Cusp as a source for high-latitude boundary layer energetic ions

Abstract

The high-altitude dayside cusp is an extremely dynamic region in geospace. Compared to the equatorial magnetosheath, orders of magnitudes enhancements of energetic ions (from 40 keV to MeV) of both the solar wind and the ionospheric origins have been observed in the high-altitude dayside cusp. These measured cusp energetic ions were associated with a dramatic decrease and large fluctuations in the local magnetic field strength. One important question is where are these ions going? Previous studies suggested that the cusp plasma may enter into the magnetopause boundary layer and that no obvious ion heating was observed inside the high-latitude boundary layer (HLBL). During high solar wind pressure periods when the magnetosphere was compressed, the POLAR spacecraft might travel through the magnetopause boundary layer and the sheath-like region. Recent POLAR observations indicated that the energetic ions in the dayside magnetopause/magnetosheath boundary layers were a mixture of ions of solar wind and ionospheric origins, similar as in the cusp. Further comparisons of the ion energy spectra, the ion charge state composition, and the ion anisotropies suggest a likely source for the HLBL energetic ions exists in the cusp region.