Surface waves on solar wind tangential discontinuities

Abstract

We demonstrate that (tangential) discontinuities in the magnetic field direction can support MHD surface waves. The surface waves resemble the usual Alfvén wave, but there are some important differences: (1) The surface waves exhibit a low‐frequency cutoff. (2) The velocity and magnetic field fluctuations are elliptically, and sometimes circularly, polarized. They may account for the solar wind helicity spectrum. (3) The surface waves are compressive, but there are special cases where they are noncompressive. (4) The wave vector k, the local normals to the surfaces of constant phase, and the magnetic minimum variance direction (mvd) do not all coincide. (5) There is a tendency for the mvd to align itself with the mean magnetic field direction. (6) The waves can be intrinsically nonplanar. (7) Equipartition between magnetic and kinetic energies is not obeyed locally. These properties of the surface waves lead us to believe that surface waves may be common in the solar wind.