Shear flow instabilities in the earth's magnetotail

Abstract

Shear flow instability is studied in the Earth’s magnetotail by treating plasma as compressible. A dispersion relation is derived from the linearized MHD equations using the oscillating boundary conditions at the inner central plasma sheet/outer central plasma sheet (OCPS) interface and OCPS/plasma-sheet boundary layer (PSBL) interface, whereas the surface-mode boundary condition is used at the PSBL/lobe interface. The growth rates and the real frequencies are obtained numerically for near-Earth (\midX\mid\sim10-15 R E ) and far-Earth (\midX\mid\sim100 R E ) magnetotail parameters. The periods and wavelengths of excited modes depend sensitively on the value of plasma-sheet half thickness, L, which is taken as L=5 R E for quiet time and L=1 R E for disturbed time. The plasma-sheet region is found to be stable for constant plasma flows unless M A3 >1.25, where M A3 is the Alfvén Mach number in PSBL. For near-Earth magnetotail, the excited oscillations have periods of 2–20 min (quiet time) and 0.5-4 min (disturbed time) with typical transverse wavelengths of 2–30 R E and 0.5-6.5 R E , respectively; whereas for distant magnetotail, the analysis predicts the oscillation periods of \sim8-80 min for quiet periods and 2–16 min for disturbed periods.