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 Alfvénic 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. Plain Language Summary: Kelvin‐Helmholtz (K‐H) waves and magnetic reconnection are two common occurrences at the Earth's magnetopause, playing crucial roles in the transfer of mass, momentum, and energy from the solar wind to the magnetosphere. While extensive research has been conducted on the relationship between K‐H waves and magnetic reconnection under northward interplanetary magnetic field (IMF) conditions, there is a notable gap in our understanding regarding the southward IMF scenario. This study presents the first observed event of K‐H waves under southward IMF conditions, accompanied by magnetic reconnection, and provides a detailed analysis of the characteristics of both phenomena. Through our observations, we investigate the correlation between K‐H waves and magnetic reconnection during southward IMF periods. Our findings suggest that the coexistence of K‐H waves and magnetic reconnection may be attributed to the promoting effect of K‐H waves on the occurrence of magnetic reconnection. Furthermore, it is likely that magnetic reconnection suppresses the evolution of K‐H waves, which could potentially contribute to the low likelihood of observing K‐H waves under southward IMF conditions. Key Points: The first report of K‐H waves and magnetic reconnection occurring simultaneously at the magnetopause LLBL under southward IMF conditionsThe reconnection ion jets are observed at both the leading and trailing edges of the K‐H wavesFavorable conditions for reconnection are produced by K‐H waves and reconnection may in turn affect the global evolution of the K‐H waves [ABSTRACT FROM AUTHOR]