Kelvin–Helmholtz Instability at the CME–Sheath and Sheath–Solar-wind Interfaces

Abstract

Wave-like features recently observed in some coronal mass ejections (CMEs) have been associated with the presence of Kelvin–Helmholtz instability (KHI) in the low corona. Previous works found observational evidence of KHI in a CME; this was followed by numerical simulations in order to determine the magnetic field strength allowing for its existence. Here, we present the first discussion of KHI formation in the outer corona at heliocentric distances from to . We study separately the CME–sheath and sheath–solar-wind (Sh–SW) interfaces of two CMEs that propagated in the slow and fast SWs. Mapping the velocities, densities, and magnetic field strengths of the CMEs, sheaths, and SWs in the CME’s flanks, we solve the Chandrasekhar condition for KHI formation. Calculations show that KHI formation is more likely in a CME propagating in a slow SW (CME 1) than that propagating in a fast SW due to the large shear flow between the CME and the slow SW. Comparing the interfaces for both CME cases, we note that the Sh–SW interface of CME 1 is more conducive to the instability because of the similar strengths of the magnetic field necessary for KHI formation and of the SW magnetic field.