According to a study led by Dr. YE Jing from Yunnan Observatories of the Chinese Academy of Sciences and Dr. CAI Qiangwei from Luoyang Normal University, there might exist a compressed interface in the Supra-Arcade Fans (SAF) above solar flares, where plasma environment shows remarkable changes in density, temperature and turbulence.

The study was published in The Astrophysical Journal on April 18.

It is well known that a distributed and hot structure named supra-arcade fan exists above post-flare loops during the solar eruption. The location of the SAF is spatially consistent with various emission sources. However, the plasma evolution features in the SAF and the origin of the hard X-ray source are still poorly understood.

Based on observations from the Interface Region Imaging Spectrograph and Hinode for the flare on September 10, 2017, the researchers found that the SAF presented discontinuous characters during the rising process, which indicated a nonuniform magnetic reconnection process. Additionally, the temperature and density increased spatially at the upper boundary of the SAF, which implied that a compressed interface may exist in the SAF.

With the help of numerical experiments and synthetic extreme ultraviolet (EUV) images, the researchers concluded that the interface could be related to termination shocks (TSs), the height of which is close to the hard X-ray source. In turn, the variations of the spectral profiles might be useful tools for identifying TSs from EUV spectral observations.

"This study provides us both observational and theorical evidences that the interface related to TSs might be detected by EUV observations in the future," said Dr. YE.