Space Sciences Laboratory, University of California, Berkeley, CA 94720

We present case and statistical studies of magnetohydrodynamic (MHD) fluctuations observed by the Wind and Geotail spacecraft in the Earth's dawn and dusk magnetosheath. A case study with simultaneous IMP 8 interplanetary magnetic field observations confirms theoretical predictions that most of the fluctuations originate in the solar wind rather than at the magnetopause and that the fluctuations can be almost absent from the outer magnetosheath. A second case study indicates that northward plasma velocity perturbations observed in the prenoon magnetosheath correspond to southward velocity perturbations in the postnoon magnetosheath, implying that the MHD perturbations apply a torque to the magnetosphere. A statistical survey reveals that the fluctuations almost invariably propagate antisunward in the magnetosheath, independent of their propagation direction relative to the interplanetary magnetic field.

J. Geophys. Res., Vol. 105, No. A1, , 129-137, 2000