Power and spectral index anisotropy of the entire inertial range of turbulence in the fast solar wind

We measure the power and spectral index anisotropy of magnetic field fluctuations in fast solar wind turbulence from scales larger than the outer scale down to the ion gyroscale, thus covering the entire inertial range. We show that the power and spectral indices above the outer scale of turbulence are approximately isotropic. The turbulent cascade causes the power anisotropy at smaller scales manifested by anisotropic scalings of the spectrum: close to k−5/3 across and k−2 along the local magnetic field, consistent with a critically balanced Alfvénic turbulence. By using data at different radial distances from the Sun and calculating the radial dependence of the ratio of the outer scale to the ion gyroscale, we show that the width of the inertial range does not change with heliocentric distance. At the smallest scales of the inertial range, close to the ion gyroscale, we find an enhancement of power parallel to the magnetic field direction coincident with a decrease in the perpendicular power. This is most likely related to energy injection by ion kinetic modes such as the firehose instability and also marks the beginning of the kinetic range, sometimes called the dissipation range, of solar wind turbulence.