Energetic particle bursts and substorm activity observed by GEOTAIL at X = −32 RE

Abstract

On February 9, 1995, the EPIC instrument on board the Japanese spacecraft GEOTAIL observed energetic electron and ion bursts in the near-magnetotail (X = −32 RE). The observations occurred during a period where the BZ component of IMF was turned southward 90 minutes prior to the bursts (WIND data located at X = 193 RE). The two bursts, were observed shortly after 0430 UT. and were separated by a short time interval (∼10 minutes).The energetic bursts were followed by intense substorm activity, which was recorded by GEOTAIL, as well as by ground magnetometers and geosynchronous satellites. The first interval of substorm activity started almost immediately with the first energetic electron burst and the second some 40 minutes after the second burst. During the first substorm GEOTAIL observed a strong tailward flow of ions, which lasted for almost 10 minutes and a flow reversal wasn't recorded by GEOTAIL. During the second activity, the BZ component of IMF turned positive 30 minutes before the onset (60-min time delay is already calculated), GEOTAIL observed a continuous tailward-dawnward flow followed by a short simultaneous earthward-tailward flow which was followed by a final flow reversal and earthward flow for the subsequent 10 minutes.Following Hones' model of substorms, we would expect the formation of a neutral line in the plasma sheet. Minutes after the substorm onset a structure of closed loops develops from reconnected plasma sheet field lines and flows tailward over GEOTAIL. At the beginning of the recovery phase at Earth, the substorm neutral line should have moved past GEOTAIL to a more tailward location, accounting for the northward turning of the field and the reversal of plasma flow at that time. The observations are especially interesting in view of the fact that although the northward turning was recorded by GEOTAIL, the plasma reversal was recorded more than an hour after the tailward flow of ions. During the interval between the tailward and earthward flows, the satellite recorded a new intensification in ion flow together with ground signatures of the intensification of the current wedge which indicates a second substorm activity.