Large amplitude Alfven waves tend to be unstable to parametric instabilities which result in a decay process of the initial wave into different daughter waves depending upon the amplitude of the fluctuations and the plasma beta. The propagation angle with respect to the mean magnetic field of the daughter waves plays an important role in determining the type of decay. In this paper, we revisit this problem by means of multi-dimensional hybrid simulations. In particular, we study the decay and the subsequent nonlinear evolution of large-amplitude Alfven waves by investigating the saturation mechanism of the instability and its final nonlinear state reached for different wave amplitudes and plasma beta conditions. As opposed to one-dimensional simulations where the Decay instability is suppressed for increasing plasma beta values, we find that the decay process in multi-dimensions persists at large values of the plasma beta via the filamentation/magnetosonic decay instabilities. In general, the decay process acts as a trigger both to develop a perpendicular turbulent cascade and to enhance mean field-aligned wave-particle interactions. We find indeed that the saturated state is characterized by a turbulent plasma displaying a field-aligned beam at the Alfven speed and increased temperatures that we ascribe to the Landau resonance and pitch angle scattering in phase space.

中文翻译：

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参数不稳定性在湍流生成和质子加热中的作用：平行传播阿尔文波的混合模拟

大振幅阿尔芬波往往不稳定到参数不稳定性，这导致初始波衰减过程为不同的子波，这取决于波动的幅度和等离子体β。相对于子波平均磁场的传播角度在确定衰减类型方面起着重要作用。在本文中，我们通过多维混合模拟重新审视这个问题。特别是，我们通过研究不稳定性的饱和机制及其在不同波幅和等离子体 beta 条件下达到的最终非线性状态，研究了大振幅阿尔芬波的衰减和随后的非线性演化。与一维模拟相反，随着等离子体β值的增加，衰减不稳定性被抑制，我们发现多维衰减过程通过丝状化/磁声衰减不稳定性在等离子体β的大值下持续存在。一般来说，衰变过程既可以触发垂直湍流级联，也可以增强平均场对齐的波粒相互作用。我们确实发现饱和状态的特征是湍流等离子体以阿尔文速度显示场对齐光束，并且温度升高，我们将其归因于相空间中的朗道共振和俯仰角散射。