Is there a near-Earth neutral line?

Abstract

High correlations between southward turnings of the IMF, increases of open flux in the tail lobes, and magnetic activity confirm dayside magnetic reconnection plays a fundamental role in magnetospheric substorms. The absence of a time delay between expansion phase onset and the decrease in tail lobe flux suggest that nightside reconnection is involved in the expansion. At 20–30 R e, the disappearance of the center of the plasma sheet and the appearance of tailward flows threaded with southward fields strongly suggest reconnection occurs earthward of this distance. Recent AMPTE/CCE and DMSP observations suggest that current disruption begins near synchronous orbit on closed field lines and expands tailward. Auroral images from the VIKING spacecraft support this. Reports of a lack of correlation between plasma and field changes at the ISEE spacecraft and onsets has led some researchers to reject the hypothesis that reconnection is the cause of the expansion. To counter these reports we present magnetic field and plasma data from substorms observed on an outbound pass by ISEE-2. We show that the tail field responds in the manner predicted by the near-earth neutral line (NENL) model, growing in strength and tilting earthward in the growth phase, and decreasing and tilting upward in the expansion phase. Near the earth the plasma sheet gradually thins in the growth phase while further away it suddenly thins at expansion onset. In the expansion phase the plasma sheet rapidly recovers close to the earth, but its recovery is delayed until the substorm recovery phase further from the earth. We explain many of the recent reports of disagreement between the NENL model and observations by a combination of factors. These include: Large changes in the location of a spacecraft relative to the neutral sheet as the size of the magnetosphere changes with dynamic pressure, or as the central meridian and latitude of substorm onset change due to inherent variability; Differences between the outbound and inbound portions of the ISEE orbit; Change in shape of successive orbits in GSM coordinates. Most important is multiple substorm onsets. Successive intensifications cause quite different effects at a spacecraft as the disturbances moves relative to the spacecraft. We conclude that rather than one x-line, there are probably multiple, localized pairs of x- and o-type lines formed in the near-earth plasma sheet during the substorm expansion.