Emission of Magnetosound From MHD-Unstable Shear Flow Boundaries in Solar-Terrestrial environment.

Kelvin-Helmholtz Instability (KHI) is a type of instability that may occur along hydrodynamic and magnetohydrodynamic (MHD) shear flow boundaries. KHI is an important process along the interface of the heliopause, in the solar atmosphere, in planetary magnetospheres, and in remote astrophysical objects such as accretion disks near neutron stars and black holes. Due to its significant effects on fluid mixing and energy and momentum transfer in-between two different flowing media, KHI has been studied extensively in the past. Surprisingly, the emission of MHD waves from a shear flow boundary as an effect of the KHI has received little attention. Here the emission of propagating MHD waves from the boundaries of flow channels that are unstable to KHI in solar-terrestrial environment is investigated. We found that KHI and MHD wave emission are two competing processes. The fastest growing modes of the KHI surface waves do not coincide with efficient wave energy transport away from a velocity shear boundary. MHD wave emission is inefficient when growth rates of KHI surface waves are maximum, which corresponds to the situation where the ambient magnetic field is perpendicular to the flow channel velocity vector. The efficiency of wave emission increases with increasing magnetic field tension, which in Earth's magnetosphere likely dominates along the nightside magnetopause tailward of the terminator, and within earthward Bursty Bulk Flows (BBFs) in the inner plasma sheet. MHD wave emission may also dominate in Supra-Arcade Downflows (SADs) in the solar corona. Our results suggest that efficient emission of propagating MHD waves along BBF and SAD boundaries can potentially explain observations of deceleration and stopping of BBFs and SADs.