MULTIFRACTAL SCALING OF THE KINETIC-ENERGY FLUX IN SOLAR-WIND TURBULENCE

The geometrical and scaling properties of the energy flux of the turbu lent kinetic energy in the solar wind have been studied. Using present experimental technology in solar wind measurements we cannot directly measure the real volumetric dissipation rate, epsilon(t), but are con strained to represent it by its surrogate he energy flux near the diss ipation range at the proton gyro scale. There is evidence for the mult ifractal nature of the so defined dissipation field epsilon(t), a resu lt derived from the scaling exponents of its statistical moments. The generalized dimension D-q has been determined and reveals that the dis sipation field has a multifractal structure, which is not compatible w ith a scale-invariant cascade. The related multifractal spectrum f(alp ha) has been estimated for the first time for MHD turbulence in the so lar wind. Its features resemble those obtained for turbulent fluids an d other nonlinear multifractal systems. The generalized dimension D, c an for turbulence in high-speed streams be fitted well by the function al dependence of the p-model with a comparatively large parameter p(1) = 0.87, indicating a strongly intermittent multifractal energy cascad e. The experimental value for D-p/3 used in the scaling exponent s(p) of the velocity structure function gives an exponent that can describe some of the observations. The scaling exponent mu of the autocorrelat ion function of epsilon(t) has also been directly evaluated, being 0.3 7. Finally, the mean dissipation rate was determined, which could be u sed in solar wind heating models.