Electron Energization in the Evolution of the Kelvin–Helmholtz Instability

Abstract The Kelvin–Helmholtz (K-H) instability is ubiquitous in space and astrophysical environments, playing a vital role in the transport of mass, momentum, and energy. Although it has been shown that the K-H instability can energize the electrons, the mechanisms of electron energization remain unclear. In this paper, two-dimensional full particle-in-cell simulations are used to explore the physical mechanisms governing electron energy variation during the evolution of the K-H instability. By tracking the temporal evolution of electron energy and incorporating the guiding-center theory, we quantitatively analyze the contributions from the parallel electric field, betatron, and Fermi mechanisms to electron energization. The results demonstrate that as the instability develops, vortices roll up and expand, with electron energization dominated by the betatron mechanism. Subsequently, small-scale magnetic reconnection disrupts the vortices, and the parallel electric field becomes the primary driver of electron energization. The vortices eventually merge into a larger vortex, and the betatron mechanism regains dominance due to the roll-up of the large-scale vortex. The Fermi mechanism contributes minimally to electron energization and even exhibits a cooling effect during vortices' expansion. This study elucidates the primary pathways of electron energization during the K-H instability evolution, providing theoretical insights into energy dissipation mechanisms in magnetized plasmas under velocity shear conditions.