Sunward polar cap convection

Abstract

This is a study of the conditions for which there is sunward convection in the central polar cap. The study of convection direction uses digital ionosonde measurements from two central polar cap stations (Resolute Bay, corrected geomagnetic (CG) latitude 83.3°N, and Eureka, CG latitude 88.67°N) and presents statistical results about pure sunward convection events. The sunward convection events in the central cap region are observed only when the interplanetary magnetic field (IMF) Bz is >2 nT. For weak northward IMF (<3 nT), sunward convection events are confined to the region near the noon cusp. For strong northward IMF conditions (>3 nT), sunward convection occurs at all local times, indicating the expansion of reverse convection to most of the central polar cap. There are very notable IMF By effects on the pattern of sunward convection. For IMF By positive the axis of the pattern of reverse convection appears to rotate toward the postnoon, whereas for IMF By negative the axis is rotated toward the prenoon. These axis orientations are compatible with the expected locations of lobe cell merging. Another significant IMF By effect is that the duration of sunward convection intervals when IMF By is negative is about the same as the duration of the northward IMF intervals, whereas for positive IMF By the sunward convection intervals usually last only for a short duration (∼1 hour). However, if the northward IMF interval is much longer than 1 hour, there is a sequence of these ∼l‐hour sunward convection events. The convection pattern during northward IMF appears to be compatible with a four‐cell form.