Evolution and its consequences of the magnetopause Kelvin-Helmholtz instability: MMS event on 8 September 2015 revisited

Abstract:

The Kelvin-Helmholtz (KH) instability is known to grow along the Earth's magnetopause, but its role in transporting solar wind mass and energy into the magnetosphere remains to be understood. On 8 September 2015, the Magnetospheric Multiscale (MMS) spacecraft, located at the postnoon magnetopause, encountered thin low-shear current sheets at the trailing edge of the KH waves, where KH-induced reconnection, one of the plasma transport processes, was occurring [Eriksson et al., 2016; Li et al., 2016; Vernisse et al., 2016]. The event occurred during a prolonged period of northward interplanetary magnetic field, and was characterized by an extended region of the low-latitude boundary layer (LLBL) immediately earthward of the KH unstable magnetopause, which appeared to have been formed through magnetopause reconnection poleward of the cusp. In this LLBL, MMS observed plasma turbulence, an alternative agent for the plasma transport [Stawarz et al., 2016], and cold electrons possibly of ionosphere origin [Wilder et al., 2016], despite that magnetic field lines threading the LLBL would have been detached from the ionosphere a few tens of minute before the observation. In the present study, we revisit this KH-wave event and address the questions of how the KH instability got excited, how the current sheets at the KH wave trailing edges were generated, what is the origin of the turbulence seen within the KH vortices, and how the cold plasma populations got access to and reached the LLBL. Our analysis suggests that MMS was not at most KH-unstable latitudes but on their southern side, and the observed current sheets with a systematic pattern of magnetic field variations result from three-dimensional (3D) development of the KH instability. An event-driven 3D kinetic simulation also show that the KH-induced reconnection can significantly contribute to the plasma transport, in addition to poleward-of-the-cusp reconnection.