Boundary layer plasma flows from high-latitude reconnection in the summer hemisphere for northward IMF: THEMIS multi-point observations

Abstract

[1] On 2008-07-11, the THEMIS spacecraft, separated both longitudinally and radially, traversed the dayside low-latitude boundary layer (LLBL) under extended northward IMF. They detected southward flows of magnetosheath plasma from magnetopause reconnection poleward of the northern cusp, which were cold-dense, and had southward velocity ∼100 km/s and longitudinal extent >3 RE. These features all agree with a global MHD simulation of the magnetosphere for similar conditions, in which under large geomagnetic dipole tilt, an LLBL forms via poleward-of-the-cusp reconnection first in the summer hemisphere and later in the other. Contrary to the simulation, however, the observed LLBL was mostly magnetically closed, characterized by balanced field-aligned and anti-field-aligned electron fluxes, and was less thick (≤0.5 RE). The former suggests comparable reconnection rate in both hemispheres, while the latter suggests the actual reconnection rate being lower, and/or the plasma transport toward the magnetotail being faster, than in the simulation.