Magnetotail Convection in Geomagnetically Active Times 1. Distance to the Neutral Lines

Abstract

The distances to the magnetic neutral lines in the magnetotail are derived from the observations of the convection in geomagnetically active times (Kp larger than 3). Observations at seven orbit segments of GEOTAIL that are distributed fromx =-36 to-169Re, and longer than 24 hours are used. For each data set the ratio between the earthward-moving northward magnetic flux and the tailward-moving southward magnetic flux is derived. The ratio varies systematically with distance, and suggests that the distant neutral line tends to be located at about 140 Re while the near-Earth neutral line tends to be formed inside 50 Re. Mainly the open field lines are reconnected at the distant neutral line, while mainly the closed field lines are reconnected at the near-Earth neutral line. The northward magnetic flux transported earthward from the distant neutral line is several times larger than the net southward flux transported tailward from the near-Earth neutral line; the open magnetic flux that is reconnected at the distant neutral line is substantially greater than that reconnected at the near-Earth neutral line. Observations of the field-aligned component of the flow velocity suggests that sometimes these two neutral lines exist together and produce the quasi-stagnant plasmoid.