The Earth's plasma sheet as a laboratory for flow turbulence in high-beta MHD

The bulk flows and magnetic-field fluctuations of the plasma sheet are investigated using single-point measurements from the ISEE-2 Fast Plasma Experiment and fluxgate magnetometer. Ten several-hour-long intervals of continuous data (with 3 s and 12 s time resolution) are analysed. The plasma-sheet flow appears to be strongly 'turbulent' (i.e. the flow is dominated by fluctuations that are unpredictable, with rms velocities much greater than mean velocities and with field fluctuations approximate to mean fields). The flow velocities are typically sub Alfvenic. The flow-velocity probability distribution P(upsilon) is constructed, and is found to be well fitted by exponential functions. Autocorrelation functions A(tau) are constructed, and the autocorrelation times tau(corr) for the flow velocities are found to be about 2 min. From the flow measurements, an estimate of the mixing length in the plasma sheet is produced, yielding L-mix approximate to 2 Earth radii; correspondingly, the plasma-sheet material appears to be well mixed in density and temperature. An eddy viscosity for the plasma sheet is also estimated. Power spectra, which are constructed from the v(t) and B(t) time series, have portions that are power laws with spectral indices that are near the range of those expected for turbulence theories. The plasma sheet may provide a laboratory for the study of turbulence in parameter regimes different from that of solar-wind turbulence: the plasma sheet is a beta much greater than 1, hot-ion plasma, and the turbulence may be strongly driven rather than well developed. The turbulent nature of the flow and the disordered nature of the magnet ic field hare implications for the transport of plasma-sheet material, for the penetration of the solar-mind electric field into the plasma sheet, and for the calculation of particle orbits in the magnetotail.