Multispacecraft measurement of anisotropic correlation functions in solar wind turbulence

Multispacecraft measurements in the solar wind are used to determine the field-aligned anisotropy of magnetohydrodynamic inertial range turbulence. The ratio of the parallel to perpendicular correlation lengths is measured by using time-lagged two-point correlations to construct a spatial autocorrelation function. The mean ratio obtained,, is significantly greater than unity and therefore consistent with solar wind fluctuations 1.79 +/- 0.36 being anisotropic with energy predominantly in wavevectors perpendicular to the large-scale mean magnetic field. In analyzing eight 40 - 60 minute intervals of multipoint magnetic field data from the four Cluster spacecraft, the degree of variation in the ratio of the parallel to perpendicular correlation lengths about the mean was larger than expected. This variation does not appear to be correlated with the solar wind velocity or the plasma beta. The ratio of parallel to perpendicular correlation lengths was also uncorrelated between different field components.