Unveiling the Velocity-Space Signature of Ion Cyclotron Damping Using Liouville Mapping

Abstract

Ion cyclotron damping is a key mechanism for the dissipation of electromagnetic wave energy in weakly collisional plasmas. This study presents a combined approach using Liouville mapping and the field-particle correlation technique to investigate qualitatively and quantitatively the velocity-space signature of ion cyclotron damping. Liouville mapping offers a computationally efficient way to predict perturbations to the particle velocity distribution function using single-particle trajectories in prescribed electromagnetic fields. One may apply the field-particle correlation technique to these perturbed velocity distributions to reveal the unique velocity-space signatures of the secular energy transfer rate associated with specific wave-particle interactions. We validate this method by reproducing known Landau damping signatures for kinetic Alfv\'en waves, and then we apply this method to ion cyclotron waves where ion cyclotron damping dominates. The resulting velocity-space signature reveals distinct energization features of ion cyclotron damping : (i) a quadrupolar pattern in the perpendicular $(v_x, v_y)$ plane; and (ii) a localized energization near the $n = 1$ resonant velocity in gyrotropic $(v_\parallel, v_\perp)$ velocity-space. The quantitative patterns remain unchanged as the ion plasma beta $\beta_i$ is varied, ultimately showing minimal $v_\perp$ dependence on $\beta_i$ of the velocity-space signature at the $n = 1$ resonant velocity. This work provides a systematic study of how the ion cyclotron damping signature varies with $\beta_i$, offering a practical foundation to identify ion cyclotron damping using kinetic simulation data or spacecraft data.