A survey of MHD waves in the magnetosheath: International Solar TerrestialProgram observations

We present case and statistical studies of magnetohydrodynamic (MHD) fluctu ations observed by the Wind and Geotail spacecraft in the Earth's dawn and dusk magnetosheath. A case study with simultaneous IMP 8 interplanetary mag netic field observations confirms theoretical predictions that most of the fluctuations originate in the solar wind rather than at the magnetopause an d 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 pert urbations in the postnoon magnetosheath, implying that the MHD perturbation s apply a torque to the magnetosphere. A statistical survey reveals that th e fluctuations almost invariably propagate antisunward in the magnetosheath , independent of their propagation direction relative to the interplanetary magnetic field.