[论文解读] How big is the Sun: Solar diameter changes over time

J. P. Rozelot, А. Г. Косовичев+1 more|arXiv (Cornell University)|Apr 18, 2018

Solar and Space Plasma Dynamics参考文献 11被引用 3

一句话总结

本文利用 SOHO/MDI 和 SDO/HMI 数据（1996–2017 年）中的太阳振荡 f 模态，分析太阳直径的变化，发现地震半径与太阳活动呈反同步变化。研究结果支持国际天文学联合会（IAU）修订的太阳半径，并为驱动太阳活动周期的次光球层磁场提供了新见解。

ABSTRACT

The measurement of the Sun's diameter has been first tackled by the Greek astronomers from a geometric point of view. Their estimation of ~1800", although incorrect, was not truly called into question for several centuries. The first pioneer works for measuring the Sun's diameter with an astrometric precision were made around the year 1660 by Gabriel Mouton, then by Picard and La Hire. A canonical value of the solar radius of 959".63 was adopted by Auwers in 1891. Despite considerable efforts during the second half of the XXth century, involving dedicated space instruments, no consensus was reached on this issue. However, with the advent of high sensitivity instruments on board satellites, such as the Michelson Doppler Imager (MDI) on Solar and Heliospheric Observatory (SoHO) and the Helioseismic and Magnetic Imager (HMI) aboard NASA's Solar Dynamics Observatory (SDO), it was possible to extract with an unprecedented accuracy the surface gravity oscillation f modes, over nearly two solar cycles, from 1996 to 2017. Their analysis in the range of angular degree l=140-300 shows that the so-called "seismic radius" exhibits a temporal variability in anti-phase with the solar activity. Even if the link between the two radii (photospheric and seismic) can be made only through modeling, such measurements provide an interesting alternative which led to a revision of the standard solar radius by the International Astronomical Union in 2015. This new look on such modern measurements of the Sun's global changes from 1996 to 2017 gives a new way for peering into the solar interior, mainly to better understand the subsurface fields which play an important role in the implementation of the solar cycles.

研究动机与目标

• 利用高精度日震数据研究太阳直径的长期变化。
• 评估地震半径与太阳活动周期之间的关系。
• 通过建模评估光球层半径与地震半径测量之间的一致性。
• 为国际天文学联合会（IAU）提供更新的太阳半径约束。
• 增进对影响太阳活动周期动力学的次光球层磁场的理解。

提出的方法

• 利用 SOHO/MDI 和 SDO/HMI 仪器自 1996 年至 2017 年的数据，分析 f 模态振荡。
• 聚焦于角阶数 l = 140–300，以提取地震半径的变化。
• 将地震半径趋势与太阳活动指数进行比较，以识别相位关系。
• 利用建模方法关联光球层半径与地震半径，尽管存在观测差异。
• 应用高灵敏度卫星仪器，实现半径测量前所未有的精度。
• 跨近两个太阳周期的时间序列分析，以检测长期趋势。

实验结果

研究问题

• RQ1 通过地震 f 模态推断的太阳直径如何随时间变化？
• RQ2 地震半径与太阳活动之间是否存在可测量的反相位关系？
• RQ3 地震半径测量在多大程度上能约束真实的太阳半径？
• RQ4 次光球层磁场在多大程度上影响观测到的半径变化？
• RQ5 日震数据能否提供一种可靠的替代光球层直径测量的方法？

主要发现

• 地震半径与太阳活动呈反相位变化，活动增强时半径减小。
• 观测到的地震半径变化与与磁场相关的次光球层动力学变化一致。
• 该分析支持国际天文学联合会（IAU）于 2015 年采纳的修订太阳半径值。
• SOHO/MDI 和 SDO/HMI 的高精度数据使检测细微的长期直径变化成为可能。
• 结果表明日震学在探测太阳内部结构和活动周期方面具有巨大潜力。
• 本研究揭示地震半径并非恒定，挑战了以往太阳尺寸稳定的假设。