Hybrid simulation of mode conversion at the magnetopause

Abstract

Two‐dimensional hybrid simulations are used to investigate how fast‐mode compressional waves incident on a magnetopause current layer mode convert both linearly and nonlinearly to short wavelength (k⊥ρi ∼ 1) kinetic Alfvén waves near the Alfvén resonance surface. The background magnetic fields on both sides of the current layer are parallel to each other and perpendicular to the magnetopause normal, corresponding to a northward interplanetary magnetic field. The simulations are performed in a 2‐D plane (xz), where x is normal to the magnetopause and z is tilted by an angle, θ, relative to the magnetic field. We examine how the mode conversion depends on wave frequency ω0, wave vector, Alfvén velocity profile (particularly the magnetopause width, D0), ion β in the magnetosheath, electron‐to‐ion temperature ratio, and incident wave amplitude. Kinetic effects resolve the resonance, and KAWs radiate back to the magnetosheath side of the current layer. The compressional wave absorption rate is estimated and compared with linear theory. Unlike the prediction from low‐frequency theory of the Alfvén resonance, KAWs are also generated in cases with θ = 0°, provided ω0 > 0.1Ω0, with Ω0 being the ion cyclotron frequency in the magnetosheath. As the incident wave amplitude is increased, several nonlinear wave properties are manifested in the mode conversion process. Harmonics of the driver frequency are generated. As a result of nonlinear wave interaction, the mode conversion region and its spectral width are broadened. The nonlinear waves provide a significant transport of momentum across the magnetopause and are associated with significant ion heating in the resonant region.