Emission of magnetosound from MHD‐unstable shear flow boundaries

Abstract

The emission of propagating MHD waves from the boundaries of flow channels that are unstable to the Kelvin‐Helmholtz Instability (KHI) in magnetized plasma is investigated. The KHI and MHD wave emission are found to be two competing processes. It is shown that 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 found to be 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.

Magnetohydrodynamic Modeling Investigations of Kelvin–Helmholtz Instability and Associated Magnetosonic Wave Emission along Coronal Mass Ejections

“…We found that the KHI growth rate for this set of parameters starts at normalized flow speeds close to zero, peaks at flow speed V 01 ; 1.64, and decreases thereafter. These results agree with the general KHI growth features when the background magnetic fields are perpendicular to the flow (e.g., Turkakin et al 2016). We found that MHD wave emission due to the KHI starts to occur for flow velocities V 01 2.5, which is larger than the flow speed at which the maximum KHI growth occurs.…”

“…This means that the chosen velocities correspond to the velocities after the KHI growth reaches its peak and starts decreasing similar to what is seen in Figure 2. This result is also found in previous studies (e.g., Turkakin et al 2016).…”

“…This suggests that the MHD wave emission due to the KHI does not hold for flow velocity values at and before the point where the KHI growth rate peaks, which is at V 01 ; 1.64 for the current setup of background parameters as seen in Figure 2. The fact that the wave emission is not possible until the flow velocity becomes larger than a certain value after the peak of the KHI growth agrees with previous studies (Turkakin et al 2014(Turkakin et al , 2016.…”

“…The magnetic pressure and tension forces act on the flowing media and introduce a variety of interactions between the flow and perturbations (Taroyan & Ruderman 2011). When the direction of the shear flow is perpendicular to the magnetic field, there is no lower cutoff velocity at which the KHI onset occurs and the growth rates are maximized (Turkakin et al 2016;Hillier et al 2018). When the direction of the shear flow is parallel to the magnetic field, the lower cutoff velocity increases and the KHI growth rate decreases (Turkakin et al 2016).…”

“…Foullon et al (2011) discovered evidence of the KHI development on the boundary of a CME inside the magnetized corona, where there exists magnetic fluxrope-like properties. In a recent study on the linear evolution of the KHI along MHD shear flow boundaries, it was suggested that MHD wave emission due to the action of the KHI may be possible along various MHD shear flow boundaries in the solar corona (Turkakin et al 2016). Depending on the background magnetic field values and orientation, MHD waves may be emitted in either or both sides of the plasma boundaries of these shear flow regions.…”