A MULTISATELLITE STUDY OF A PSEUDO-SUBSTORM ONSET IN THE NEAR-EARTH MAGNETOTAIL

This paper reports the multisatellite and ground observations of two p seudo-substorm onset events that occurred successively at 0747 UT and 0811 UT, May 30, 1985, with more attention to the 0747 UT onset. The d istinguishing features of the 0747 UT event are as follows. (1) The su bstorm-associated tail reconfiguration started in a very localized reg ion in the near-Earth magnetotail. (2) The magnitude of the current di sruption decreased markedly as the disruption region expanded tailward . (3) On the ground the onset of a very small negative bay (approximat ely 40 nT) was observed simultaneously with the onset of the current d isruption, but over a much wider local time sector than the near-Earth tail reconfiguration. Positive bay onsets at mid-latitudes also had a longitudinally wide distribution. From these features we infer that i n the present event the current disruption took place filamentarily ne ar AMPTE/CCE at approximately 8.8 R(E). It is also inferred that pseud o-substorm onsets are distinguished from standard substorm onsets by t he absence of a global expansion of the current disruption, and that t he spatial scale of the onset region in the magnetosphere is not a maj or difference between the two. The present study suggests that the spa tial distribution of the magnetic distortion before onsets is an impor tant factor to determine the expansion sr-ale of the current disruptio n. It is also suggested that the current disruption is basically an in ternal process of the magnetosphere.