选址与日冕观测组知识库

高温低密日冕磁化等离子体介质可承载多种波动模式的传播.本文主要介绍低日冕中两类常见的极紫外波动现象:大尺度极紫外波和准周期快磁声波.大尺度极紫外波是低日冕中全球性传播的大尺度扰动现象,它通常与耀斑、日冕物质抛射等剧烈太阳爆发活动紧密相关. 20世纪60年代,大尺度扰动现象(莫尔顿波)首先在太阳色球层被观测到,相应的理论模型预言了低日冕中也必然存在与莫尔顿波相关的大尺度扰动现象.直到20世纪90年代,空间望远镜才探测到与莫尔顿波类似的日冕大尺度波动现象(大尺度极紫外波).然而,关于大尺度极紫外波的物理本质和激发机制长期以来一直存在着巨大分歧.得益于近年来空间和地面太阳望远镜的高(时间、空间)分辨、多波段、多视角观测数据,目前人们对大尺度极紫外波的激发和物理本质有了更深入和较为完备的认识.近年的高分辨观测还揭示了日冕中的另一类波动现象,即准周期快磁声波.本文将总结近年来人们对两类波动的研究进展,指出目前研究中存在的重点和难点问题,并展望未来可能的研究方向. 

The hot, tenuous coronal plasma medium can support the propagation of various kinds of magnetohydrodynamic (MHD) waves, such as Alfvén waves, fast and slow magnetosonic waves. These MHD waves are important for understanding the enigmatic problems of coronal heating and the acceleration of solar winds, as well as the basic physical property of the solar atmosphere and the physics behind solar eruptions. In this review, we mainly introduce two types of fast-propagating extreme ultraviolet (EUV) magnetosonic waves in the corona, namely, the large-scale (global) EUV waves and the quasi-periodic fast-propagating (QFP) magnetosonic waves. EUV waves are large-scale propagating disturbances in the corona; they are intimately related to violent solar eruptions such as flares, coronal mass ejections (CME), and radio type II bursts. In history, large-scale propagating disturbances were firstly discovered in the chromosphere in 1960s, and they were called Moreton waves. In theory, the dense chromosphere can not support the fast propagating of Moreton waves. Therefore solar physicists explained Moreton waves as the chromosphere responses of fast magnetosonic or shock waves in the corona, although people did not detected corona waves in that era due to the lack of coronal observations. Until the 1990s, the long-expected similar large-scale fast-propagating coronal disturbances called EUV waves were observed by the Extreme ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliosphereic Observatory (SOHO). EUV waves were initially thought to be fast mode magnetosonic waves driven by flare pressure pulses, and can be regarded as the coronal counterparts of Moreton waves. However, latter observations raised questions about their driving mechanism and physical nature. For their driving source it was unclear that EUV waves are driven by flare pressure pulses or CMEs. For their physical nature it was unclear that EUV waves are true MHD waves or apparent motions caused by reconfiguration of large-scale coronal magnetic fields or other mechanisms. Thanks to the high spatiotemporal resolution and multi-angle observations provided by the Solar Terrestrial Relations observatory (STEREO) and the Solar Dynamics Observatory (SDO) in recent years, we have achieved a deeper and complete understanding about the generation and physical properties of EUV waves. A common consensus reached in recent years is that at least two types of EUV waves can be detected during the eruption of a CME. One is a fast mode magnetosonic wave or shock at a speed ranged from several hundred to more than one thousand km/s, which corresponds to the coronal counterpart of a chromosphere Moreton wave; the other one whose physical nature is unclear, propagates following the fast one with a low speed generally below 500 km/s. For the driving sources of EUV waves, the majority of high resolution observations showed that they are driven by the lateral expansion of CMEs, and a few studies suggested that they can also be excited by other mechanisms such as flare pressure pulses, sudden loop expansion caused by ambient solar eruptions, coronal jets, and expanding motions of unwinding helical structure of filaments. The high spatiotemporal resolution observations taken by the SDO discovered a new type of fast mode waves called QFP waves, which have multiple wavefronts and generally propagate along magnetic field lines with a speed in the range of several hundred to more than 2000 km/s, and their periods are often similar to the quasi-periodic pulsations in the associate flares. Here we present a summary of the recent research progress about the two types of EUV waves in the corona, and try to point out the key and difficult issues, as well as the possible research topics in the future.