ON THE FORMATION AND EVOLUTION OF PLASMOIDS - A SURVEY OF ISEE-3 GEOTAIL DATA

ISEE 3 magnetometer and electron plasma measurements from the 1983 Geotail Mission were surveyed to determine the magnetic and plasma properties of plasmoids and their evolution with distance downtail. Events were selected on the basis of a bipolar magnetic signature in either the geocentric solar magnetospheric B(z) and/or B(y) component; most had B(z) bipolar signatures. We found 366 events consistent with this signature while ISEE 3 was in the plasma sheet. ISEE 3 observed plasmoids all along its trajectory whenever it was in the plasma sheet. Plasmoids are characterized by high-speed plasma flow. Plasmoid length was determined using both the magnetometer and the electron plasma velocity data. We found the average length of plasmoids is 16.7 +/- 13.0 R(E), significantly smaller than previous estimates. Many plasmoids have a well-defined magnetic core field, characterized by a field strength maximum at the center of the pass through the structure. Plasmoids appear to be relatively stable structures once their formation process is complete. The size, velocity, magnetic core strength, and B(z) field amplitude of plasmoids do not depend on distance beyond 100 R(E) downtail. The average electron temperature inside plasmoids drops by a factor of 2 and the electron density increases by a factor of 2 as plasmoids propagate from near Earth distances (within 100 R(E) of the Earth) to the deep tail. We conclude that the stable size of the plasmoids, the density increase and the temperature decrease are consistent with a flux of cold electrons into the plasmoid. The strong correlation of interplanetary magnetic field B(y) an hour before the event with the strength and direction of B(y) observed inside plasmoids, the existence of events with the bipolar signature in both the B(y) and B(z) components, and the possible mass flux all are consistent with plasmoids being ''open'' magnetic structures.