Some low‐altitude cusp dependencies on the interplanetary magnetic field

Abstract

Although it has become well established that the low‐altitude polar cusp moves equatorward during intervals of southward interplanetary magnetic field (IMF B z <0), many other important aspects of the cusp's response to IMF components are not as well investigated. An algorithm for identifying the cusp proper was applied to 12,569 high‐latitude dayside passes of the DMSP F7 satellite (which is in a nearly circular polar orbit at ∼838 km altitude), and the resulting cusp positioning data were correlated with the IMF (IMF data were available for about 25% of the cases). It was found that the peak probability of observing the cusp shifts prenoon for B y negative (positive) in the northern (southern) hemisphere and postnoon for B y positive (negative) in the northern (southern) hemisphere. The B y induced shift is much more pronounced for southward than for northward B z , a result that appears to be consistent with elementary considerations from, for example, the antiparallel merging model. No interhemispherical latitudinal differences in cusp positions were found that could be attributed to the IMF B x component. As expected, the cusp latitudinal position correlated reasonably well (0.70) with B z when the IMF had a southward component; the previously much less investigated correlation for B z northward proved to be only 0.18, suggestive of a half‐wave rectifier effect. The ratio of cusp ion number flux precipitation for B z southward to that for B z northward was 1.75±0.12. The statistical local time (full) width of the cusp proper was found to be 2.1 hours for B z northward and 2.8 hours for B z southward.