Turbulent Dynamo in a Weakly Ionized Medium
Turbulent Dynamo in a Weakly Ionized Medium
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DOI：
10.3847/1538-4357/aafbe8
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发表时间：
2019-01
期刊：
影响因子：
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通讯作者：
Siyao Xu;S. Garain;D. Balsara;A. Lazarian
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文献类型：
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作者：
Siyao Xu;S. Garain;D. Balsara;A. Lazarian
小尺度湍流发电机是宇宙磁化过程的重要组成部分。在部分电离的天体物理等离子体中，磁能的发电机增长强烈依赖于离子与中性离子之间的耦合状态和离子-中性碰撞阻尼效应。Xu和Lazarian从理论上预测了弱电离介质中紊流发电机的一个新的阻尼阶段。通过对三维双流体发电机的仿真，首次从数值上确定了发电机阻尼阶段磁场强度的物理条件和线性增长规律。发电机放大磁场的特征长度为阻尼尺度，阻尼尺度随时间增加，在足够低的电离率和较弱的初始磁场条件下，最大涡流周转约8次后可达到湍流注入尺度。由于离子与中性之间的弱耦合，中性携带的大部分湍流能量不能转化为磁能，导致发电机末端的磁场相对较弱。这一结果对部分电离星际介质中磁场的增长和银河宇宙射线的激波加速具有重要意义。
The small-scale turbulent dynamo is an important process contributing to the cosmic magnetization. In partially ionized astrophysical plasmas, the dynamo growth of magnetic energy strongly depends on the coupling state between ions and neutrals and the ion-neutral collisional damping effect. A new damping stage of turbulent dynamo in a weakly ionized medium was theoretically predicted by Xu & Lazarian. By carrying out a 3D two-fluid dynamo simulation, we have for the first time numerically confirmed the physical conditions and the linear-in-time growth of the magnetic field strength of the damping stage of a dynamo. The dynamo-amplified magnetic field has a characteristic length as the damping scale, which increases with time and can reach the injection scale of turbulence after around eight turnover times of the largest eddy, given sufficiently low ionization fraction and a weak initial magnetic field. Due to the weak coupling between ions and neutrals, most turbulent energy carried by neutrals cannot be converted to magnetic energy, resulting in a relatively weak magnetic field at the end of the dynamo. This result has important implications for the growth of magnetic fields in the partially ionized interstellar medium and shock acceleration of Galactic cosmic rays.