VELOCITY-SHEAR-INDUCED MODE COUPLING IN THE SOLAR ATMOSPHERE AND SOLAR WIND: IMPLICATIONS FOR PLASMA HEATING AND MHD TURBULENCE

We analytically consider how velocity shear in the corona and solar wind can cause an initial Alfvén wave to drive up other propagating signals. The process is similar to the familiar coupling into other modes induced by non-WKB refraction in an inhomogeneous plasma, except here the refraction is a consequence of velocity shear. We limit our discussion to a low-beta plasma, and ignore couplings into signals resembling the slow mode. If the initial Alfvén wave is propagating nearly parallel to the background magnetic field, then the induced signals are mainly a forward-going (i.e., propagating in the same sense as the original Alfvén wave) fast mode, and a driven signal propagating like a forward-going Alfvén wave but polarized like the fast mode; both signals are compressive and subject to damping by the Landau resonance. For an initial Alfvén wave propagating obliquely with respect to the magnetic field, the induced signals are mainly forward- and backward-going fast modes, and a driven signal propagating like a forward-going Alfvén wave but polarized like the fast mode; these signals are all compressive and subject to damping by the Landau resonance. A backward-going Alfvén wave, thought to be important in the development of MHD turbulence, is also produced, but it is very weak. However, we suggest that for oblique propagation of the initial Alfvén wave the induced fast-polarized signal propagating like a forward-going Alfvén wave may interact coherently with the initial Alfvén wave and distort it at a strong-turbulence-like rate.