INTERMITTENT TURBULENCE IN SOLAR-WIND FROM THE SOUTH POLAR HOLE

The magnetic fields measured by the Ulysses spacecraft are used to stu dy solar wind turbulence in the fast solar wind from the south polar h ole. The spacecraft was at about 46 deg south latitude and 3.9 AU. For a magnetic field with a Gaussian distribution the power spectrum (sec ond-order structure function) is sufficient to completely characterize the turbulence. However, the actual distribution is non-Gaussian so t hat the effects of intermittency must be taken into account. The obser ved spectral exponents include effects of intermittency and cannot be directly compared with the standard second-order spectral theories suc h as the Kolmogorov and Kraichnan theories. To permit a better compari son of the observations with the theoretical models, we study the stru cture characteristics of the data. We find the exponents of the second -order structure functions (power spectra) and the higher-order normal ized structure functions for the components of the magnetic fields. We show that these sets of exponents can be approximately described by t wo basic numbers: the spectral exponent and the intermittency exponent . The intermittency exponent characterizes correlation properties of t he energy cascade from large to small scales. Before comparing the obs ervations to the theoretically expected values, a reduction must be ma de to the observed spectral exponent. The amount of the reduction depe nds on both the intermittency exponent and the model of the energy cas cade assumed in the turbulence theory. We reduce the measured spectral indices according to a simple model for Alfven turbulence that is des cribed here. We then compare our reduced spectral indices with second- order spectral theory. The reduced spectral indices for the period ran ge of 1 min to about a half hour are remarkably constant and in good a greement with the value of 3/2. Thus our treatment is self-consistent, Our tentative conclusion is that the high-frequency turbulence appear s to agree with the model of random-phased Alfven waves. This tentativ e conclusion must be tested by further theoretical and observational w ork.