Thinning of the plasma sheet during substorms. Technical report

Concurrent measurements of plasma by the SCATHA satellite in near-synchronous orbit and by ISEE 2 in the near magnetotail are used to examine the phenomenon of plasma-sheet thinning associated with substorms. Results show that thinning occurs in two stages, thought to be attributable to two different processes. For 30-60 minutes before expansive phase offset, gradual thinning occurred over the region from geosynchronous orbit out to 20 RE in the tail. This was attributable to the tail-like configuration thought to lead to neutral line formation and the beginning of magnetic reconnection within the plasma sheet. At expansive phase onset, extreme thinning was seen and is thought to occur as a portion of the plasma sheet (a plasmoid), severed by magnetic connection at a near-earth neutral line, quickly departs tailward. Earthward of the neutral line, the plasma sheet expanded. Results generally support the neutral-line model of substorms.