The polar cusp Original Research Article

The polar cusp is a region in which the magnetosheath plasma has direct access to the ionosphere. It exists whether the interplanetary magnetic field is northward or southward. In a non-reconnecting magnetosphere the location of the cusp depends on the shape of the magnetopause but when the magnetosphere reconnects with either southward or northward interplanetary magnetic field the location of the cusp is altered. Since the polar cusp was discovered simultaneously at both low and high altitudes in 1971, its exploration has been mainly carried out at low altitudes. The launch of the POLAR spacecraft has allowed similar exploration of the high altitude polar cusp in the northern hemisphere. Where the high altitude properties of the cusp have direct counterparts at low altitudes, we find consistency between the measurements in the two regions. The invariant latitude of the cusp depends on the tilt angle of the cusp producing a significant north-south asymmetry at the solstices. The cusp moves equatorward when the IMF turns southward but also moves somewhat equatorward for increasingly northward IMF. As the IMF By component becomes more negative, the cusp moves to earlier local times in the northern hemisphere. This is consistent with a motion of the reconnection site away from the noon meridian when the IMF is not due southward. When it is positive, the northern cusp moves to later local times. When the solar wind dynamic pressure increases the polar cusp becomes wider in both local time and latitude. Surprisingly simple models such as the vacuum model of Tsyganenko predict the cusp location better than the more recent Tsyganenko 96 empirical model. MHD models appear to be quite successful in predicting the location of the cusp, its magnetic configuration and plasma properties.