NON-GAUSSIAN PROBABILITY-DISTRIBUTIONS OF SOLAR-WIND FLUCTUATIONS

The probability distributions of field differences Delta x(tau) = x(t + tau) - x(t), where the variable x(t) may denote any solar wind scala r field or vector field component at time t, have been calculated from time series of Helios data obtained in 1976 at heliocentric distances near 0.3 AU. It is found that for comparatively lone time lag tau, ra nging from a few hours to 1 day, the differences are normally distribu ted according to a Gaussian. For shorter time lags, of less than ten m inutes, significant changes in shape are observed. The distributions a re often spiker and narrower than the equivalent Gaussian distribution with the same standard deviation, and they are enhanced for large, re duced for intermediate and enhanced for very small values of Delta x. This result is in accordance with fluid observations and numerical sim ulations. Hence statistical properties are dominated at small scale ta u by large fluctuation amplitudes that are sparsely distributed, which is direct evidence for spatial intermittency of the fluctuations. Thi s is in agreement with results from earlier analyses of the structure functions of Delta x. The non-Gaussian features are differently develo ped for the various types of fluctuations. The relevance of these obse rvations to the interpretation and understanding of the nature of sola r wind magnetohydrodynamic (MHD) turbulence is pointed out, and contac t is made with existing theoretical concepts of intermittency in fluid turbulence.