Transpolar arcs - Earth's high latitude auroras

Astronomy Unit Seminars
Dr Robert Fear (U. Southampton)
David Burgess
October 3rd, 2014 at 14:30
GO Jones Room 410

The Earth's auroras typically form in an oval configuration around the magnetic poles, encircling a dim region called the polar cap. The most vivid auroral displays, and many of the dynamical processes in the Earth's magnetosphere, occur when the interplanetary magnetic field (IMF) is southward. However, the behaviour of the magnetosphere and aurora when the IMF is northward is far more poorly understood. One particularly puzzling phenomenon is the transpolar arc - an auroral feature which extends into the polar cap from the night side of the main auroral oval. A multitude of possible mechanisms has been proposed to explain the formation of transpolar arcs, and these mechanisms differ in the predictions that they make. In this seminar we review the mechanisms which have been suggested, and discuss two recent statistical studies which demonstrate that transpolar arcs are formed by the occurrence of magnetic reconnection in the magnetotail which closes magnetic flux in the nightside magnetosphere. Unlike the standard Dungey Cycle paradigm, this newly-closed flux is unable to convect back to the day side, and so it builds up in the magnetotail and the precipitation on these field lines gives rise to the transpolar arc. This conclusion is confirmed by a case study conjunction between a transpolar arc and the Cluster spacecraft which observe the source plasma for the transpolar arc at very high altitudes.