IMF draping around the Geotail: IMP 8 observations

Abstract

This paper gives maps of magnetosheath magnetic field lines in a roughly 12RE thick annulus circling the tail approximately 30RE 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.

IMP 8 magnetosheath field comparisons with models

“…They also showed that the angle between the IMF direction and the separatrix in the magnetosheath where two topologically dierent ®elds (from the magnetosphere and from the magnetosheath) meet increases as the IMF changes its orientation from north to south. These results agrees well with the results of Kaymaz et al (1992) and encourage us to make the comparisons given in this work. We give here the ®rst comprehensive comparison between magnetosheath data and Fedder-Lyon MHD model.…”

“…Because the magnetic forces become dominant in the inner dayside magnetosheath and because the geomagnetic ®eld interconnects with the IMF and violates the gas dynamic model assumption of an impenetrable magnetopause boundary, magnetic reconnection particularly aects the ®eld draping geometry at the magnetopause as shown in Kaymaz et al (1992) for IMFs in the equatorial plane. Further, the ®eld structure is dierent qualitatively and quantitatively for northward and southward IMFs.…”

“…Our analysis here focuses on these cross-¯ow components of the magnetosheath magnetic ®eld only. Kaymaz et al (1992) discussed the attributes of observed magnetosheath ®eld draping patterns in terms of their departures from symmetry with respect to the IMF direction. They pointed out that the draping appears most symmetric for northward and southward cases, while the equatorial IMF case is distorted by a twisted ®eld draping pattern in the magnetosheath close to the magnetopause.…”

“…The observations used here are described in detail in Kaymaz et al (1992). Brie¯y, the 5-minute averaged magnetometer data from IMP 8 for 1978±1982 were obtained from the data archive at the NSSDC.…”

IMP 8 observations of a poleward plasma circulation: Evidence of entry into the plasma mantle?

“…A more global study by Kaymaz et al [1992] examines the magnetic field data from IMP 8 and ISEE-3 and shows that the draping pattern of magnetic field lines in the magnetosheath is heavily dependent on the interplanetary magnetic field (IMF) orientation. They also conclude that northward and southward IMF directions have significant and different effects on the character of the magnetic field inside the magnetosheath.…”