Damped Kinetic Alfv\'en Waves in Earth's Magnetosheath: Numerical Simulations and MMS Observations

Abstract

The Earth's magnetosheath provides a high $\beta$ (ratio of electron thermal pressure to magnetic pressure) plasma environment where kinetic Alfv\'en waves (KAWs) strongly influence turbulence and energy dissipation. This study investigates how Landau damping modifies the nonlinear evolution of KAWs by solving a modified nonlinear Schr\"odinger equation that captures both dispersive and nonlinear effects. Without Landau damping, modulational instability drives rapid self-focusing into intense magnetic filaments, producing a turbulent cascade with $k_\perp^{-5/3}$ scaling in the inertial range ($k_\perp\rho_i<1$) that transitions to $k_\perp^{-8/3}$ at sub-ion scales ($k_\perp\rho_i>1$), here $k_\perp$ is the wavevector component perpendicular to the background magnetic field and $\rho_i$ the ion thermal gyroradius. When Landau damping is included, magnetic structures are significantly suppressed, and the spectrum steepens to $k_\perp^{-11/3}$ in the sub-ion range while the inertial range maintains $k_\perp^{-5/3}$ scaling. The damping acts across all scales through resonant wave-particle interactions, efficiently transferring energy from waves to particles. Direct comparison with Magnetospheric Multiscale (MMS) spacecraft observations shows that the observed kinetic range spectral slope falls between our undamped and damped simulation limits, consistent with an intermediate damping regime in magnetosheath turbulence. This agreement confirms that Landau damping is one of the primary mechanisms controlling turbulent energy dissipation at kinetic scales in collisionless plasmas.