AMPTE CCE-SCATHA SIMULTANEOUS OBSERVATIONS OF SUBSTORM-ASSOCIATED MAGNETIC FLUCTUATIONS/

This study examines substorm-associated magnetic field fluctuations ob served by the AMPTE/CCE and SCATHA satellites in the near-Earth tail. Three tail reconfiguration events are selected, one event on August 28 , 1986, and two consecutive events on August 30, 1986. The fractal ana lysis was applied to magnetic field measurements of each satellite. Th e result indicates that (1) the-amplitude of the fluctuation of the no rth-south magnetic component is larger, though not overwhelmingly, tha n the amplitudes of the other two components ana (2) the magnetic fluc tuations do have a characteristic timescale, which is several times th e proton gyroperiod. In the examined events the satellite separation w as less than 10 times the proton gyroradius. Nevertheless, the compari son between the AMPTE/CCE and SCATHA observations indicates that (3) t here was a noticeable time delay between the onsets of the magnetic fl uctuations' at the two satellite positions, which is too long to ascri be to the propagation of a fast magnetosonic wave, End (4) the coheren ce of the magnetic fluctuations was low in the August 28, 1986, event and the fluctuations had different characteristic timescales in the fi rst event of August 30, 1986, whereas some similarities can be found f or the second event of August 30, 1986. Result 1 indicates that pertur bation electric currents associated with the magnetic fluctuations ten d to flow parallel to the tail current sheet and are presumably relate d to the reduction of the tail current intensity. Results 2 and 3 sugg est that the excitation of the magnetic fluctuations;and therefore the trigger of the tail current disruption is a kinetic process in which ions play an important role. It is inferred from results 3 and 4 that the characteristic spatial scale of the associated instability is of t he order of the proton gyroradius or even shorter, and therefore the t ail current disruption is described as a system of chaotic filamentary electric currents. However, result 4 suggests that the nature of the tail current disruption can vary from event to event.