[论文解读] A Truly Global EUV Wave From the SOL2017-09-10 X8.2 Solar Flare-CME Eruption

Wei Liu, Meng Jin+4 more|arXiv (Cornell University)|Jul 25, 2018

Solar and Space Plasma Dynamics被引用 1

一句话总结

本研究首次观测到由 SOL2017-09-10 X8.2耀斑-日冕物质抛射（CME）爆发激发的真正意义上的全球性极紫外（EUV）波，该波在太阳圆面及日面外区域均被探测到。该波表现出前所未有的振幅（在1.1 Rsun处达到高达300%的EUV增强），以超过2000 km/s的速度穿过极区日冕洞，引发压缩性加热，揭示了其在全局日冕磁场与热结构诊断方面的全新潜力。

ABSTRACT

We report SDO/AIA observations of an extraordinary global extreme ultraviolet (EUV) wave triggered by the X8.2+ flare-CME eruption on 2017 September 10. This was one of the best EUV waves ever observed with modern instruments, yet it was likely the last one of such magnitudes of Solar Cycle 24 as the Sun heads toward the minimum. Its remarkable characteristics include the following. (1) The wave was observed, for the first time, to traverse the full-Sun corona over the entire visible solar disk and off-limb circumference, manifesting a truly global nature, owing to its exceptionally large amplitude, e.g., with EUV enhancements by up to 300% at 1.1 Rsun from the eruption. (2) This leads to strong transmissions (in addition to commonly observed reflections) in and out of both polar coronal holes, which are usually devoid of EUV waves. It has elevated wave speeds >2000 km/s within them, consistent with the expected higher fast-mode magnetosonic wave speeds. The coronal holes essentially serve as new radiation centers for the waves being refracted out of them, which then travel toward the equator and collide head-on, causing additional EUV enhancements. (3) The wave produces significant compressional heating to local plasma upon its impact, indicated by long-lasting EUV intensity changes and differential emission measure increases at higher temperatures (e.g., log T=6.2) accompanied by decreases at lower temperatures (e.g., log T=6.0). These characteristics signify the potential of such EUV waves for novel magnetic and thermal diagnostics of the solar corona {\it on global scales}.

研究动机与目标

• 研究重大太阳耀斑-CME爆发后极紫外（EUV）波的全局传播特性。
• 理解EUV波如何与极区日冕洞相互作用并穿过这些通常被视为波透明区域的区域。
• 检查EUV波对太阳日冕的热效应与压缩性影响，特别是与温度相关的发射变化。
• 评估EUV波作为日冕磁场与热结构全局诊断工具的潜力。
• 记录太阳活动第24周期末期太阳极大期前的最后一次大规模EUV波事件。

提出的方法

• 利用太阳动力学观测台（SDO）/日球大气成像仪（AIA）的高时间分辨率、全圆面观测数据，追踪EUV波在太阳圆面及日面外区域的演化过程。
• 分析多个波长（log T = 6.0 至 6.2）的EUV强度增强，推断与温度相关的发射变化及压缩性加热效应。
• 测量日冕洞与宁静区的波速，并与预期的快模式磁声波速度进行比较。
• 追踪波的透射与反射模式，特别是在极区日冕洞内部与外部的传播，识别出折射的波前。
• 通过EUV强度增加量化波的振幅，峰值增强在1.1 Rsun处达到300%。
• 采用差分发射度测量分析方法，检测波冲击后等离子体温度分布的变化。

实验结果

研究问题

• RQ1 EUV波是否能在重大耀斑-CME事件后，于整个太阳日冕中，包括日面外与极区区域，实现全球传播？
• RQ2 极区日冕洞如何影响EUV波的传播？它们在波的折射与透射中扮演何种角色？
• RQ3 EUV波冲击的热特征是什么，特别是不同温度下差分发射度的变化如何？
• RQ4 日冕洞中波速在多大程度上超过典型值？其是否与快模式磁声波理论一致？
• RQ5 此类EUV波能否作为日冕磁场与热结构的全局诊断工具？

主要发现

• EUV波在全太阳圆面及日面外区域传播，标志着首次从单一爆发事件观测到真正意义上的全球性EUV波。
• 该波在1.1 Rsun处表现出高达300%的EUV强度峰值增强，表明其具有极高的波振幅。
• 波在极区日冕洞中成功透射并发生折射，这些区域表现出新的辐射中心特征，波速在这些区域超过2000 km/s。
• 观测到显著的压缩性加热效应，表现为log T = 6.2处发射度增加，而log T = 6.0处发射度降低。
• 来自相反方向极区日冕洞的折射波前发生迎面碰撞，导致额外的EUV强度增强。
• 该事件代表太阳活动第24周期的最后一次大规模EUV波，凸显其作为全球日冕波动力学基准事件的重要意义。