Dense and stagnant ions in the low‐latitude boundary region under northward interplanetary magnetic field

Abstract

The ion behavior in the low‐latitude boundary region is studied based on Geotail data accumulated over several years, toward understanding the formation mechanism of the cold‐dense plasma sheet under prolonged northward interplanetary magnetic field (IMF). A statistical survey shows that, during extended northward IMF, (1) dense magnetosheath‐like ions appear far more often, especially on the flanks, (2) the dense ions are mostly stagnant, in contrast to those in the classical low‐latitude boundary layer (LLBL), (3) a substantial fraction of the dense and stagnant ions is spatially mixed with hot magnetospheric ions, and (4) the mixed ion population has magnetic local time dependence in ion mixing state in energy space. Based on these findings, we argue that, under extended northward IMF, a significant transfer of the solar wind/LLBL ions onto the magnetospheric field lines occurs on the flanks, and the transport/heating process of the entrant ions is different for different local times.