Kelvin-Helmholtz Waves and Magnetic Reconnection at the Earth's Magnetopause Under Southward Interplanetary Magnetic Field

We present Magnetospheric Multiscale (MMS) observations of a K-H wave event under southward IMF conditions, accompanied by ongoing magnetic reconnection. The nonlinear K-H waves are characterized by quasi-periodic fluctuations, the presence of low-density and high-speed ions, and variations in the boundary normal vectors at both the leading and trailing edges. Our observations reveal clear evidence of on-going magnetic reconnection through the identification of Alfvenic ion jets and the escape of energetic magnetospheric electrons. Among the 36 magnetopause current-sheet crossings in this event, 19 exhibit unambiguous signatures of reconnection at both the leading (7) and trailing (12) edges. Notably, the estimated current-sheet thicknesses at both edges are comparable to the ion-inertial scale, confirming the compression effect resulting from the large-scale evolution of the K-H waves. The reconnection jets potentially contribute to the suppression of K-H growth through boundary-layer broadening and the development of complex flow and magnetic field patterns.