General properties of field-aligned plasma flow in quasi-steady ideal MHD configurations are discussed, and explicit solutions, modeling compressible flow around a plasmoid in the distant magnetotail, are presented. The explicit solutions are based on the assumption that plasma and field grandients along the x and y directions parallel to the tail current sheet are typically much smaller than those in the z direction perpendicular to the current sheet. In contrast to the incompressible case, two different flow solutions exist in the compressible case in the subalfv¿nic regime. The different regimes are distinguished by whether the flow speed is below or above a critical speed v0, which represents the group velocity of a slow magnetosonic wave (aligned with the magnetic field) in the limit of phase propagation perpendicular to the magnetic field, where the phase speed vanishes. In the incompressible case, only the subcritical flow solution is realized in the subalfv¿nic regime.

In the compressible case, both regimes may exist simultaneously in the flow around the plasmoid. They are separated by a tangential discontinuity which may be considered as the limit of a slow shock for vanishing normal flow and normal magnetic field. The subcritical flow region surrounding the plasmoid inside of a supercritical region possibly corresponds to the layers of tailward streaming ions observed adjacent to the plasmoid proper. The signatures of a passage from the supercritical through the subcritical flow region in the vicinity of the plasmoid can be similar to those of a passage through the plasmoid itself: a reduction of Bx followed by a local enhancement in the middle of the encounter, as observed, for instance, by Hones et al. (1984) and Slavin et al. (1989). This enhancement is found for a simple loop structure within the plasmoid and hence need not be associated with multiple loops or a tail flapping as suggested by Hones et al. Field signatures outside the plasmoid are typically those of encounters of a traveling compression region: an enhancement of Bx accompanying a north-south signature of Bz. In certain parameter regimes, however, unusual signatures are also possible: a reduction of Bx with a north-south Bz signature for a close plasmoid encounter (without penetration) and a south-north signature of Bz accompanying a Bx enhancement for a distant plasmoid encounter at high latitudes. ¿ American Geophysical Union 1992