A research team led by 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.