部分电离气体中MHD turbulence的阻尼及中性粒子和离子速度的观测差异
Damping of MHD turbulence in partially ionized gas and the observed difference of velocities of neutrals and ions
摘要：
在过去的几十年里，对星际介质湍流的理论和观测研究发展迅速。超声速磁化湍流的理论以及对观测量投影效应的理解仍在发展中。在这项工作中，我们探讨了表征的湍流级联和它的阻尼观测谱线轮廓。我们解决的离子和中性速度的差异，澄清部分电离的湍流阻尼的性质。我们提供了理论论据，有利于解释的更大的多普勒展宽的线所产生的中性物种相比，离子所产生的湍流阻尼的离子在更大的尺度。此外，我们计算了一些MHD数值模拟不同的湍流制度和显式湍流阻尼，并比较了三维速度分布和合成线轮廓分布。从数值模拟，我们放置的精度，可以测量的三维色散取决于湍流音速马赫数的限制。我们发现，通过观测存在的三维速度色散测量的湍流体积和最小的二维速度色散之间没有普遍的对应关系。例如，对于亚音速湍流，在比湍流喷射尺度小得多的尺度上对应性较差，而对于超音速湍流，在与喷射尺度相当的尺度上对应性较差。我们提供了这样一个2D-3D对应的存在的物理解释，并讨论了在评估的阻尼尺度的离子，可以从观察到的不确定性。然而，我们表明，从观测到的线轮廓的速度色散的统计数据可以提供的频谱指数和湍流的能量传递率。此外，比较两个类似的模拟不同的粘性系数，它是可能的约束湍流截止尺度。这可能是特别有用的，因为它被认为是双极扩散可能是在恒星形成区域的主要耗散机制之一。在这种情况下，双极扩散尺度的确定可以用作用于确定塌陷芯中的磁场强度的补充方法。我们讨论了我们的研究结果的影响，在一个新的方法，磁场测量提出的Li和Houde（2008）。
英文摘要：
Theoretical and observational studies on the turbulence of the interstellar medium developed fast in the past decades. The theory of supersonic magnetized turbulence, as well as the understanding of projection effects of observed quantities, are still in progress. In this work we explore the characterization of the turbulent cascade and its damping from observational spectral line profiles. We address the difference of ion and neutral velocities by clarifying the nature of the turbulence damping in the partially ionized. We provide theoretical arguments in favor of the explanation of the larger Doppler broadening of lines arising from neutral species compared to ions as arising from the turbulence damping of ions at larger scales. Also, we compute a number of MHD numerical simulations for different turbulent regimes and explicit turbulent damping, and compare both the 3-dimensional distributions of velocity and the synthetic line profile distributions. From the numerical simulations, we place constraints on the precision with which one can measure the 3D dispersion depending on the turbulence sonic Mach number. We show that no universal correspondence between the 3D velocity dispersions measured in the turbulent volume and minima of the 2D velocity dispersions available through observations exist. For instance, for subsonic turbulence the correspondence is poor at scales much smaller than the turbulence injection scale, while for supersonic turbulence the correspondence is poor for the scales comparable with the injection scale. We provide a physical explanation of the existence of such a 2D-3D correspondence and discuss the uncertainties in evaluating the damping scale of ions that can be obtained from observations. However, we show that the statistics of velocity dispersion from observed line profiles can provide the spectral index and the energy transfer rate of turbulence. Also, comparing two similar simulations with different viscous coefficients it was possible to constrain the turbulent cut-off scale. This may especially prove useful since it is believed that ambipolar diffusion may be one of the dominant dissipative mechanism in star-forming regions. In this case, the determination of the ambipolar diffusion scale may be used as a complementary method for the determination of magnetic field intensity in collapsing cores. We discuss the implications of our findings in terms of a new approach to magnetic field measurement proposed by Li & Houde (2008).
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