Reflection of Alfvén waves in the solar wind

Abstract

We have revisited the problem of propagation of toroidal and linear Alfvén waves formulated by Heinemann and Olbert (1980) to compare WKB and non‐WKB waves and their effects on the solar wind. They considered two solar wind models and showed that reflection is important for Alfvén waves with periods of the order of one day and longer and that non‐WKB Alfvén waves are no more effective in accelerating the solar wind than WKB waves. There are several recently published papers that seem to indicate that Alfvén waves with periods of the order of several minutes should be treated as non‐WKB waves and that these non‐WKB waves exert a stronger acceleration force than WKB waves. The purpose of this paper is to study the origin of these discrepancies by performing parametric studies of the behavior of the waves under a variety of different conditions. In addition, we want to investigate two problems that have not been addressed by Heinemann and Olbert, namely, calculate the efficiency of Alfvén wave reflection by using the reflection coefficient and identify the region of strongest wave reflection in different wind models. To achieve these goals, we investigated the influence of temperature, electron density distribution, wind velocity, and magnetic field strength on the waves. The obtained results clearly demonstrate that Alfvén wave reflection is strongly model dependent and that the strongest reflection can be expected in models with the base temperatures higher than 106 K and with the base densities lower than 7 × 107 cm−3. In these models as well as in the models with lower temperatures and higher densities, Alfvén waves with periods as short as several minutes have negligible reflection so that they can be treated as WKB waves; however, for Alfvén waves with periods of the order of one hour or longer reflection is significant, requiring a non‐WKB treatment. We also show that non‐WKB, linear Alfvén waves are always less effective in accelerating the plasma than WKB Alfvén waves. Finally, it is evident from our results that the region of strongest wave reflection is usually located at the base of the models and hence that interpretation of wave reflection based solely on the reflection coefficient can be misleading.