Scale dependent alignment between velocity and magnetic field fluctuations in the solar wind and comparisons to Boldyrev’s phenomenological theory

Abstract

A theory of incompressible MHD turbulence recently developed by Boldyrev predicts the existence of a scale dependent angle of alignment between velocity and magnetic field fluctuations that is proportional to the lengthscale of the fluctuations to the power 1/4. In this study, plasma and magnetic field data from the Wind spacecraft are used to investigate the angle between velocity and magnetic field fluctuations in the solar wind as a function of the timescale of the fluctuations and to look for the power law scaling predicted by Boldyrev. Because errors in the velocity vector can create large errors in the angle measurements, particularly at small scales, the angle measurements are suspected to be unreliable except at the largest inertial range scales. For the data at large scales the observed power law exponents range from 0.25 to 0.34, which are somewhat larger than Boldyrev’s prediction of 0.25. The results suggest that the angle may scale like a power law in the solar wind, at least at the largest inertial range scales, but the observed power law exponents appear to differ from Boldyrev’s theory.