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

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.