A two-satellite study of nightside flux transfer events in the plasma sheet

Abstract

High time resolution measurements of magnetic and electric fields and of the kiloelectronvolt electron flux at X ∼ −21 R e in the central magnetotail were used in the study of short-duration (time scale 1 min) magnetic field events during two consecutive substorms on 23 March 1979. These events were observed at ISEE-1 and 2 (having 0.3 R e separation along the Z-coordinate), both inside and outside of the thinned plasma sheet, close (in time) to the polarity reversal of the plasma flow from tailward to earthward direction in association with the transient plasma sheet expansions. The enhanced magnetic flux closure (southward B z during tailward streaming events, northward during earthward streaming events), plus enhanced plasma flow during these events, suggest that they are nightside magnetic flux transfer events (NFTEs). The typical behaviour of the magnetic field during a NFTE (compression, rotation then field decrease), the signatures of overpressure in such structures and their close correlation with ground-based observed auroral activations and high energy particle bursts signify that NFTE structures may result from the impulsive reconnection in the plasmasheet. Systematic differences in the magnetic variations found at ISEE-1 and 2 indicate current concentration at the outer plasmasheet boundary during the passage of a NFTE. Two NFTEs appeared to have structures similar to magnetic flux ropes. The NFTE structure and associated current system, as well as the interpretation of NFTE signatures in terms of the impulsive reconnection, are discussed. During the “poleward leap” stage of one substorm, the spacecraft within the central part of the plasma sheet observed the intense earthward magnetic flux transport associated with superimposed NFTE structures. The estimated amount of this flux transfer explained well the observed amplitude of the poleward expansion of the westward electrojet in the conjugate auroral zone.