Lower-Hybrid Wave-Induced Plasma Mixing Related to Kelvin-Helmholtz Vortices During Southward IMF

We examine characteristics of the boundaries of 11 Kelvin-Helmholtz vortex crossings observed by MMS on 23 September 2017 under southward IMF conditions. At both the leading and trailing edges, boundary regions of mixed plasma are observed together with lower-hybrid wave activity. We found that thicker boundary regions feature a higher number of sub-ion scale current sheets, of which only one shows clear reconnection signatures. Moreover, the lower-hybrid waves along the vortex spine region are identified as an effective mechanism for plasma transport with an estimated diffusion coefficient of $D\approx 10^9$ $m^2/$s. Comparisons with 3D simulations performed under the same conditions as the MMS event suggest that the extension of the boundary regions as well as the number of current sheets are related to different evolutionary stages of the vortices. Such observations can be explained by changes in the upstream magnetic field conditions.