C. Pagel et A. Balogh, A study of magnetic fluctuations and their anomalous scaling in the solar wind: the Ulysses fast-latitude scan, NONL PR GEO, 8(4-5), 2001, pp. 313-330
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.