Convectively driven dynamo action in the quiet Sun
Convectively driven dynamo action in the quiet Sun
复制标题
在安静的阳光下对流驱动的发电机动作
DOI：
10.1080/03091929.2011.638067
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发表时间：
2012
影响因子：
1.3
通讯作者：
Bushby P
中科院分区：
文献类型：
--
作者：
Bushby P
对平静太阳表面的观测表明，垂直磁通量的局部集中往往在对流下降流中积累。此外，有一些证据表明，中颗粒边界是形成这些通量浓度的首选位置。这意味着这些磁场是在比颗粒尺度更大的尺度上组织起来的。平静太阳磁场特征存在的一个可能解释是，它们是由近地表对流运动不断再生的。出于这一点，我们认为发电机行动在一个可压缩的导电流体中的对流的局部笛卡尔模型。该区域的水平尺度足以模拟中尺度行为。只要磁雷诺数超过某个临界值，就会发生发电机作用。在运动学制度的存在下的mesogranules似乎是有益的发电机行动：相比类似的较小的纵横比计算，我们发现更高的运动学增长率的磁能，以及一个较低的值的临界磁雷诺数。在非线性制度的峰值磁场强度比较非常有利的观测，大大超过均分值在表面，在安静的太阳观察到的。然而，没有证据表明，介粒的存在下，显着增加发电机的饱和度在这个非线性制度，这（在最高的磁雷诺数的情况下）是在较小的域类似的计算中发现的。
Observations of the quiet solar surface indicate that localised concentrations of vertical magnetic flux tend to accumulate in the convective downflows. Furthermore, there is some evidence to suggest that mesogranular boundaries are preferred locations for the formation of these flux concentrations. This implies that these magnetic fields are organised on scales that are larger than the granular scale. One possible explanation for the existence of quiet Sun magnetic features is that they are continuously regenerated by the near-surface convective motions. Motivated by this, we consider dynamo action in a local Cartesian model of convection in a compressible electrically conducting fluid. The horizontal scale of this domain is large enough to model mesoscale behaviour. Dynamo action occurs provided that the magnetic Reynolds number exceeds some critical value. In the kinematic regime the presence of mesogranules seems to be beneficial for dynamo action: compared to similar smaller aspect ratio calculations, we find higher kinematic growth rates for the magnetic energy, as well as a lower value for the critical magnetic Reynolds number. In the nonlinear regime the peak magnetic field strengths compare very favourably to observations, greatly exceeding the equipartition value at the surface, as observed in the quiet Sun. However, there is no evidence to suggest that the presence of mesogranules significantly increases the saturation level of the dynamo in this nonlinear regime, which (in the highest magnetic Reynolds number case) is comparable to that found in similar calculations in smaller domains.
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