MAGNETOTAIL RESPONSE DURING A STRONG SUBSTORM AS OBSERVED BY GEOTAIL IN THE DISTANT TAIL

Simultaneous energetic particle and magnetic field observations from t he GEOTAIL spacecraft in the distant tail (X-GSM approximate to -150 R e) have been analysed to study the response of the Earth's magnetotail during a strong substorm (AE less than or equal to 680 nT). At geosyn chronous altitude. LANL spacecraft recorded three electron injections between 0030 UT and 0130 UT, which correspond to onsets observed on th e ground at Kiruna Ground Observatory. The Earth's magnetotail respond ed to this substorm with the ejection of five plasmoids, whose size de creases from one plasmoid to the next. Since the type of magnetic stru cture detected by a spacecraft residing the lobes, depends on the Z ex tent of the structure passing underneath the spacecraft, GEOTAIL is fi rst engulfed by a plasmoid structure; six minutes later it detects a b oundary layer plasmoid (BLP) and finally at the recovery phase of the substorm GEOTAIL observes three travelling compression regions (TCRs). The time-of-flight (TOF) speed of these magnetic structures was estim ated to range between 510 km/s and 620 kms. The length of these indivi dual plasmoids was calculated to be between 28 Re and 56 Re. The princ ipal axis analysis performed on the magnetic field during the TCR enco untered, has confirmed that GEOTAIL observed a 2-D perturbation in the X-Z plane due to the passage of a plasmoid underneath. The first larg e plasmoid that engulfed GEOTAIL was much more complicated in nature p robably due to the external, variable draped field lines associated wi th high beta plasma sheet and the PSBL flux tubes surrounding the plas moid. From the analysis of the energetic particle angular distribution , evidence was found that ions were accelerated from the distant X-lin e at the onset of the burst associated with the first magnetic structu re.