Single point spacecraft observations of the turbulent solar wind flow exhibit a characteristic nonaxisymmetric anisotropy that depends sensitively on the perpendicular power spectral exponent. We use this nonaxisymmetric anisotropy as a function of wave vector direction to test models of MHD turbulence. Using Ulysses magnetic field observations in the fast, quiet polar solar wind we find that the Goldreich-Sridhar model of MHD turbulence is not consistent with the observed anisotropy, whereas the observations are well reproduced by the “$\mathrm{\text{slab}}+2\mathrm{D}$” model. The Goldreich-Sridhar model alone cannot account for the observations unless an additional component is also present.

• Received 13 October 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.085001