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We present observations of the power spectral anisotropy in wave-vector spaceof solar wind turbulence, and study how it evolves in interplanetary space withincreasing heliocentric distance. For this purpose we use magnetic fieldmeasurements made by the Helios-2 spacecraft at three positions between 0.29and 0.9 AU. To derive the power spectral density (PSD) in $(k_\parallel,k_\bot)$-space based on single-satellite measurements is a challenging task notyet accomplished previously. Here we derive the spectrum$\rm{PSD}_{\rm{2D}}$($\rm{k}_\parallel$, $\rm{k}_\bot$) from the spatialcorrelation function $\rm{CF}_{\rm{2D}}(r_\parallel, r_\bot)$ by atransformation according to the projection-slice theorem. We find the soconstructed PSDs to be distributed in k-space mainly along a ridge that is moreinclined toward the $\rm{k}_\bot$ than $\rm{k}_\parallel$ axis, a new resultwhich probably indicates preferential cascading of turbulent energy along the$\rm{k}_\bot$ direction. Furthermore, this ridge of the distribution is foundto gradually get closer to the $\rm{k}_\bot$ axis, as the outer scale length ofthe turbulence becomes larger while the solar wind flows further away from theSun. In the vicinity of the $\rm{k}_\parallel$ axis, there appears a minorspectral component that probably corresponds to quasi-parallel Alfv\'enicfluctuations. Their relative contribution to the total spectral density tendsto decrease with radial distance. These findings suggest that solar windturbulence undergoes an anisotropic cascade transporting most of its magneticenergy towards larger $\rm{k}_\bot$, and that the anisotropy in the inertialrange is radially developing further at scales that are relatively far from theever increasing outer scale.

Radial evolution of the wave-vector anisotropy of solar wind turbulence between 0.3 and 1 AU