Velocity Shear-induced Mode Conversion in Solar Wind and Streamer Plasmas

Abstract

The possible scenario of Alfvén wave transformation into other MHD waves due to inhomogeneous flow in different regions of solar wind/corona is examined. We have chosen four examples with different plasma parameters and velocity shear parameters as representative cases for the plasma conditions. Two of the cases are representative of coronal holes and streamers at a radial distance of 1.6 R_solar. The third example examines the effect of an increased velocity shear due to the vicinity of a boundary. The fourth case represents solar wind conditions at 1 AU, where the plasma parameter is about unity. To study the wave coupling in the above described cases, we make use of ``nonmodal'' formalism, which appears to be a useful tool for wave processes taking place in a plasma with velocity shear. Our finding is that the possibility of coupling between Alfvén waves and fast magnetosonic waves exists in coronal holes while for streamer conditions the Alfvén mode couples to the slow magnetosonic mode. In the inner corona, it seems that these two possible ways of coupling do not overlap each other in the same structures but beyond the Alfvénic point, where β is close to unity, both of them can operate at the same time and the mechanism is more sensitive to the background plasma conditions.