Scale-dependent Polarization of Solar Wind Velocity Fluctuations at the Inertial and Kinetic Scales

We study the polarization properties of the velocity fluctuations in solar wind turbulence using high-resolution data from the Spektr-R spacecraft. The ratio of perpendicular to parallel velocity fluctuations in the inertial range is smaller than the equivalent ratio for magnetic fluctuations, but gradually increases throughout this range. In the kinetic range, there is a large decrease in the ratio, similar to the magnetic fluctuations. We compare the measurements to numerical solutions for a combination of kinetic Alfven waves and slow waves, finding that both the slow increase and sharp decrease in the ratio are consistent with a majority population of Alfven waves and minority population of slow waves in critical balance. Furthermore, the beta-dependence of this scale-dependent ratio can be successfully captured in the model when incorporating a beta-dependent Alfven to slow wave ratio similar to that observed in the solar wind.