冕洞针状体中的快扭曲波及其对针状体气体的加热

Spicules abound in the solar atmosphere. Recent observations indicate that part of the chromospheric material in spicules can be heated to transition region or even coronal temperatures. Furthermore, spicules often undergo transverse motions generally accepted to be fast kink waves, which carry energy fluxes sufficient to account for coronal heating in solar coronal holes. However, the available theoretical studies are generally based on assumptions far from reality. It therefore remains an open question as to whether and how fast kink waves can heat the materials in spicules in coronal holes. In the framework of Magnetohydrodynamics, we will examine, in a systematic manner, the propagation, damping, and dissipation of fast kink waves in realistic temporally varying, dynamic, and longitudinally structured spicules. This will be done with a synergetic approach combining analytical methods together with time-dependent linear and fully non-linear computations. We will quantitatively assess the heating rates due to fast kink waves and their observational consequences. The outcome of this project will help understand the energy supply to the corona in coronal holes, and will also be applicable to soft X-ray jets and network jets, which are prevalent on the Sun and often accompanied by cyclic transverse motions as well.

针状体是太阳大气中普遍存在的小尺度喷流。近期观测表明，针状体中部分色球气体可被加热至过渡区乃至日冕温度。此外，针状体常伴有被理解为快扭曲波的横向摆动，所估计的能流足供冕洞内日冕加热所需。但由于相关研究的理论基础偏离针状体现实甚远，快扭曲波能否及怎样加热冕洞针状体中太阳大气这一问题还远未解决。本项目拟在磁流体力学框架内，利用解析、线性数值计算及全三维非线性模拟手段，计入时变针状体中可观的流动及针状体内外介质的纵向分层，系统讨论这些真实物理因素如何影响针状体中快扭曲波的能量输运、衰减、向小尺度结构的转换及相应耗散，从而定量评估快扭曲波对冕洞针状体内太阳大气的加热效率。项目成果将有助于理解冕洞区日冕能量供应问题，考虑到横波在软X线喷流、网络喷流等小尺度喷流中广泛存在，项目成果还将具有普遍的应用价值。

本项目旨在“探索针状体中扭曲模的能量输运、衰减、向小尺度结构的转换及相应耗散，并由此评估大气加热效应”，因而涉及扭曲模由低层大气至日冕的传播、扭曲模被Alfven和/或cusp连续谱共振吸收、共振吸收导致小尺度结构的发育并进而产生强梯度、强梯度启动开尔文-亥姆霍兹(KH)不稳定性并最终形成KH湍流、湍动耗散与辐射冷却相抗衡这一复杂链条。着眼于这一复杂性，本项目将研究内容分为三个紧密相关的层面，取得了如下成果：1) 在具普适性的非均匀磁位形中，对磁流体集体模的一般理论特性做了较深入的探讨。首次系统梳理并评述了近20年来日冕快腊肠模的研究进展；进一步确认了非均匀介质中集体模的混合属性；数值证实了共振吸收机制的鲁棒性并讨论了相关能量平衡。2) 研究了低层大气磁结构中集体模的阻尼及其与高层大气的关联。首次表明低层大气中电阻耗散可能远比共振阻尼有效；探讨了日冕磁结构对类似针状体活动的脉冲式激发源的响应，首次展示了将本征模理论结果与时变计算所得小波谱相结合的必要性。3) 探索了磁结构中扭曲模的加热效应。给出了KH湍流串级率的唯象学解析表达式；首次探讨了磁柱中扭曲模及Alfven波对柱内大气加热的联合贡献；表明重力分层效应有助于扭曲模的耗散；首次将辐射损失自洽纳入能量方程，由此展示了扭曲模经由KH湍流的耗散确可抗衡辐射冷却。利用研究成果，项目组成员共发表第一或通讯作者论文15篇（含13篇ApJ、1篇Space Science Reviews）。