MHD SIMULATIONS OF MAGNETIC RECONNECTION IN A SKEWED 3-DIMENSIONAL TAIL CONFIGURATION

Using our three-dimensional MHD code we have studied the dynamic evolution of a non-symmetric magnetotail configuration, initiated by the sudden occurrence of (anomalous) resistivity. The initial configuration included variations in all three space dimensions, consistent with average tail observations. In addition, it was skewed due to the presence of a net cross-tail magnetic field component B(y)N with a magnitude as typically observed, so that it lacked commonly assumed mirror symmetries around the midnight meridian and the equatorial planes. The field evolution was found to very similar to that of a symmetric configuration studied earlier (e.g., Birn and Hones, 1981), indicating plasmoid formation and ejection. The most noticeable new feature in the evolution of the individual field components is a reduction of B(y) on the reconnected dipole-like field lines earthward from the reconnection region. The topological structure of the magnetic field, however, defined by the field line connections, shows remarkable differences from the symmetric case, consistent with conclusions by Hughes and Sibeck (1987) and Birn et al. (1989). The plasmoid, which is a magnetically separate entity in the symmetric case, becomes "open", connected initially with the Earth, but getting gradually connected with the interplanetary field, as reconnection of lobe field lines proceeds from the midnight region to the flanks of the tail. The separation of the plasmoid from the Earth is thus found to take a finite amount of time. When the plasmoid begins to separate from the Earth, a filamentary structure of field connections develops, not present in the spatial variation of the fields; this confirms predictions by Birn et al. (1989). A localization of the electric field parallel to the magnetic field is found consistent with conclusions on general magnetic reconnection (Schindler et al., 1988a,b; Hesse and Schindler, 1988). The effect of E//, integrated along field lines, is found to be maximal on field lines near the plasma sheet/lobe interface. The "footprints" of these regions at the near-Earth boundary show a clottiness, reflecting the filamentary structure of the field connections, but not present in the spatial structure of the field itself.