[论文解读] High-resolution spectropolarimetric observations of the temporal evolution of magnetic fields in photospheric bright points

P. H. Keys, A. Reid+8 more|arXiv (Cornell University)|Nov 19, 2019

Solar and Space Plasma Dynamics参考文献 90被引用 21

一句话总结

本研究分析了300个光球磁亮斑（MBPs）的高分辨率谱线偏振数据，以探究快速磁感应强度增强机制。利用NICOLE和SIR反演代码，识别出三种机制——对流塌缩、颗粒压缩和MBP合并——可在30–100秒内使磁场强度增强两倍，类似过程在MURaM辐射磁流体动力学模拟中也得到验证，包括尚未在观测数据中检测到的涡度驱动增强机制。

ABSTRACT

Context. Magnetic bright points (MBPs) are dynamic, small-scale magnetic elements often found with field strengths of the order of a kilogauss within intergranular lanes in the photosphere. Aims. Here we study the evolution of various physical properties inferred from inverting high-resolution full Stokes spectropolarimetry data obtained from ground-based observations of the quiet Sun at disc centre. Methods. Using automated feature-tracking algorithms, we studied 300 MBPs and analysed their temporal evolution as they evolved to kilogauss field strengths. These properties were inferred using both the NICOLE and SIR Stokes inversion codes. We employ similar techniques to study radiative magnetohydrodynamical simulations for comparison with our observations. Results. Evidence was found for fast (~30 - 100s) amplification of magnetic field strength (by a factor of 2 on average) in MBPs during their evolution in our observations. Similar evidence for the amplification of fields is seen in our simulated data. Conclusions. Several reasons for the amplifications were established, namely, strong downflows preceding the amplification (convective collapse), compression due to granular expansion and mergers with neighbouring MBPs. Similar amplification of the fields and interpretations were found in our simulations, as well as amplification due to vorticity. Such a fast amplification will have implications for a wide array of topics related to small-scale fields in the lower atmosphere, particularly with regard to propagating wave phenomena in MBPs.

研究动机与目标

• 理解驱动光球磁亮斑（MBPs）中快速磁感应强度增强的物理机制。
• 研究对流动力学、颗粒流动以及MBP相互作用在磁场强度演化中的作用。
• 将观测结果与辐射磁流体动力学模拟（MURaM）进行对比，以验证物理解释。
• 确定MBP生命周期中磁场增强事件的时间特性及其重复性。
• 评估快速增强对太阳光球层中波传播与能量传输的影响。

提出的方法

• 利用地基望远镜获取日面中心安静太阳区域的高分辨率全 Stokes 谱线偏振数据。
• 应用自动化特征追踪算法，长期监测300个MBPs，提取其物理属性的时序演化。
• 使用NICOLE和SIR反演代码，从Stokes参数推断视向磁感应强度和速度。
• 分析视向速度、磁感应强度与MBP形态变化之间的时空相关性。
• 将观测到的MBP动力学与MURaM辐射磁流体动力学模拟生成的合成数据进行直接对比。
• 在模拟中量化涡度与磁通量压缩效应，并在观测中搜索类似特征。

实验结果

研究问题

• RQ1 驱动MBPs中快速磁感应强度增强的物理机制是什么？这些过程在多长的时间尺度上发生？
• RQ2 对流下冲、颗粒膨胀以及MBP合并在观测到的MBPs磁场强度增加中分别起到何种作用？
• RQ3 MURaM模拟在多大程度上再现了MBPs中观测到的磁场增强过程？
• RQ4 涡度是否是MBPs中磁感应强度增强的重要驱动力？其在观测数据中是否可检测？
• RQ5 MBPs在生命周期内发生弱场与强场状态转换的频率如何？这对它们的演化动力学有何含义？

主要发现

• MBPs中的磁感应强度平均在30–100秒内增强两倍，表明存在快速增强过程。
• 对流塌缩是最常见的机制，其特征为视向速度先增加及MBP尺寸缩小。
• 颗粒压缩通过MBP的椭圆形形变及在颗粒扩张期间的空间压缩促进磁场增强。
• MBP合并导致磁场强度增加，可能源于磁通量叠加及合并结构中压缩增强。
• 涡度驱动增强在MURaM模拟中被观测到，但在当前观测数据中尚未检测到，提示需要更高分辨率的研究。
• 单个MBP生命周期内发生多次增强事件，弱场与强场状态之间的转换时间在33–99秒之间。