基于冕流波观测对延伸日冕磁场的推测

It remains a difficult issue in both space and solar physics to directly measure the magnetic field in the solar corona. This is particularly true as far as the extended corona is concerned. Observations indicate that, in response to the lateral impingement on streamer helmets by coronal mass ejections, streamer stalks may experience some cyclic transverse motions dubbed “streamer waves”. Interpreted as Magnetohydrodynamic (MHD) fast kink waves in magnetic slabs, streamer waves can offer a rather unique way for deducing the magnetic field in streamer stalks. However, seismological studies along this line of thinking have yet to consider the effects due to the longitudinal stratification and a continuous transverse distribution of the physical parameters. This project will work in the framework of ideal MHD, and examine how the longitudinal and transverse inhomogeneities influence fast kink waves in magnetic slabs. Of particular importance will be the spatial distribution and temporal damping of the associated perturbations. With the necessary theoretical toolkit at hand, we will then seismologically exploit streamer waves, making use of all available measurements rather than just their apparent propagation speeds. We will be able to deduce not only the height dependence of the magnetic field in streamer stalks but also how the magnetic field is distributed transversely. This project will also assess the relative importance of “resonant absorption” and “dispersion” in the temporal damping of streamer waves. The theoretical toolkit to be developed will provide a rather generic description for the three-dimensional propagation of fast kink waves in magnetic slabs that are inhomogeneous in both the longitudinal and transverse directions.

日冕磁场测量是空间和太阳物理学中核心而困难的问题之一，延伸日冕磁场的测量是该问题的重要组成。观测表明，冕盔受到日冕物质抛射撞击后，冕流茎可能会横向往复摆动，这就是“冕流波”。它被理解为磁片中的快扭曲行波，其测量结合磁流体力学波动理论，可以推测冕流茎磁场。基于该思路的现有研究，都忽略了冕流茎介质的纵向分层及横向连续分布这些现实特征。本项目将在理想磁流体力学框架内，考察磁片介质的横向及纵向非均匀性对快扭曲模的影响，特别关注扰动量的空间分布及时间衰减，进而将理论成果应用于冕流波观测，力图利用冕流波的所有已知观测特征而不局限于其传播速度的测量，将不仅获得冕流茎中磁场随高度的变化，还将推测磁场在横向上的空间分布特征。将明确冕流波的衰减机制，厘清“共振吸收”与“色散”机制在冕流波衰减中的相对重要性。其理论储备将对横向及纵向都不均匀的磁片中三维传播的快扭曲模提供很具通用性的理论描述。

天基日冕仪成像观测表明，作为高速日冕物质抛射侧向撞击的后果，冕流茎会呈现准周期性横向往复摆动。这一“冕流波”的观测已被用于冕震学实践，但相关实践的若干理论基础还有待探讨。本项目以“冕流波”的冕震学价值为驱动，将冕流茎视为片状位型，以“磁片参数空间分布更可能是连续而非间断的”这一认识为入手点，开展了如下工作。1) 探讨了非零气压对日冕磁片中快模波色散特性的影响，发现非零气压的效应通常可通过将零气压理论中的Alfven速理解为非零气压理论中的横向快波速这一简便方式来替代。2) 探讨了日冕磁片中脉冲式局域扰动激发的色散波列的理论特性，发现时变计算结果的小波谱具有重要的定量冕震学价值，从而建议在冕流波成像观测数据分析中，横向位移时间序列的小波谱而非时间序列本身更便于冕震学实践。3) 探讨了共振吸收对横向连续非均匀的磁片中离面传播的快扭曲波的影响，在波矢空间勾勒了“共振吸收”可能起作用的区域。