Proton aurora in the cusp during southward IMF

Abstract

One of the most distinct aurorae in the high‐latitude dayside region occurs at the footprint of the cusp. The Spectrographic Imager SI‐12 channel on the IMAGE spacecraft allows for long‐duration observations of the proton aurora in this region and thus enables morphological and quantitative studies of the behavior of the proton aurora under different external solar wind conditions. Here we extend an earlier study of the proton aurora in the cusp during northward interplanetary magnetic field (IMF) conditions [Frey et al., 2002] to southward IMF conditions. We find a very similar dependence of the cusp aurora's location in magnetic local time (MLT) on the IMF By component, with locations at prenoon for negative By and postnoon for positive By. We also confirm the previously established dependence of the proton aurora brightness on the solar wind dynamic pressure. However, for similar solar wind pressures the proton aurora is brighter for the southward IMF case than for the northward IMF case. The other major difference between southward and northward IMF conditions is the very strong dependence of the latitude location on the magnitude of the IMF Bz for southward IMF. We discuss both differences in terms of the location of the reconnection site at the magnetopause that is connected to the ionosphere along cusp magnetic field lines.