Quasi-periodic fast-mode propagating (QFP) wave trains in the corona have been studied intensively in the past decade, thanks to the full-disk, high spatiotemporal resolution, and widetemperature coverage observations taken by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). In AIA observations, QFP wave trains are seen to consist of multiple coherent and concentric wavefronts emanating successively near the epicenter of the accompanying flares; they propagate outwardly either along or across coronal loops at fast-mode magnetosonic speeds from several hundred to more than 2000 km s−1, and their periods are in the range of tens of seconds to several minutes. Based on the distinct different properties of QFP wave trains, they might be divided into two distinct categories including narrow and broad ones. For most QFP wave trains, some of their periods are similar to those of quasi-periodic pulsations (QPPs) in the accompanying flares, indicating that they are probably different manifestations of the same physical process. Currently, candidate generation mechanisms for QFP wave trains include two main categories: pulsed energy excitation mechanism in association with magnetic reconnection and dispersion evolution mechanism related to the dispersive evolution of impulsively generated broadband perturbations. In addition, the generation of some QFP wave trains might be driven by the leakage of three and five minute oscillations from the lower atmosphere. As one of the new discoveries of SDO, QFP wave trains provide a new tool for coronal seismology to probe the corona parameters, and they are also useful for diagnosing the generation of QPPs, flare processes including energy release and particle accelerations.
Prof. SHEN Yuandeng from the Yunnan Observatories of the Chinese Academy of Sciences has observed a quasi-periodic fast-propagating (QFP) wave train along open magnetic field lines in the outer corona at a height of about two to four solar radii in the white-light images. The images were taken by the Large Angle Spectroscopic Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO).
Their findings were published in Astronomy & Astrophysics on Sept. 8.
QFP wave trains are usually observed in the low corona using extreme ultraviolet images, and can be divided into narrow and broad QFP wave trains propagating along and perpendicular to magnetic field, respectively. Both types are tightly associated with periodic pulsations in flares, but their excitation mechanisms are still an open question.
The researchers found a magnetic breakout topology in the low corona eruption source region, which is composed of a large high-lying loop hosting three low-lying loops. Different to typical breakout reconnection which occurs between the middle low-lying and the high-lying large loops, the reconnection occurred between the two sided low-lying loops.
“Such reconnection does not result in a coronal mass ejection in the outer corona, since the reconnection continuously generates new reconnected high-lying large loops so that the confining ability of the high-lying loop becomes more and more strong. Therefore, the eruption in the low corona should be a failed eruption,” said Prof. SHEN.
This process also leads to successive stretching and expanding of the newly formed reconnected high-lying loops, which can naturally excite the observed QFP wave train owing to the pulsed increase of outward magnetic and plasma pressures.
This work presents the first white-light imaging observation of QFP wave trains in the outer corona, and provides the detailed energy releasing process in the low corona magnetic reconnection that subsequently excited the observed QFP wave train.