© European Geosciences Union 1996
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 (∣X∣~10–15 RE) and far-Earth (∣X∣~100 RE) 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 RE for quiet time and L=1 RE for disturbed time. The plasma-sheet region is found to be stable for constant plasma flows unless MA3>1.25, where MA3 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 RE and 0.5–6.5 RE, respectively; whereas for distant magnetotail, the analysis predicts the oscillation periods of ~8–80 min for quiet periods and 2–16 min for disturbed periods.