宁静太阳中磁场的对流增强
Convective intensification of magnetic fields in the quiet Sun
摘要：
在宁静太阳的光球层中，经常可以在粒间通道中观测到千高斯强度的磁场。这样的磁场比均分场$B_e$更强，对应于与光球对流的动能密度相匹配的磁能密度，并且与施加等于环境气体压力的磁压力的磁场$B_p$相当。我们提出了一个理想化的数值模型的三维可压缩磁对流在光球，磁雷诺数的值的范围。在没有磁场的情况下，对流是高度超临界的，其特征是一种剧烈的、随时间变化的"颗粒状“运动。当弱磁场施加在对流上时，磁通量被扫入对流下降流中，在那里形成局部浓度。除非这一过程被磁扩散显著抑制，否则所产生的磁场通常比$B_e$大得多，并且这些通量元件中的高磁压导致它们被部分抽空。这些通量元素中的一些包含显著大于$B_p$的超强磁场。这些场由气体的热压力和对流运动的动压力的组合所包含，并且它们不断地发展。由于磁场和对流之间的非线性相互作用，这些超强磁场得以发展;它们不能用与周围环境保持整体压力平衡的薄通量管内的“对流湍流”来解释。
英文摘要：
Kilogauss-strength magnetic fields are often observed in intergranular lanes at the photosphere in the quiet Sun. Such fields are stronger than the equipartition field $B_e$, corresponding to a magnetic energy density that matches the kinetic energy density of photospheric convection, and comparable with the field $B_p$ that exerts a magnetic pressure equal to the ambient gas pressure. We present an idealised numerical model of three-dimensional compressible magnetoconvection at the photosphere, for a range of values of the magnetic Reynolds number. In the absence of a magnetic field, the convection is highly supercritical and is characterised by a pattern of vigorous, time-dependent, ``granular'' motions. When a weak magnetic field is imposed upon the convection, magnetic flux is swept into the convective downflows where it forms localised concentrations. Unless this process is significantly inhibited by magnetic diffusion, the resulting fields are often much greater than $B_e$, and the high magnetic pressure in these flux elements leads to their being partially evacuated. Some of these flux elements contain ultra-intense magnetic fields that are significantly greater than $B_p$. Such fields are contained by a combination of the thermal pressure of the gas and the dynamic pressure of the convective motion, and they are constantly evolving. These ultra-intense fields develop owing to nonlinear interactions between magnetic fields and convection; they cannot be explained in terms of ``convective collapse'' within a thin flux tube that remains in overall pressure equilibrium with its surroundings.
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