On the minimum variance direction of magnetic field fluctuations in the azimuthal velocity structure of the solar wind

Abstract

Studies of waves in the interplanetary medium often assume that the direction of minimum variance is parallel (or anti‐parallel) to the direction of the wave normal vector . In an azimuthally structured solar wind, geometrical optics predicts that the vectors of magneto‐hydrodynamic (MHD) waves should be refracted in a characteristic manner. The present study considers the properties of microscale magnetic field fluctuations for divers classes of large‐scale azimuthal velocity structures in the solar wind. Using 168.75 sec and 22.5 min variance matrices obtained by Mariner 5, we find that the direction of minimum variance does not exhibit any characteristic refraction pattern with respect to large scale velocity structures in the solar wind. The distribution of minimum variance directions is always strongly peaked at and symmetric about the average field direction, and shows no tendency to refract towards a more radial configuration. These observational results suggest that either the predictions of geometrical optics are incorrect, or that the direction of minimum variance is not indicative of the vector direction. We favor the latter interpretation.