IMF draping around the Geotail: IMP 8 observations

This paper gives maps of magnetosheath magnetic field lines in a roughly 12R_E thick annulus circling the tail approximately 30R_E downwind from Earth. There are maps for northward and southward IMF and for quasi‐equatorial IMF (the usual case). The maps are based on IMP 8 data taken during the 127 orbits (four years) when ISEE 3 measured the upstream IMF. In these four years, IMP 8's orbit swept the mapped annulus eight times. The maps project the field onto a plane perpendicular to the sun‐earth line, after correcting for solar wind aberration and tail flaring. Each shows draping around the tail: The field enters from one side, bifurcates at the tail boundary, “flows” around in both directions, rejoins at the opposite side, and exits. A new finding is that, contrary to expectations, the draping pattern is rotated relative to the plane formed by the IMF and the aberrated x‐axis. For example, it is rotated relative to the equatorial plane when the IMF lies in the equatorial plane. The degree of rotation varies from essentially zero for strongly northward and southward IMF to a peak ∼17° for moderately southward IMF. The sense of rotation is clockwise for toward IMF sectors and counterclockwise for away IMF sectors—the same as the dayside merging line. We also find that the tail is fatter and the magnetosheath field less ordered for strongly southward IMF than for strongly northward IMF. The new findings are understandable in terms of dayside magnetic merging.