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arXiv 0903.4904astro-ph.SRastro-ph.HE
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阿尔文-回旋涨落的级联和阻尼:应用于太阳风Turbulence

Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence

Yanwei Jiang, Siming Liu, Vahé Petrosian

摘要:

磁场的存在将导致各向异性的能量级联和天体物理湍流的耗散。利用扩散近似和线性耗散率,数值研究了太阳等离子体中Alfvén回旋涨落的级联和阻尼。对于各向同性的情况下,稳态湍流谱在惯性范围内几乎是各向同性的,并且可以用谱指数为-3/2的单一幂律函数拟合,类似于Iroshnikov-Kraichnan现象。在磁流体动力学范围之外,动力学效应使光谱在较高波数处变得较软。在耗散范围内,湍流谱在波数处截止,其中阻尼率变得与级联率相当,并且截止波数随波传播方向而变化。各向同性模型的角平均湍流谱类似于破缺的幂律。考虑多普勒效应后,该模型自然地再现了太阳风中幂律湍流谱的破裂,并预测了较高的破裂频率总是伴随着较软的耗散范围谱,这可能是由于湍流强度、等离子体β的倒数和/或太阳风速度与平均磁场之间的夹角的增加造成的。这些预测可以通过与更精确的观测进行详细的比较来检验。

英文摘要:

It is well-recognized that the presence of magnetic fields will lead to anisotropic energy cascade and dissipation of astrophysical turbulence. With the diffusion approximation and linear dissipation rates, we study the cascade and damping of Alfvén-cyclotron fluctuations in solar plasmas numerically. For an isotropic case the steady-state turbulence spectra are nearly isotropic in the inertial range and can be fitted by a single power-law function with a spectral index of -3/2, similar to the Iroshnikov-Kraichnan phenomenology. Beyond the MHD regime the kinetic effects make the spectrum softer at higher wavenumbers. In the dissipation range the turbulence spectrum cuts off at the wavenumber, where the damping rate becomes comparable to the cascade rate, and the cutoff wavenumber changes with the wave propagation direction. The angle averaged turbulence spectrum of the isotropic model resembles a broken power-law. Taking into account the Doppler effects, the model naturally reproduces the broken power-law turbulence spectra observed in the solar wind and predicts that a higher break frequency always comes along with a softer dissipation range spectrum that may be caused by the increase of the turbulence intensity, the reciprocal of the plasma β, and/or the angle between the solar wind velocity and the mean magnetic field. These predictions can be tested by detailed comparisons with more accurate observations.

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