MMS observations of the KHI during southward IMF and comparison to 2D and 3D simulations

The Kelvin-Helmholtz instability (KHI) is one of the main drivers of plasma transport across Earth’s magnetopause. Statistical studies have shown that it occurs much more frequently during periods of northward interplanetary magnetic field (IMF). Here we present MMS observations of the instability during southward IMF on September 23, 2017.

Two MMS intervals featuring plasma parameters fulfilling the instability criterion for KH waves are studied. A boundary normal vector analysis indicates the presence of linear waves and a magnetosheath side crossing of a vortex in these intervals. Correspondingly, clear signatures of Low Density Faster Than Sheath (LDFTS) plasma, in general located at the magnetosheath side of vortices, are found indicating a rolled-up vortex structure. Specific variations of the ion bulk velocity and the total pressure strengthen the argument for the detection of linear waves. Interestingly, the vortex-like event features a constant total pressure, which is explained by a magnetosheath side crossing of a vortex structure.

The MMS observations are compared to simulation results from 2D and 3D fully kinetic PIC simulations performed using the plasma parameters observed around the two MMS events. A linearity analysis of the fastest growing mode of the 2D simulation results suggests the detection of the vortex-like event in the early nonlinear phase.

The simulation further demonstrates that the secondary instabilities such as the lower-hybrid drift instability (LHDI) and the Rayleigh-Taylor instability (RTI) grow near the edge of the non-linearly developed KH vortex and strongly disturb the vortex structure. The elongated vortex arm due to the RTI together with disturbances of the vortex structure can also lead to the observed constant total pressure in MMS data. Given the above quantitative consistencies of the simulation and the MMS observations in the earlier growth phase of the KHI, these results suggest that the secondary modes may reduce the observation probability of KH wave/vortex structures during southward IMF.