Research progress on coronal extreme ultraviolet waves

Abstract

<p indent="0mm">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.