Asymmetric Kelvin‐Helmholtz propagation at Saturn's dayside magnetopause

Abstract

AbstractAt Saturn's magnetopause, the shear flows are maximized (minimized) in the prenoon (postnoon) sector due to the rapid planetary rotation and the corotating magnetodisc. As such, the prenoon sector is expected to be more Kelvin‐Helmholtz (KH) unstable than the postnoon sector; however, in situ Cassini data analyses showed that the evidence of KH activity favors the postnoon sector. In this study, we use a two‐dimensional MHD simulation to demonstrate that fast‐growing KH modes strongly deform and diffuse the boundary layer on a time scale of a few minutes in the prenoon sector. Therefore, the KH observational signature is difficult to identify by spacecraft in the diffused boundary layer. KH vortices originating in the subsolar region (roughly from 10 to 14 local times) are transported to the postnoon sector and the wavelength is enlarged due to the gradient of shear flow, which is a plausible reason why KH events are more often observed in the postnoon sector. The prediction of the local boundary normal direction distribution as a function of spacecraft inward/outward crossing in the postnoon sector suggested by our simulation is qualitatively consistent with Cassini in situ observational results. We also discuss the impact of this dawn‐dusk asymmetric Kelvin‐Helmholtz evolution on magnetic reconnection at Saturn's magnetopause boundary.