We present observations of a plasma depletion layer in front of an interplanetary magnetic cloud and analyze them quantitatively using a recent magnetohydrodynamic (MHD) theory of flow around magnetic clouds driving 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 expanding, and with a frontside boundary speed of ≈ 900 km s−1 it is one of the fastest ever seen at Earth orbit. We find a simple variation of field and plasma parameters in the sheath region behind the shock. Along the spacecraft trajectory the sum of the gas and plasma pressures is approximately constant, with a plasma beta less than unity. As the frontside boundary of the magnetic cloud is approached, the magnetic field strength increases while the density and temperature 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 underlying cause is the same: the stretching of interplanetary magnetic field lines around the ejecta. The plasma depletion layer extends throughout the sheath region for which data are available, that is, for approximately 3 out of 5.7 hours. The sheath terminates at a clear discontinuity at the cloud's surface. To compare the observations with MHD theory, 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 and (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 across the sheath. This paper is intended as a first attempt at modeling plasma depletion layers associated with interplanetary magnetic clouds.
