Probing the Production of Extreme-ultraviolet Late-Phase Solar Flares by Using the Model Enthalpy-Based Thermal Evolution of Loops

TL;DR

This study uses the EBTEL model to numerically investigate the mechanisms behind EUV late-phase solar flares, distinguishing between long-lasting cooling and secondary heating effects on emission characteristics.

Contribution

It introduces a new diagnostic method based on EUV light curve shapes and compares the hydrodynamic evolutions of different late-phase flare mechanisms.

Findings

Both mechanisms can produce EUV late-phase flares matching observations.

Cooling processes differ in timing: radiative cooling for long-lasting cooling, conductive cooling for secondary heating.

Most flares are not EUV late due to energy input partition and emission factors.

Abstract

Recent observations in extreme-ultraviolet (EUV) wavelengths reveal an EUV late phase in some solar flares that is characterized by a second peak in warm coronal emissions ($\sim3$~MK) several tens of minutes to a few hours after the soft X-ray (SXR) peak. Using the model enthalpy-based thermal evolution of loops (EBTEL), in this paper we numerically probe the production of EUV late-phase solar flares. Starting from two main mechanisms of producing the EUV late phase, i.e., long-lasting cooling and secondary heating, we carry out two groups of numerical experiments to study the effects of these two processes on the emission characteristics in late-phase loops. In either of the two processes an EUV late-phase solar flare that conforms to the observational criteria can be numerically synthesized. However, the underlying hydrodynamic and thermodynamic evolutions in late-phase loops are different between the two synthetic flare cases. The late-phase peak due to a long-lasting cooling process always occurs during the radiative cooling phase, while that powered by a secondary heating is more likely to take place in the conductive cooling phase. We then propose a new method for diagnosing the two mechanisms based on the shape of EUV late-phase light curves. Moreover, from the partition of energy input, we discuss why most solar flares are not EUV late flares. Finally, by addressing some other factors that may potentially affect the loop emissions, we also discuss why the EUV late phase is mainly observed in warm coronal emissions.