Generation mechanism of ultra-low frequency electromagnetic oscillations in the plasma sheet boundary layer

  1. Space Research Institute

High-energy ion flows aligned along magnetic field are usually observed in the Plasma Sheet Boundary Layer
(PSBL) of the earth’s magnetotail. Multisatellite Cluster experimental data have shown that the ion flows are
accompanied with magnetic field fluctuations. Their typical frequencies and wavelengths are 0.004 – 0.02 Hz
and 5 - 20 RE accordingly. Kelvin-Helmholz (K-H) instability developing in a plasma flow of finite thickness is
considered to cause such oscillations. Using analytical analysis of the three layered plasma model the general
dispersion equation is derived, with no restrictions imposing on sound and Alfven velocities. The analysis also
allows for arbitrary orientation of the magnetic fields and plasma flows. Studying K-H instability development
for different sound velocities showed that waves with wavelengths longer or of the order of flow thickness were
unstable for any propagation angle, even temperature being zero, unlike the case of one boundary between two
semi-infinite plasmas. It is found that long-wavelength oscillations can be unstable when on the one side of the
flow the flow velocity is less then sum of Alfven velocity in the flow and Alfven velocity in surrounding plasma.
Also eigen mode structure patterns were obtained.