolar arcs form and the IMF BY component, which acts in opposite senses in the Northern and Southern hemispheres. However, this correlation is weak if the IMF is only averaged over the hour before the first emergence of the arc and becomes stronger if the IMF is averaged 3–4 h beforehand. This is consistent with a mechanism where the magnetic local time at which the arc first forms depends on the BY component in the magnetotail adjacent to the plasma sheet, which is determined by the IMF BY component during intervals of dayside reconnection in the hours preceding the first emergence of the arc. We do not find evidence for the triggering of arcs by an IMF BY sign change.
DOI Link: 10.1029/2011JA017209
ISSN: 0148-0227
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Copyright © 2012 by the American Geophysical Union. All rights reserved. Archived with reference to Usage Permissions granted to authors, available at
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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