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We observe the anisotropy of the power spectral tensor of magnetic fieldfluctuations in the fast solar wind for the first time. In heliocentric RTNcoordinates the power in each element of the tensor has a unique dependence onthe angle between the magnetic field and velocity of the solar wind (\theta)and the angle of the vector in the plane perpendicular to the velocity (\phi).We derive the geometrical effect of the high speed flow of the solar wind pastthe spacecraft on the power spectrum in the frame of the plasma P(k) to arriveat the observed power spectrum P(f,\theta,\phi) based on a scalar fielddescription of turbulence theory. This allows us to predict the variation inthe \phi direction and compare it to the data. We then transform theobservations from RTN coordinates to magnetic-field-aligned coordinates. Theobserved reduced power spectral tensor matches the theoretical predictions wederive in both RTN and field-aligned coordinates which means that the localmagnetic field we calculate with wavelet envelope functions is an accuraterepresentation of the physical axis of symmetry for the turbulence and impliesthat on average the turbulence is axi-symmetric. We also show that we canseparate the dominant toroidal component of the turbulence from the smaller butsignificant poloidal component and that these have different power anisotropy.We also conclude that the magnetic helicity is anisotropic and mostlytwo-dimensional, arising from wavevectors largely confined to the planeperpendicular to B.

Power Anisotropy in the Magnetic Field Power Spectral Tensor of Solar Wind Turbulence