Generation and role of turbulence in Kelvin-Helmholtz vortices at Earth’s magnetopause

The Kelvin-Helmholtz (KH) instability at Earth's magnetopause and associated turbulence are suggested to play a role in the transport of mass and momentum from the solar wind into Earth’s magnetosphere, which forms the basis of dynamic phenomena in the magnetosphere such as geomagnetic storms and aurora. We analyze magnetopause KH vortices observed by NASA's Magnetospheric Multiscale (MMS) spacecraft under northward solar wind magnetic field conditions in order to reveal the role of the nonlinear development of the KH instability in the plasma transport (Nakamura et al., Nature Commun., 2017) and in the turbulence generation. The magnetopause is an ideal region for the study of the generation and cascading process of turbulence in space plasmas, because well-studied solar wind turbulence is often fully developed, except for the inner heliospheric region close to the Sun. We discuss the relative contributions of the KH instability and magnetic reconnection on the dayside high-latitude magnetopause, another candidate mechanism for the turbulence generation, to the excitation or amplification of electromagnetic turbulent fluctuations observed in the KH vortices. The mode of the electromagnetic turbulence is analyzed with a method based on Ampere's law, developed by Bellan (JGR, 2016), that allows us to estimate the wave vector k as a function of frequency in the spacecraft frame based on single-spacecraft measurements of the magnetic field and current density, the latter being the quantity that became accurately observable only recently thanks to the state-of-the-art, high time-resolution plasma measurements by the MMS mission. Our analysis suggests that the electromagnetic turbulence is not due to propagating waves but caused by advection along the background plasma flow of interweaved magnetic flux tubes generated through three-dimensional vortex induced reconnection. The turbulence spectra characterized by power laws, observed even in an early nonlinear phase of the KH instability, may result from a combination of standard forward cascade at the MHD scales and rapid energy injection into ion scales through vortex induced reconnection, as suggested by Franci et al. (ApJL, 2017).