Characterization of satellite crossings through Kelvin-Helmholtz structures

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. This instability represents a way for plasmas to give rise to a turbulent scenario and to convert the energy due to the large-scale motion of the shear flow into heat. Indeed, the evolution of the KH instability is characterized by the nonlinear coupling of different modes, which tends to generate smaller and smaller vortices along the shear layer. Recently, both kinetic simulations and in situ measurements, focusing on the kinetic effects during the nonlinear phase of the instability, have shown the generation of strong current sheets between well-developed vortices, and temperature anisotropy and agyrotropy at both ion and electron scales, in accordance with the multi-scale nature of the phenomenon [1, 2]. In the near Earth’s environment, spacecraft observations have highlighted an evolution of KH vortices along the flanks of the Earth’s magnetopause and an enhancement of rolled-up vortices as they propagate tailward [3]. Moreover, the evolution of the KH instability strongly depends on the solar wind conditions, such as the velocity and magnetic field orientation, as shown in fluid simulations [4]. In this context, we introduce a new quantity, the so-called mixing parameter, which can identify the vortex boundaries and distinguish the evolution of KH structures crossed by the satellite. The mixing parameter exploits the well distinct particle energies which characterize the magnetosphere and magnetosheath plasmas by using only single-spacecraft measurements [5]. [1] Settino, A., et al. (2020) The Astrophysical Journal, 901(1), 17. [2] Settino, A., et al. (2021) The Astrophysical Journal, 912(2), 154. [3] Lin, et al. (2014) Journal of Geophysical Research, 119(9), 7485-7494. [4] Nykyri, K. (2013) Journal of Geophysical Research, 118(8), 5068-5081. [5] Settino, A., et al. (2022) Journal of Geophysical Research: Space Physics, 127, e2021JA029758.