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arXiv 1302.4340astro-ph.SR
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2.5维非均匀MHD等离子体中Alfven波引起的有质动力效应

On ponderomotive effects induced by Alfven waves in inhomogeneous 2.5D MHD plasmas

J. O. Thurgood, J. A. McLaughlin

备注：
Pre-print, Accepted for publication in Solar Physics Feb 2013
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摘要：

当阿尔芬波振幅的空间梯度不为零时，非线性磁压梯度作用于介质（有质动力）。我们通过分析非均匀B和ρ的一般情况下的非线性波方程中的源项来研究非均匀2.5维MHD等离子体中这种力的性质，并考虑支持非线性数值模拟。我们的方程表明，有两个不同的类的有质动力效应引起的阿尔芬波一般2.5维MHD，每一个都有纵向和横向的表现形式; i）几何效应：在脉冲几何相对于背景磁场的影响，使波维持同空间的扰动，纵向和横向的子扰动-我们报告的横向扰动的第一次。ii）梯度（c_A）效应：在脉冲传播通过非均匀区域（其中阿尔芬速度分布中的非零梯度为非零）的情况下，非线性磁压梯度用于加速等离子体。横向梯度（相位混合区域）激发独立传播的快磁声波（概括Nakariakov等人1997，Solar Physics，175，93的结果），并且纵向梯度沿着场扰动（因此在β = 0中产生静态扰动，并且在β 1 = 0中产生慢波）。我们还证明，由于非线性洛伦兹力的模式转换是一个单向的过程，并不作为一种机制，非线性地产生阿尔芬波由于传播磁声波。我们的结论是，这些有质动力效应引起的阿尔芬波在任何MHD介质中传播，并有可能有显着的后果，提供系统的能量传输和耗散方面的动力学是足够的非线性和不均匀的。

英文摘要：

Where spatial gradients in the amplitude of an Alfven wave are non-zero, a nonlinear magnetic-pressure gradient acts upon the medium (the ponderomotive force). We investigate the nature of such a force in inhomogeneous 2.5D MHD plasmas by analysing source terms in the nonlinear wave equations for the general case of inhomogeneous B and ρ, and consider supporting nonlinear numerical simulations. Our equations indicate there are two distinct classes of ponderomotive effect induced by Alfven waves in general 2.5D MHD, each with both a longitudinal and transverse manifestation; i) Geometric Effects: Gradients in the pulse geometry relative to the background magnetic field cause the wave to sustain cospatial disturbances, the longitudinal and transverse daughter disturbances - where we report on the transverse disturbance for the first time. ii) Grad (c_A) Effects: Where a pulse propagates through an inhomogeneous region (where the non-zero gradients in the Alfven-speed profile are non-zero), the nonlinear magnetic-pressure gradient acts to accelerate the plasma. Transverse gradients (phase mixing regions) excite independently propagating fast magnetoacoustic waves (generalising the result of Nakariakov et al. 1997, Solar Physics, 175, 93) and longitudinal gradients perturb along the field (thus creating static disturbances in β = 0, and slow waves in β \= 0). We additionally demonstrate that mode conversion due the nonlinear Lorentz force is a one-way process, and does not act as a mechanism to nonlinearly generate Alfven waves due to propagating magnetoacoustic waves. We conclude that these ponderomotive effects are induced by an Alfven wave propagating in any MHD medium, and have the potential to have significant consequences on the dynamics of energy transport and aspects of dissipation provided the system is sufficiently nonlinear and inhomogeneous.

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