Alfvén wave refraction in high-speed solar wind streams

Abstract

We use a simple physical theory to calculate the variation of Alfven wave amplitudes and the wave refraction in a schematic model for a high-speed solar wind stream. The results are as follows. (1) The wave amplitudes 〈δB²〉1/2 are larger in the compression region of the stream than in the rarefaction region. (2) The relative amplitudes 〈δB²〉1/2/B0 are larger in the rarefaction region than in the compression region, this result indicating that nonlinear effects may be more important in the rarefaction region. (3) The azimuthal velocity gradient in the stream leads to the result that k is no longer nearly radial at 1 AU, in contrast to predictions based on a spherically symmetric solar wind structure. In the rarefaction region, k turns into the direction of B0, whereas in the compression region, k turns away from the direction of B0. This predicted result in the rarefaction region agrees with direct in situ observations at 1 AU. (4) Waves that start near the sun with different k all tend to be refracted into the same direction by the time that they reach 0.5 AU. This result indicates that plane wave analyses will be appropriate beyond 0.5 AU.