Laboratory Study of Turbulence in Collision-Free Shocks

Abstract

In an earlier paper the authors describe a plasma wind tunnel that produces a steady collision-free flow. A shock wave standing in front of a magnetic obstacle was produced in this flow. The changes in magnitude and direction of the magnetic field across the shock wave were found to be in agreement with values predicted using the magnetohydrodynamic Rankine-Hugoniot relations. This work has been extended using electric field measurements to determine the over-all flow patterns. The electron density has been measured with a laser interferometer, and the density ratio across the shock wave has been found to be consistent with the shock wave equations. The power spectra of the magnetic and electric fluctuations have been measured. These fluctuations are consistent with whistler waves standing in the shock front. The power spectrum shows large variations with position in the shock with the highest frequencies appearing near the toe of the shock wave. The amplitude and frequency variation of the power spectrum is quite similar to space data where the latter is available. The electric field fluctuations are found to be proportional to the magnetic fluctuations with a proportionality constant equal to the flow velocity. The absolute magnitude of the magnetic turbulence is too low to absorb all of the dissipation required for the shock wave and a simple model using wave-particle interactions is suggested.