Double cusp: Model prediction and observational verification

Recent modeling of the entry of solar wind plasma into the magnetosphere and ionosphere has adequately simulated the large-scale particle precipitation features in the observed cusp, mantle, polar rain, and open-field line low-latitude boundary layer regions. The assumption of a simple dawn-dusk electric field limited the models to the near-noon region and southward interplanetary magnetic field (IMF) case. Here, we present an improved model that incorporates the electric field obtained from statistical convection patterns. When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of a double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower-latitude cusp ions originate from low-latitude magnetosheath, whereas the higher-latitude ions originate from the high-latitude magnetosheath. The lower-latitude cusp is located in the region of weak azimuthal E x B drift, resulting in a dispersionless cusp, as would be observed from a typical meridional trajectory of a polar-orbiting satellite. The higher-latitude cusp is located in the region of strong azimuthal and poleward E x B drift. Because of a significant poleward drift, the higher-latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. This prediction was subsequently confirmed in a large case study with Defense Meteorological Satellite Program (DMSP) data. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF B, and small negative IMF B-z, 30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double-cusp result is consistent with the following new statistical results: (1) the cusp latitudinal width increases with \ IMF B-y\ and (2) the cusp equatorward boundary moves to lower latitude with increasing \ IMF B-y\.