Low-latitude boundary layer formation by magnetic reconnection

Abstract

Magnetic reconnection at the dayside magnetopause allows the direct transport of plasma into and out of the magnetosphere along the interconnected magnetic field, thereby forming boundary layers both inside and outside the magnetopause. The low-latitude portion of this boundary layer is referred to as the low-latitude boundary layer (LLBL) inside the magnetopause and as the magnetosheath boundary layer (MSBL) outside the magnetopause. As the intermixing plasma encounters the magnetopause current layer, it can be substantially modified through particle acceleration, heating, and partial reflection and transmission. As a result of these effects, the plasma in the LLBL is characterized by densities and temperatures intermediate between those in the neighboring magnetosheath and magnetosphere. The LLBL formation is quite different under southward and northward interplanetary magnetic field (IMF) conditions. When the IMF is southward, the LLBL forms directly at low latitudes where reconnection is occurring. During northward IMF, the LLBL forms through the remote occurrence of reconnection at high latitudes. In this paper, we present direct observations of the plasma heating and intermixing at high latitudes where reconnection is occurring under northward IMF. We demonstrate that the local entry and heating of the magnetosheath plasma across the high-shear, high-latitude magnetopause is sufficient to account for the observed LLBL properties at low latitudes.