THE EARTH 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 Pla sma Experiment and fluxgate magnetometer. Ten several-hour-long interv als of continuous data (with 3 s and 12 s time resolution) are analyse d. The plasma-sheet flow appears to be strongly 'turbulent' (i.e. the flow is dominated by fluctuations that are unpredictable, with rms vel ocities much greater than mean velocities and with field fluctuations approximate to mean fields). The flow velocities are typically sub Alf venic. The flow-velocity probability distribution P(upsilon) is constr ucted, and is found to be well fitted by exponential functions. Autoco rrelation functions A(tau) are constructed, and the autocorrelation ti mes tau(corr) for the flow velocities are found to be about 2 min. Fro m the flow measurements, an estimate of the mixing length in the plasm a sheet is produced, yielding L-mix approximate to 2 Earth radii; corr espondingly, the plasma-sheet material appears to be well mixed in den sity and temperature. An eddy viscosity for the plasma sheet is also e stimated. Power spectra, which are constructed from the v(t) and B(t) time series, have portions that are power laws with spectral indices t hat are near the range of those expected for turbulence theories. The plasma sheet may provide a laboratory for the study of turbulence in p arameter regimes different from that of solar-wind turbulence: the pla sma sheet is a beta much greater than 1, hot-ion plasma, and the turbu lence may be strongly driven rather than well developed. The turbulent nature of the flow and the disordered nature of the magnet ic field h are implications for the transport of plasma-sheet material, for the p enetration of the solar-mind electric field into the plasma sheet, and for the calculation of particle orbits in the magnetotail.