Researchers Reveal Thermodynamics of Supra-arcade Fans above Solar Flares

Chinese Academy of Sciences

A study published in The Astrophysical Journal and mainly by Dr. CAI Qiangwei from Luoyang Normal University and Dr. YE Jing from Yunnan Observatories of the Chinese Academy of Sciences reported the numerical work about the supra-arcade fans (SAFs). The findings implied that the formation and evolution of the SAF is related to the turbulence in the flare current sheet.

It is found in many observations that a distributed structure named as supra-arcade fan exists above post-flare loops in solar eruptions. The location of the SAF is spatially consistent with various emission sources. Termination shocks (TSs) that are often regarded as an efficient driver for particle acceleration possibly exist in the SAF.

In order to study the thermodynamical manifestations of the SAF, and the potential detection of TSs in extreme ultraviolet (EUV) images, the researchers performed the high-resolution magnetohydrodynamical (MHD) simulations based on the standard flare model with thermal conduction and radiative cooling. The results showed that the SAF is formed by the reconnection outflows, and it starts oscillating when the tearing mode develops.

Using the streamline tracking method, they found that the motion history and temperature evolution of plasmas inside the SAF indicated that the mass of the SAF comes from the corona, and the plasmas are heated in the reconnecting current sheet.

Besides, the researchers found that the height of the SAF undergoes a decent-ascent trend during the impulsive and the decay phases, which could be explained by the unbalance of the Lorentz force and the pressure force inside the magnetic loops. The motion of the SAF probably explains the observed motion of the hard X-ray sources.

Based on the synthetic EUV images, they showed that TSs are possibly identified in Atmospheric Imaging Assembly wavelengths under some observational conditions. For instance, due to the projection effect and the limitation of observational instrument, the shock surface could change from vertical to tilted in the realistic situation, resulting in the unobvious characteristics of TSs.

This study helps people to better understand the details hidden in the SAF and provides some flexible ways to demonstrate the theory in the future.

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