Seasonal and interplanetary magnetic field-dependent polar cap contraction during substorm expansion phase

Abstract

Recent observations from simultaneous imaging in two hemispheres have shown that the polar caps can attain considerably different shapes as the auroral ovals contract during the substorm expansion phase. We use images from 2770 substorms to study the evolution of the polar cap boundary location statistically. We show that, during the first 26 min after substorm expansion phase onset, the polar cap boundary location depends on seasons, interplanetary magnetic field (IMF) By, and IMF Bx. For different signs of By, with By > 3 nT, the asymmetry in polar cap boundary observed at onset increases during the expansion phase, consistent with an increase in tail reconnection of field lines with asymmetrical footpoints. When Bx > 2 nT and By < 2 nT, the polar cap boundary dawnward of the onset propagates slightly farther poleward compared to negative Bx conditions. In the sunlit hemisphere, the polar cap boundary evolves from a pronounced equatorward displacement at onset to an almost reversed displacement during the expansion phase, compared to substorms observed in darkness. Substorms in the dark hemisphere also have a much more pronounced bulge than substorms in the sunlit hemisphere. If the interpretation of the poleward auroral boundary as being coincident with the open/closed field line boundary (OCB) is correct, the seasonal differences in OCB locations imply seasonal differences in the ionospheric convection during the substorm expansion phase. © Copyright 2010 by the American Geophysical Union.