Abstract
Two-dimensional (2-D) and three-dimensional (3-D) hybrid simulations are carried out for mode conversion from fast mode compressional wave to kinetic Alfvén waves (KAWs) at the inhomogeneous magnetopause boundary. For cases in which the incident fast wave propagates in the xz plane, with the magnetopause normal along x and the background magnetic field pointing along z, the 2-D (xz) simulation shows that KAWs with large wave number k x ρ i ∼ 1 are generated near the Alfvén resonance surface, where ρ i is the ion Larmor radius. Several nonlinear wave properties are manifest 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. In the 3-D simulation, after this first stage of the mode conversion to KAWs with large k x , a subsequent generation of KAW modes of finite k y is observed in the later stage, through a nonlinear parametric decay process. Since the nonlinear cascade to k y can lead to massive transport at the magnetopause, the simulation results provide an effective transport mechanism at the plasma boundaries in space as well as laboratory plasmas.
Highlights
For cases in which the incident fast wave propagates in the xz plane, with the magnetopause normal along x and the background magnetic field pointing along z, the 2-D simulation shows that kinetic Alfven waves (KAWs) with large wave number kxρi ∼ 1 are generated near the Alfven resonance surface, where ρi is the ion Larmor radius
The efficiency of the mode conversion process can be greatly enhanced over the MHD limit and for a broad range of frequency can capture most of the wave power in the magnetopause boundary layer where it is converted into kinetic Alfven waves
The 2-D simulation shows that as the compressional wave reaches the magnetopause boundary from the magnetosheath, strong KAWs are excited at the location where the Alfven resonance condition ω = k VA(x)(1 − ω2/Ω2i ) is satisfied
Summary
Two-dimensional (2-D) and three-dimensional (3-D) hybrid simulations are carried out for mode conversion from fast mode compressional wave to kinetic Alfven waves (KAWs) at the inhomogeneous magnetopause boundary. For cases in which the incident fast wave propagates in the xz plane, with the magnetopause normal along x and the background magnetic field pointing along z, the 2-D (xz) simulation shows that KAWs with large wave number kxρi ∼ 1 are generated near the Alfven resonance surface, where ρi is the ion Larmor radius. The magnetopause current sheet in slab geometry is assumed to be centered at x = 0 in the middle of the simulation domain, separating two uniform plasma regions of the magnetosheath (x < 0) with a high density and low magnetic field strength and magnetosphere (x > 0) with a low density and high magnetic field.
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