STATISTICAL-ANALYSIS OF THE PLASMOID EVOLUTION WITH GEOTAIL OBSERVATIONS

Plasmoids in the Earth's magnetotail were studied statistically, using low energy particle (LEP) and magnetic field (MGF) data from the Geot ail spacecraft. Their evolution along the tail axis from X-GSM' = -16 to -210 R-E was investigated with 824 plasmoid events. Their dependenc e on Y-GSM' was studied as well to derive the three-dimensional struct ure of the plasmoids. (The coordinates are aberrated to remove the ave rage effects of the orbital velocity of the Earth about the Sun.) We d efined a plasmoid as a structure with rotating magnetic fields and enh anced total pressure. In the near tail (X-GSM' greater than or equal t o -50 R-E), there was a tendency for the plasmoids to be observed in t he premidnight sector around the tail axis (\Y-GSM' - 3\ less than or equal to 10 R-E), while they were observed widely (\Y-GSM'\ less than or equal to 20 R-E) in the middle tail (-50 > X-GSM' greater than or e qual to -100 R-E) and in the distant tail (-100 R-E > X-GSM'). The pla smoids expanded in the +/-Y-GSM' direction with typical velocities of +/-130 +/- 100 km/s in the near tail. This strongly supports the view that plasmoids are initially formed at the near-Earth neutral line whi ch has a limited extent in the Y-GSM' direction. The plasmoids acceler ated in the downtail direction from 400 +/- 200 km/s to 700 +/- 300 km /s from the near to the middle tail. Then, it is suggested that they d ecelerated to 600 +/- 200 km/s as they traveled to the distant tail. T he ion temperature inside plasmoids was 4.5 +/- 2 keV in the near and middle tail, and then rapidly decreased to 2 +/- 1 keV from the middle to the distant tail region. The ion temperature in the distant tail w as 2 times higher than the values deduced previously. Typical plasmoid dimensions were estimated to be 10 R-E (length) x 40 R-E (width) x 10 R-E (height) in the middle and distant tail. The energy carried by ea ch plasmoid was similar to 2 x 10(14) J in the middle tail, half of wh ich was lost from the middle to the distant tail. Inside plasmoids,the thermal energy flux exceeded the bulk energy flux and Poynting flux. The energy released tailward in the course of a substorm was estimated to be roughly 10(15) J.