Normals of non‐WKB Alfvén waves in the solar wind

Abstract

A theoretical treatment of wave normals of monochromatic non‐WKB Alfvén waves in the solar wind is developed. The method is based on the one‐fluid ideal MHD equations governing small‐amplitude, undamped, poloidal Alfvén waves propagating in the solar equatorial plane of a Weber and Davis spiral field. Numerical results for waves propagating from the sun to 1 AU are presented. The purpose of the calculations is to determine whether the inclusion of non‐WKB effects can significantly alter the geometrical optics result of Völk and Alpers that the wave normals tend to align themselves with the radial direction as the waves propagate through the solar wind. The conclusion is that there are differences, depending on the wave period, but that they are relatively small: the wave normals at 1 AU of waves of any period which have evolved from a reasonable distribution of normals in the corona are confined to within about 20° from the radial direction. The implication is that any lack of radial alignment is unlikely to be explained by appeal to finite wavelength effects.