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Origin of nonlinearity and plausible turbulence by hydromagnetic transient growth in accretion disks: Faster growth rate than magnetorotational instability

机译:吸积盘中水磁瞬态增长引起的非线性和合理湍流的起源:比磁旋转不稳定性更快的增长速度

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摘要

We investigate the evolution of hydromagnetic perturbations in a small section of accretion disks. It is known that molecular viscosity is negligible in accretion disks. Hence, it has been argued that a mechanism, known as magnetorotational instability (MRI), is responsible for transporting matter in the presence of a weak magnetic field. However, there are some shortcomings, which question the effectiveness of MRI. Now the question arises, whether other hydromagnetic effects, e.g., transient growth (TG), can play an important role in bringing nonlinearity into the system, even at weak magnetic fields. In addition, it should be determined whether MRI or TG is primarily responsible for revealing nonlinearity in order to make the flow turbulent. Our results prove explicitly that the flows with a high Reynolds number (Re), which is the case for realistic astrophysical accretion disks, exhibit nonlinearity via TG of perturbation modes faster than that by modes producing MRI. For a fixed wave vector, MRI dominates over transient effects only at low Re, lower than the value expected to be in astrophysical accretion disks, and low magnetic fields. This calls into serious question the (overall) persuasiveness of MRI in astrophysical accretion disks.
机译:我们调查了一小部分吸积盘中水磁扰动的演化。已知在吸积盘中分子粘度可忽略不计。因此,已经争论了一种机制,称为磁旋转不稳定性(MRI),其在弱磁场存在下负责物质的运输。但是,存在一些缺点,这对MRI的有效性提出了质疑。现在出现的问题是,即使在弱磁场下,其他水磁效应(例如瞬态增长(TG))是否也可以在将非线性引入系统中起重要作用。此外,应确定MRI或TG是否主要负责揭示非线性,以使流动湍流。我们的结果明确证明,现实的天体物理增生盘就是这种情况,具有高雷诺数(Re)的流动通过摄动模式的TG表现出比产生MRI的模式更快的非线性。对于固定波矢量,MRI仅在低Re时才优于瞬态效应,该Re低于预期在天体吸积盘和低磁场中的值。这就引起了一个严重的问题,即天文学的吸积盘中MRI的(总体)说服力。

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