NASA Goddard Space Flight Center, Laboratory for Extraterrestrial Physics, Greenbelt, Maryland 20771

This paper reports new evidence for the existence of plasmoids with force-free flux rope magnetic topologies. Motivated by the face that force-free magnetic flux ropes have intense axial fields at their centers, the ISEE 3 observations have been searched for plasma sheet intervals in which the magnetic field intensity exceeds that in the lobes by ³ 10% for a minute or longer. A total of 39 "high field regions" were found which met this simple criterion. Further examination showed that they nearly always correspond to the core regions of plasmoids; i.e., intervals of bipolar B_z with durations of minutes, fast tailward flows and clear substorm associations. A new, more realistic plasmoid magnetic field model which represents the core region as a non-linear force-free flux rope was developed and validated using these events. Finally, observations suggesting that plasmoids evolve toward quasi-force-free flux rope configurations as they move down the tail are presented.

Geophys. Res. Lett., 22, No. 15, 2061-2064, Aug. 1995