Search for the magnetic neutral line in the near-Earth plasma sheet, 1. Critical reexamination of earlier studies on magnetic field observations

Abstract

Magnetic field observations (Explorer 34 and 35) in the magnetotail, which have been presented previously by several workers as supporting evidence for the conjectured formation of a magnetic X type neutral line in the near-earth plasma sheet, have been critically reexamined with simultaneous energetic electron data. It is found that negative B/sub z/ in these events is observed either in the tail lobe or adjacent to the plasma sheet boundary, where the magnitude B/sub T/ of the total magnetic field is much greater than B/sub z/ and energetic electron fluxes are low or absent. Thus the substorm time variations of the B/sub z/ component reported by the earlier studies can be described as a slight southward dipping (approx.5/sup 0/ to 10/sup 0/) of the magnetic field vector. This change is partly the spatial variation associated with thinning of the plasma sheet, which shifts the satellite adjacent to the plasma sheet boundary, where the magnetic field has a slight flaring tendency. A large dip angle is sometimes observed near the time of the boundary crossing and thus is a very transient phenomenon, lasting for only a few minutes. Such observations are far from conclusive in establishing the large-scale change of themore » magnetic field in the plasma sheet as implied by the neutral line formation in the near-earth plasma sheet. An extensive examination of magnetotail data from Imp 6 and other satellites will be presented in this series of papers. (AIP)« less