Anisotropy of Solar Wind Turbulence between Ion and Electron Scales

The anisotropy of turbulence in the fast solar wind, between the ion and electron gyroscales, is directly observed using a multispacecraft analysis technique. Second order structure functions are calculated at different angles to the local magnetic field, for magnetic fluctuations both perpendicular and parallel to the mean field. In both components, the structure function value at large angles to the field S(perpendicular to) is greater than at small angles S(parallel to): in the perpendicular component S(perpendicular to)/S(parallel to) = 5 +/- 1 and in the parallel component S(perpendicular to)/S(parallel to) > 3, implying spatially anisotropic fluctuations, k(perpendicular to) > k(parallel to). The spectral index of the perpendicular component is -2.6 at large angles and -3 at small angles, in broad agreement with critically balanced whistler and kinetic Alfven wave predictions. For the parallel component, however, it is shallower than -1.9, which is considerably less steep than predicted for a kinetic Alfven wave cascade.