GENERATION OF LOW FREQUENCY OSCILLATIONS
IN THE PLASMA SHEET BOUNDARY LAYER

T.M. Burinskaya, M.M. Shevelev, E.E. Grigorenko

Space Research Institute RAS, Moscow, Russia


Multipoint Cluster observations have revealed that during the motion of highly accelerated (up to 2500 km/sec) 
field-aligned ion flows in the plasma sheet boundary layer the related propagation of magnetic oscillations with 
a phase velocity of the order of the local Alfven speed is observed. Since the Kelvin-Helmholtz (K-H) instability
 is a very probable candidate for the excitation of such Alfven type disturbances, a general stability analysis is 
performed for the K-H instability in a three-layered system, when a background magnetic field is directed parallel
 to the plasma flow velocity. Solutions of the dispersion equation for the compressible plasma have shown that there is no 
upper critical sonic Mach number (MS=2) for oscillations propagating along the magnetic field, contrary to the common
 case of two plasmas moving relative to each other. For sonic Mach numbers higher than 2, the instability arises in 
a limited range of wave numbers, thus fixing the upper and lower cut off frequencies for the wave spectra. The structure 
of eigenmode has a damped-oscillatory character in the direction perpendicular to the background magnetic field and 
a kink-like type along the magnetic field. The comparison of obtained theoretical results with experimental data is discussed.