OBSERVATIONS OF PLASMAS ASSOCIATED WITH THE MAGNETIC SIGNATURE OF A PLASMOID IN THE DISTANT MAGNETOTAIL

The three-dimensional electron and positive ion velocity distributions were examined for four events for which the magnetic signature of pla smoids occurred at the Geotail spacecraft position in Earth's distant magnetotail. The proton bulk parameters and the electron velocity dist ributions for one of the events are presented in order to characterize the qualitative character of the set of events. The north-to-south tu rnings of the magnetic field that were taken for evidence of the passa ge of a plasmoid were often accompanied by a significant strong core o f B-y that was centered on the transition of B-z from north to south. The magnetic signatures thus appeared to be more indicative of flux ro pes than of classical plasmoids with weak core fields. The proton and electron bulk velocities in these magnetic structures are not equal, a situation that provides an earthward current. The plasma beta was typ ically one or greater. Counterstreaming of low-energy electrons was of ten found on the magnetic field lines around the central magnetic core and suggested the existence of a closed field line topology in these regions. These events appeared to be the tailward flowing debris from acceleration processes occurring in the vicinity of a separator positi oned nearer to Earth.