INTERMITTENCY AND ALIGNMENT IN STRONG RMHD TURBULENCE
INTERMITTENCY AND ALIGNMENT IN STRONG RMHD TURBULENCE
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DOI：
10.1088/0004-637x/807/1/39
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发表时间：
2014-03
期刊：
影响因子：
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通讯作者：
B. Chandran;A. Schekochihin;A. Mallet
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文献类型：
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作者：
B. Chandran;A. Schekochihin;A. Mallet
本文建立了一个具有零交叉螺旋度的间歇性、三维、强、约化磁流体动力学（RMHD）湍流的解析模型。我们把波动幅度有一个对数泊松分布，并纳入模型的一个新的现象尺度依赖的动态调整之间的Elsässer变量z ±？> .我们发现结构函数Δ z λ ±？>尺度为λ 1 − β n？>，其中Δ z λ ±？>是z ±的变化？>距离λ？>垂直于磁场。我们计算出β的值为<$0.69，这是基于我们的假设，即最强的相干结构是二维的，其体积填充因子为<$λ？> .这种结构-功能标度的两个结果是，总能量功率谱为1.52？波动的峰度是<$λ − 0.27？> .我们的模型解决了以不同方式定义的对齐角度表现出不同缩放的问题。具体地说，我们发现，能量加权平均角之间的速度和磁场波动是<$λ 0.21？>，Δ z +？>而Δ z −？>是λ 0.10吗？，以及Δ z +？>而Δ z −？>没有能量加权时为‡ [ ln（L / λ）]-1 / 2？>当L / λ <$1？>，其中L是外尺度。我们还对RMHD湍流进行了直接数值模拟。在我们的模型中的标度是类似的标度在这个模拟中，以及在缓慢的太阳风中观察到的结构功能标度。
We develop an analytic model of intermittent, three-dimensional, strong, reduced magnetohydrodynamic (RMHD) turbulence with zero cross helicity. We take the fluctuation amplitudes to have a log-Poisson distribution and incorporate into the model a new phenomenology of scale-dependent dynamic alignment between the Elsässer variables z ± ?> . We find that the structure function ⟨ ∣ Δ z λ ± ∣ n ⟩ ?> scales as λ 1 − β n ?> , where Δ z λ ± ?> is the variation in z ± ?> across a distance λ ?> perpendicular to the magnetic field. We calculate the value of β to be ≃ 0.69 based on our assumption that the most intense coherent structures are two-dimensional with a volume filling factor ∝ λ ?> . Two consequences of this structure-function scaling are that the total-energy power spectrum is ∝ k ⊥ − 1.52 ?> and the kurtosis of the fluctuations is ∝ λ − 0.27 ?> . Our model resolves the problem that alignment angles defined in different ways exhibit different scalings. Specifically, we find that the energy-weighted average angle between the velocity and magnetic-field fluctuations is ∝ λ 0.21 ?> , the energy-weighted average angle between Δ z + ?> and Δ z − ?> is ∝ λ 0.10 ?> , and the average angle between Δ z + ?> and Δ z − ?> without energy weighting is ∝ [ ln ( L / λ ) ] − 1 / 2 ?> when L / λ ≫ 1 ?> , where L is the outer scale. We also carry out a direct numerical simulation of RMHD turbulence. The scalings in our model are similar to the scalings in this simulation as well as the structure-function scalings observed in the slow solar wind.