In Situ Kinetic Analysis of Kelvin-Helmholtz Instability through a Mixing Parameter

The Kelvin-Helmholtz (KH) instability is a ubiquitous space plasma process which can develop and grow in the presence of strong velocity shears, such as at the Earth's magnetopause. The evolution of the KH instability is characterized by the nonlinear coupling of different modes, which tend to generate smaller and smaller vortices along the shear layer. Such a process leads to the conversion of the energy due to the large-scale motion of the shear flow into heat contributing to the local heating and the generation of a turbulent environment. On the other hand, it allows the entry of the dense and cold solar wind plasma into the tenuous and hot magnetosphere; thus favoring the mixing of these two different regions.

In this context, we introduce a new quantity, the so-called mixing parameter, which can identify the vortex boundaries and distinguish among the type of KH structures crossed by the spacecraft. The mixing parameter exploits the well distinct particle energies which characterize the magnetosphere and magnetosheath plasmas by using only single-spacecraft measurements [1]. The mixing parameter is therefore used to conduct a statistical analysis of the evolution of KH structures observed by the Magnetospheric Multiscale mission in the near Earth's environment. Moreover, we directly compare such a quantity with kinetic simulations of KH instability, thus straightening our interpretation of the mixing parameter features and results. The identification of KH structures can allow us to better understand the role played by such instability in the local heating of plasma and in the development of turbulence under different solar wind conditions and for different locations at the Earth's magnetopause.

[1] Settino, A., et al. (2022) Journal of Geophysical Research: Space Physics, 127, e2021JA029758.