Evolution of auroral asymmetries in the conjugate hemispheres during two substorms

Abstract

Five hours of simultaneous global imaging data from the conjugate hemispheres are used to examine the dynamics of the auroral substorm. Earlier studies have demonstrated that substorm onset locations in the two hemispheres are systematically displaced due to the orientation of the interplanetary magnetic field. In this paper we present, for the first time, how the asymmetric auroras induced by the IMF orientation at substorm onset disappears during the expansion phase. We suggest that this is the large scale manifestation of auroral arcs as being the sites of magnetic stress release. Magnetic stress on field lines with asymmetric footpoints can lead to a net hemispherical difference in parallel electric field strength which implies that the auroras move with different speeds in the two hemispheres to release the magnetic stress. The relative velocity can be derived from the potential between the hemispheres. During expansion phase the twisted magnetic fields are rectified, bringing the closed magnetic field lines back to the configuration defined by the Earth's interior. Copyright © 2011 by the American Geophysical Union.