A study of magnetic fluctuations and their anomalous scaling in the solar wind: the Ulysses fast-latitude scan

The solar wind is a highly turbulent and intermittent medium at frequencies between 10(-4) and 10(-1) Hz. Power spectra are used to look at fluctuatio ns in the components of the magnetic field at high frequencies over a wide range of latitudes. Results show steady turbulence in the polar regions of the Sun and a more varied environment in the equatorial region. The magneti c field fluctuations exhibit anomalous scaling at high frequencies. Various models have been proposed in an attempt to better understand the scaling n ature of such fluctuations in neutral fluid turbulence. We have used the Ul ysses fast latitude scan data to perform a wide ranging comparison of three such models on the solar wind magnetic field data: the well-known P model, in both its Kolmogorov and Kraichnan forms, the lognormal cascade model an d a model adapted from atmospheric physics, the G infinity model. They were tested by using fits to graphs of the structure function exponents g(q), b y making a comparison with a non-linear measure of the deviation of g(q) fr om the non-intermittent straight line, and by using extended self similarit y technique. over a large range of heliolatitudes. Tests of all three model s indicated a high level of intermittency in the fast solar wind, and showe d a varied structure in the slow wind, with regions of apparently little in termittency next to regions of high intermittency, implying that the slow w ind has no uniform origin. All but one of the models performed well, with t he lognormal and Kolmogorov P model performing the best over all the tests, indicating that inhomogeneous energy transfer in the cascade is a good des cription. The Kraichnan model performed relatively poorly, and the overall results show that the Kraichnan model of turbulence is not well supported o ver the frequency and distance ranges of our data set. The G infinity model fitted the results surprisingly well and showed that there may very well b e important universal geometrical aspects of intermittency over many physic al systems.