MAGNETIC FLUCTUATIONS ASSOCIATED WITH TAIL CURRENT DISRUPTION - FRACTAL ANALYSIS

The objective of the present study is to assess the mechanism of subst orm-associated tail current disruption on the basis of magnetic field observations in the near-Earth tail. We examined 15 events observed by the Charge Composition Explorer (CCE) of the Active Magnetospheric Pa rticle Tracer Explorers (AMPTE), with an emphasis on the August 28, 19 86, event. In these events the satellite observed magnetic fluctuation s to start almost simultaneously with ground substorm onsets, strongly suggesting that these fluctuations are related to the trigger of subs torms. In this study we applied the new method, fractal analysis, to t hese fluctuations. This method enables us to examine fluctuations quan titatively and to pick up characteristic timescale(s) of fluctuations, even if fluctuations are far from sinusoidal. The results are summari zed as follows: (1) Whereas before the onset of tail current disruptio n, magnetic fluctuations are suppressed in each of the magnetic compon ents, after the onset, the magnitude of the H (north-south) component fluctuations is about 30% larger than the magnitudes of the fluctuatio ns of the other components. (2) The magnetic fluctuations have a chara cteristic timescale, which is several times the proton gyroperiod. The first result suggests that observed magnetic fluctuations are actuall y related to changes in the tail current intensity, that is, tail curr ent disruption. This result also indicates that the microprocess of ta il current disruption should be described in terms of turbulent pertur bation electric currents, although away from the onset region the effe cts of tail current disruption may be approximated by those of an orde rly decrease in the tail current intensity. The second result strongly suggests that tail current disruption is driven by a certain instabil ity, which grows most rapidly around that characteristic time scale, a nd in which ions should play an important role.