Polar Observations of Cusp Electrodynamics: Evolution from 2- to 4-Cell Convection Patterns

Abstract

The dayside magnetosphere quickly responds to changes in the polarities of IMF B _Y and/or B _Z . Within a few minutes of the changes reaching the magnetopause, characteristic optical [1] and plasma convection signatures [2] appear in the ionospheric projection of the cusp. Global ionospheric convection patterns at high geomagnetic latitudes, however, represent a mixture of IMF conditions over the previous half hour. Maezawa [3] first reported observing magnetic perturbations during sustained periods of northward IMF whose explanation required sunward convection in the central polar cap. Electric and magnetic fields measured by the S3–2 [4]. Atmospheric Explorer [5] and MAGSAT [6] satellites suggested that with IMF B _Z > 0 and B _Y ≈ 0, a four-cell convection patterns evolves. This convection pattern consists of two cells in the polar cap, whose polarity is opposite to the adjacent, standard negative potential (clockwise) afternoon and positive (counter-clockwise) morning cells. The polar cap convection cells are driven by magnetic merging at the poleward boundary of the cusp [7]. The residual, standard-polarity pair of cells at auroral latitudes are weak and probably are related to the low latitude boundary layer (LLBL) [8].