Geotail Observations of an Unusual Magnetotail under Very Northward IMF Conditions

Abstract

On March 29, 1993, the Geotail spacecraft was 134R_E downstream from the earth and within 12R_E of the axis of the magnetotail whose orientation was calculated from the solar wind direction measured by IMP 8. The IMF had a magnitude near 12 nT and remained steady and roughly 50° northward for 9 hours. Geotail moved back and forth between two regions, an interior region with characteristics similar to the tail and an exterior region with characteristics similar to the magnetosheath. The flow in the dense rapidly flowing exterior region was tilted 8° toward the tail axis rather than parallel to a cylindrical tail. Calculated boundary normals were primarily in the solar magnetospheric Y direction but with a small X component consistent with a slightly flaring or cylindrical tail. The unusual flow direction in the exterior region along with the presence of slower, less dense, tailward flowing plasma on taillike field lines inside the boundary suggest that the exterior flow has a component across the boundary and into the tail. These data support accumulating evidence from MHD simulations that suggest that the magnetotail assumes a very different configuration when the IMF remains very northward for periods of at least several hours. In this unusual configuration, field lines which would normally extend into the distant tail instead close on the flanks and nearer to the earth, creating a distant tail with less open flux and a reduced Y dimension.