High-altitude cusp flow dependence on IMF orientation: A 3-year Cluster statistical study

摘要

We report on the statistical properties of the plasma flows measured by the Cluster spacecraft in the high-altitude cusp region of the Northern Hemisphere as a function of the interplanetary magnetic field (IMF) orientation, with selected clock angle intervals. The technique uses a magnetic field model, taking into account the actual solar wind conditions and level of geomagnetic activity, in order to model the magnetopause and cusp displacements as a function of these conditions. The distributions of the magnetic field vector show a clear consistency with the IMF clock angle intervals chosen and demonstrate that the technique used here fixes the positions of the cusp boundaries adequately. The antisunward convection observed in the exterior cusp suggests that this region is statistically quite convective under southward IMF, while for northward IMF the region appears more stagnant. The presence of large parallel (downward) flows at the equatorward edge of the cusp shows that plasma penetration occurs preferentially at the dayside low-latitude magnetopause for southward IMF conditions; in contrast, under northward IMF the results are suggestive of plasma penetration from the poleward edge of the cusp, combined with a substantial sunward convection, but no flows are observed at all at the dayside boundary with the plasma sheet. The comparison of the measured flow speed with the Alfvén speed suggests that the magnetosheath adjacent to the external boundary is more sub-Alfvénic, even for high magnetic latitudes, under northward IMF than under southward IMF. This result is consistent with the preference for the plasma depletion layer to develop under such conditions. The transverse plasma convection in the exterior cusp appears to be controlled by the IMF B Y component as well; for dawnward (duskward) IMF orientations the convection is preferentially directed toward dusk (dawn). These results are interpreted as strong arguments in favor of the cusp being structured, at large scales, by the occurrence of magnetic reconnection at the high-latitude magnetopause for northward IMF and at the low-latitude magnetopause for southward IMF.