Resonant Absorption and Fluctuations via Hybrid Simulations on Coronal Loops. II. Kinetic Description

Abstract The kinetic characteristics of resonant absorption of combined left- and right-hand polarized waves with finite amplitude are investigated. The plasma consists of a magnetized 2D slab with linearly inhomogeneous density layers. Using a 2D-3V particle-in-cell-hybrid code, we simulate this system with different layer thicknesses and different angles of the background magnetic field relative to the simulation plane. Resonant absorption excites counterpropagating kinetic Alfvén waves (KAWs) inside the layers, with frequencies consistent with large-scale kink modes. They are observed to interact nonlinearly to generate a parallel electric field, which subsequently produces density structures and accelerates protons. This causes strong heating and flat-topped distribution functions. We derive an analytical estimate for this parallel field that reproduces the most relevant signals of the dispersion relations during and after resonant absorption. The transverse particle dynamics are driven by the cross-field drift, causing the transverse temperature to oscillate and grow exponentially due to small-scale fluctuations. Therefore, the proton distribution functions are largely shaped nonresonantly by the KAW activity.