[论文解读] Long-term trends of magnetic bright points: I. Number of MBPs at disc centre

D. Utz, R. Müller+6 more|arXiv (Cornell University)|Nov 24, 2015

Solar and Space Plasma Dynamics参考文献 56被引用 3

一句话总结

本研究利用八年的Hinode/SOT G-band数据（2006–2014）分析了磁性亮点（MBPs）的长期趋势，发现赤道附近的MBP数量随太阳周期变化，存在约2.5年的延迟，并显著受各半球活动差异的调制。结果表明，赤道附近的MBPs与黑子活动带相关，最大MBP数量的高达60%在太阳活动极小期仍持续存在，暗示其与全球和局部发电机过程均有关联。

ABSTRACT

Context. The Sun shows an activity cycle that is caused by its varying global magnetic field. During a solar cycle, sunspots, i.e. extended regions of strong magnetic fields, occur in activity belts that are slowly migrating from middle to lower latitudes, finally arriving close to the equator during the cycle maximum phase. While this have been well known for centuries, much less is known about the solar cycle evolution of small-scale magnetic fields. Aims. To address this question, we study magnetic bright points (MBPs) as proxies for such small-scale, kG solar magnetic fields. This study is based on a homogeneous data set that covers a period of eight years. Methods. An automated MBP identification algorithm was applied to the synoptic Hinode/SOT G-band data over the period November 2006 to August 2014, i.e. covering the decreasing phase of Cycle 23 and the rise, maximum, and early decrease of Cycle 24. This data set includes, at the moment of investigation, a total of 4 162 images, with about 2.9 million single MBP detections. Results. After a careful preselection and monthly median filtering of the data, the investigation revealed that the number of MBPs close to the equator is coupled to the global solar cycle but shifted in time by about 2.5 years. Furthermore, the instantaneous number of detected MBPs depends on the hemisphere, with one hemisphere being more prominent, i.e. showing a higher number of MBPs. After the end of Cycle 23 and at the starting point of Cycle 24, the more active hemisphere changed from south to north. Conclusions. These findings suggest that there is indeed a coupling between the activity of MBPs close to the equator with the global magnetic field. The results also indicate that a significant fraction of the magnetic flux that is visible as MBPs close to the equator originates from the sunspot activity belts.

研究动机与目标

• 调查作为磁性亮点（MBPs）示踪的小尺度磁场是否表现出与黑子相似的长期活动周期行为。
• 确定MBP数量与全球太阳磁场周期之间的时序关系，特别是在太阳活动周23向24过渡期间。
• 评估半球不对称性对MBP分布与活动的影响。
• 评估太阳极小期MBP活动的持续性是否表明存在超越全球周期的局部发电机贡献。

提出的方法

• 对2006年11月至2014年8月期间收集的4,162幅Hinode/SOT G-band合成图像应用了自动MBP检测算法。
• 采用月度中值滤波和数据预筛选以减少噪声，并从短期波动中分离出长期趋势。
• 分析MBP数量随时间与纬度的变化，重点关注赤道区域及半球差异。
• 将MBP数量的时间演化与太阳黑子数及太阳周期相位进行比较，以评估相位延迟与相关性。
• 评估仪器老化效应，以确保所观测到的趋势为物理现象而非数据退化所致。
• 通过随时间比较南北半球MBP数量来量化半球不对称性。

实验结果

研究问题

• RQ1 在太阳圆面中心，磁性亮点（MBPs）数量是否存在长期周期？若存在，其与黑子周期的关系如何？
• RQ2 MBP活动周期是否相对于黑子周期存在时间延迟？这种延迟可能由何原因解释？
• RQ3 在太阳周期中，MBPs的分布与数量在南北半球之间如何变化？
• RQ4 在太阳活动极小期，最大MBP活动的多少比例仍能持续存在？这对其小尺度磁通量起源意味着什么？

主要发现

• 赤道附近MBP数量与全球太阳周期呈强相关性，但存在约2.5年的延迟。
• 在太阳活动周23与24之间的极小期附近，更活跃的半球由南半球转向北半球，表明半球主导地位发生了反转。
• 在±7°纬度处观察到MBP数量的明显峰值，与太阳周期末期黑子活动带的位置一致。
• 即使在太阳极小期，最大MBP探测率的高达60%仍可被观测到，表明存在持续的小尺度磁通量源。
• 所观测到的趋势与半球不对称性对仪器老化效应具有鲁棒性，表明其为真实物理现象。
• 结果表明，赤道MBPs的显著部分起源于衰减的黑子活动带，而残余活动可能源自局部表面发电机过程。