An unambiguous large-scale quasi-periodic coronal wave consisting of multiple wave fronts has been found by Dr. SHEN Yuandeng from Yunnan Observatories, Chinese Academy of Sciences, which occurred on 2012 April 24 and was accompanied by the eruptions of a GOES C3.7 flare and a rotating filament. This finding, published in The Astrophysical Journal recently, makes us a better understanding the intriguing phenomenon of large-scale coronal waves.

A coronal wave appears as a single circular propagating bright front in Extreme Ultraviolet wavelengths, and most recent high-resolution observations have indicated that it is fast-mode magnetosonic wave driven by coronal mass ejections (CMEs). However, any disturbance in the corona can launch waves in theory; therefore, there must be other driving mechanism of coronal waves besides CMEs.

It is reasonable that a CME drives a coronal wave ahead of it, and due to the continuous driving of the CME, the wave can last for tens of minutes to hours. Now, the finding of quasi-periodic coronal waves leaves us a question: how large-scale quasi-periodic coronal waves are excited?

"By comparing the periods of the flare, the filament’s unwinding helical structures, and the waves, we found that the periods of the wave and the unwinding filament’s helical structures are almost the same.” Dr. SHEN said, “Therefore, we propose that this large-scale quasi-periodic wave should be excited by the expanding motion of the filament’s helical structures.”

In addition to the excitation mechanism, the quasi-periodic wave on April 24 also provides a rare opportunity for researchers to test different seismology methods that are usually used to estimate coronal parameters.

Since a part of the wave fronts propagated along a group of closed loop system and caused the kink oscillation of the latter, researchers, for the first time, estimated the magnetic field strength of the loop by taking advantage of the properties of the oscillation and the quasi-periodic wave. The two methods yield similar results, indicating that these seismology methods are reasonable.

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