A Walén test of low-frequency MHD waves in the magnetosheath observed by Geotail

Data measured in the magnetosheath by the Geotail satellite are used to test for the existence of a proportional relation between the temporal changes in magnetic field, ΔB, and the temporal changes in plasma velocity, ΔV. We have compared the ratio |ΔV/ΔB| with that which would be anticipated from magneto-hydrodynamic wave theory. When the plasmaʹs β exceeds 1, the ratio often significantly differs from that expected from any wave mode in isotropic plasmas. The existence of anisotropies in plasma distribution functions and consequent modifications of |ΔV/ΔB| is strongly suggested. Under the assumption that the modifications of |ΔV/ΔB| are predominantly caused by such temperature anisotropies, β||/β⊥ is evaluated over the range from 1 to 3. These anisotropies of plasma temperature may be caused by the adiabatic expansion of magnetic flux tubes in the magnetosheath.