STRUCTURE AND KINETIC-PROPERTIES OF PLASMOIDS AND THEIR BOUNDARY REGIONS

Based on GEOTAIL/LEP observations in the distant magnetotail, this pap er reports on several new features of velocity distribution functions of electrons and ions within a plasmoid and at its boundary. Here we u se the term 'plasmoid' in a wider meaning than usual in spite of the p resence of significant magnetic B-y fields. In the lobe, as expected f rom MHD simulations of magnetic reconnection processes, cold plasmas a re pushed away from the plasma sheet before the arrival of plasmoids, while after the plasmoid passage the convection is enhanced toward the normal direction to the plasma sheet. The cold ions flow into the pla smoid along magnetic field lines, are heated and accelerated perpendic ularly at the boundary, and finally merge with hot plasmas deeper insi de the plasmoids. Deep inside plasmoids, however, the ion distribution functions often show the existence of counterstreaming ion beams, whi le the simultaneously measured electron distribution functions show a flat-top distribution. It is noted that the presence of the counterstr eaming ions is a fine structure along magnetic field lines inside the whole distribution convecting tailward with speeds of 500-900 km/s. Th e relative velocity of the two components along the magnetic field lin e reaches 1000-1500 km/s, which is much higher than the local Alfven s peed. Each component has an anisotropic distribution with respect to i ts center in the velocity space; the perpendicular temperature is seve ral times higher than the parallel temperature. We conclude that these counterstreaming ions are most likely of lobe origin, and they have n ot had time enough for thermalization. They might have entered the pla smoid from the northern and southern lobes, being heated and accelerat ed through slow-mode shocks at the boundaries. Hence, these field line s are open, and both ends are connected to the northern and southern l obes. This phenomenon is observed predominantly in the latter part of the plasmoid after southward turning of the magnetic field, especially after the plasma bulk speed has increased stepwise and the B-y/B-z fi eld magnitudes have attained the peak value. It is also observed even near the neutral sheet, where the magnetic B-x field is very small, bu t significant B-y and/or B-z fields exist. Since the tailward flow spe ed becomes faster associated with the above phenomenon, these open fie ld lines would be draped around the leading (core) part of plasmoids. The compression due to the draping may increase the field intensity.