Evidence for magnetic field reconnection at the Earth's magnetopause

Abstract

Eleven passes of the ISEE satellites through the frontside terrestrial magnetopause (local time 0900–1700; GSM latitude 2–43°N) have been identified, where the plasma velocity in the magnetopause and boundary layer was substantially larger than it was in the magnetosheath. This paper examines the nature of the plasma flow, magnetic field, and energetic particle fluxes in these regions, with a view to determining whether the velocity enhancements can be explained by magnetic‐field reconnection. The principal question is whether the observed difference in tangential plasma velocity Δv between a point in the magnetopause or boundary layer and a reference point in the adjacent magnetosheath, had the direction and magnitude Δv theory produced by the Maxwell stresses in the magnetopause, assuming that the magnetosheath plasma moved across that boundary. Except for its sign, Δv theory is shown to be independent of the normal magnetic field component B n and flow component υ n . For the 11 cases, the average ratio |Δv|/|Δv| theory was in the range 0.6–1.2, with a composite average of 0.8. The average angular error was <25°, with a composite average of 10°. The plasma results would require 10 of the crossings to have been located north of the reconnection line ( B n < 0), and one (at 2.4° N latitude) south of it ( B n > 0). The B n values obtained from minimum variance analysis of the magnetic data were mostly poorly determined, but in general their signs were consistent with the plasma results. The flow velocity across the magnetopause was also poorly determined but it had a negative (inward) composite average as expected. In several cases energetic magnetospheric particles with the proper flow anisotropy and, in one case, reflected magnetosheath particles, were observed outside but adjacent to the magnetopause. All of these results support the reconnection hypothesis. The energetic particles were also used to identify the outer separatrix surface. In one case it was possible to conclude from its location relative to the magnetopause that the reconnection site was in the vicinity of the equatorial plane and not in the cusp. The electric field tangential to the magnetopause is inferred to be in the range 0.4–2.8 mV/m.