Structure and Kinetic Properties of Plasmoids and Their Boundary Regions

Abstract

Based on GEOTAIL/LEP observations in the distant magnetotail, this paper reports on several new features of velocity distribution functions of electrons and ions within a plasmoid and at its boundary. Here we use the term'plasmoid' in a wider meaning than usual in spite of the presence of significant magnetic By fields. In the lobe, as expected from MHD simulations of magnetic reconnection processes, cold plasmas are 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 plasmoid along magnetic field lines, are heated and accelerated perpendicularly at the boundary, and finally merge with hot plasmas deeper inside the plasmoids. Deep inside plasmoids, however, the ion distribution functions often show the existence of counterstreaming ion beams, while the simultaneously measured electron distribution functions show a flat-top distribution. It is noted that the presence of the counterstreaming ions is a fine structure along magnetic field lines inside the whole distribution convecting tailward with speeds of 500-900 km/s. The relative velocity of the two components along the magnetic field line reaches 1000-1500 km/s, which is much higher than the local Alfven speed. Each component has an anisotropic distribution with respect to its center in the velocity space; the perpendicular temperature is several times higher than the parallel temperature. We conclude that these counterstreaming ions are most likely of lobe origin, and they have not had time enough for thermalization. They might have entered the plasmoid from the northern and southern lobes, being heated and accelerated through slow-mode shocks at the boundaries. Hence, these field lines are open, and both ends are connected to the northern and southern lobes. 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 Ba/B, field magnitudes have attained the peak value. It is also observed even near the neutral sheet, where the magnetic Bs field is very small, but significant By and/or B. fields exist. Since the tailward flow speed becomes faster associated with the above phenomenon, these open field lines would be draped around the leading (core) part of plasmoids. The compression due to the draping may increase the field intensity.

Transient and localized processes in the magnetotail: a review

“…Zong et al (1997) reported quasi-periodic energetic ion bursts in the deep tail (X GSM =−96 R E ) associated with plasmoid-like structures with bipolar signature in the B z -component and in the B y -component. Mukai et al (1996) reported details of the counter-streaming ions and electron distributions found within a plasmoid and at its boundary. Frank et al (1995) directly determined the field-aligned current densities using the plasma measurement in the core of a magnetotail flux rope with a cross-sectional size of 0.7 R E , at X GSM ∼96 R E .…”

Dynamic processes and kinetic structure of collisionless reconnection at the dayside magnetopause: comparison between GEOTAIL observations and computer simulations

“…These beams are often observed when a satellite crosses the plasma sheet boundary (PSBL) (Parks et al, 1998;Lui et al, 1994;Frank et al, 1994;Eastman et al, 1984). Another class of observed non-Maxwellian distribution functions is for counterstreaming ions (CSIs) along the magnetic field line, often observed inside the plasmamoids/flux ropes (Mukai et al, 1996;Hoshino et al, 1997). However, detailed observations of particle distribution functions in the vicinity of the reconnection layer are still rare.…”

Structure of the near-Earth plasma sheet during tailward flows

“…The slow shocks were observed in the distant tail (X<−100 R E ) on the lobe-plasma sheet boundary (e.g. Saito et al, 1995) and on the plasmoid boundaries at X∼−100 R E (Mukai et al, 1996). Recently, Eriksson et al (2004) reported observations of reconnection related shocks in the near-Earth tail.…”

“…Zong et al, 2004), the term "plasmoid" is usually used for closed magnetic structures (magnetic islands), manifested as the northsouth bipolar variation of the magnetic field, embedded into tailward bulk flow (e.g. Mukai et al, 1996;Ieda et al, 1998).…”