Evidence for a flux transfer event generated by multiple X-line reconnection at the magnetopause

Magnetic flux transfer events (FTEs) are signatures of unsteady magnetic reconnection, often observed at planetary magnetopauses. Their generation mechanism, a key ingredient determining how they regulate the transfer of solar wind energy into magnetospheres, is still largely unknown. We report THEMIS spacecraft observations on 2007-06-14 of an FTE generated by multiple X-line reconnection at the dayside magnetopause. The evidence consists of (1) two oppositely-directed ion jets converging toward the FTE that was slowly moving southward, (2) the cross-section of the FTE core being elongated along the magnetopause normal, probably squeezed by the oppositely-directed jets, and (3) bidirectional field-aligned fluxes of energetic electrons in the magnetosheath, indicating reconnection on both sides of the FTE. The observations agree well with a global magnetohydrodynamic model of the FTE generation under large geomagnetic dipole tilt, which implies the efficiency of magnetic flux transport into the magnetotail being lower for larger dipole tilt

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 similar to 100 km/s and longitudinal extent >3 R-E. 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 R-E). 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. Citation: Hasegawa, H., et al. (2009), Boundary layer plasma flows from high-latitude reconnection in the summer hemisphere for northward IMF: THEMIS multi-point observations, Geophys. Res. Lett., 36, L15107, doi: 10.1029/2009GL039410