The magnetic and plasma structure of flux transfer events

Abstract

Flux transfer events have been interpreted to be manifestations of transient reconnection of the magnetospheric and magnetosheath magnetic fields at the Earth's magnetopause. In this study we determine the interior structure of flux transfer events by examining high‐resolution magnetic field and plasma distribution functions from the ISEE 2 spacecraft. The sampling time and cadence of these data are more than adequate to resolve the rapidly changing plasma regimes and to avoid spatial aliasing. From these data we have confirmed the existence of two distinct regions within a flux transfer event (FTE), a central core and a field‐draping region, and these two regions have been found within FTEs observed both on the magnetosheath side and the magnetospheric side of the magnetopause. The boundaries between the two regions are apparent in both the field and the plasma data. In the field‐draping region the plasma distribution functions show little difference from those in the neighboring magnetosheath or magnetosphere, and the magnetic field consistently exhibits signatures expected from field‐line draping around a flux tube. Within the central core region, plasma appears to be a mixture of the magnetosheath and magnetospheric components. In FTEs observed in the magnetosphere, we find transmitted magnetosheath plasma and a strong depletion of hot magnetospheric plasma. In FTEs observed in the magnetosheath, we find an outflow of magnetospheric plasma mixed with the magnetosheath plasma. These signatures unambiguously show the reconnection of the interplanetary magnetic field and the Earth's magnetic field. Thus our observations indicate that the magnetic field lines within the central core region are open allowing the inflow of cold magnetosheath plasma and the outflow of hot magnetospheric plasma through the open flux tube. The reconnection picture is further supported by the observation of separate electron and ion edges at the trailing boundary of a northward moving flux tube, expected as time‐of‐flight effects on newly reconnected field lines. Thus our observations are consistent with the reconnected open flux tube interpretation for FTEs.