SHEAR-FLOW INSTABILITY NEAR THE CUSP RESONANCE IN MAGNETOPAUSE BOUNDARY-LAYER

Shear flow instability is studied in the planar magnetopause boundary layer region by treating the plasma as compressible. A necessary crite rion for instability near the cusp resonance is obtained analytically. The criterion depends on plasma beta, Alfven Mach number M(A) and the ratio of the scale lengths of the gradients in the flow and Alfven ve locities. The instability at the cusp resonance layer can be excited r ather easily for the low beta plasma and for shear flow scale length s maller than the typical scale length over which Alfven velocity varies . The growth rate for instability is obtained for any beta from a cubi c equation. The unstable modes may contribute to the ULF wave activity at the magnetopause.