POSSIBLE PLASMA DEPLETION LAYER AHEAD OF AN INTERPLANETARY EJECTA

We present observations of a plasma depletion layer in front of an int erplanetary magnetic cloud and analyze them quantitatively using a rec ent magnetohydrodynamic (MHD) theory of flow around magnetic clouds dr iving shocks. The magnetic field and plasma measurements were made by the ISEE 3 and IMP 8 on September 28-29, 1978. The magnetic cloud is e xpanding, and with a frontside boundary speed of approximate to 900 km s(-1) it is one of the fastest ever seen at Earth orbit. We find a si mple variation of field and plasma parameters in the sheath region beh ind the shock. Along the spacecraft trajectory the sum of the gas and plasma pressures is approximately constant, with a plasma beta less th an unity. As the frontside boundary of the magnetic cloud is approache d, the magnetic field strength increases while the density and tempera ture simultaneously decrease. These are defining characteristics of a plasma depletion layer, similar to that often seen in the terrestrial magnetosheath adjacent to the magnetopause. This suggests that the und erlying cause is the same: the stretching of interplanetary magnetic f ield lines around the ejecta. The plasma depletion layer extends throu ghout the sheath region for which data are available, that is, for app roximately 3 out of 5.7 hours. The sheath terminates at a clear discon tinuity at the cloud's surface. To compare the observations with MHD t heory, we model the ejecta as a magnetic flux rope of locally straight cylindrical geometry. We consider two limiting cases separately: (1) bulk motion relative to the surrounding medium but without expansion a nd (2) expansion into the ambient medium but without bulk motion. The predictions of both these limiting theories yield good agreement with observed variations of plasma parameters and magnetic field strength a cross the sheath. This paper is intended as a first attempt at modelin g plasma depletion layers associated with interplanetary magnetic clou ds.