Implications for additional plasma heating driving the extreme-ultraviolet late phase of a solar flare with microwave imaging spectroscopy

TL;DR

This study investigates the late-phase EUV emission in a solar flare, linking it to additional plasma heating driven by nonthermal electrons, using microwave imaging spectroscopy to analyze the timing and origin of the emission.

Contribution

It provides new insights into the origin of the EUV late phase by correlating microwave observations with EUV data, highlighting the role of additional heating processes.

Findings

Microwave enhancement occurs about three minutes after the main flare peak.

Radio sources are located at the footpoints of ELP loops.

Spectral analysis indicates non-Maxwellian electron distributions.

Abstract

Extreme-ultraviolet late phase (ELP) refers to the second extreme-ultraviolet (EUV) radiation enhancement observed in certain solar flares, which usually occurs tens of minutes to several hours after the peak of soft X-ray emission. The coronal loop system that hosts the ELP emission is often different from the main flaring arcade, and the enhanced EUV emission therein may imply an additional heating process. However, the origin of the ELP remains rather unclear. Here we present the analysis of a C1.4 flare that features such an ELP, which is also observed in microwave wavelengths by the Expanded Owens Valley Solar Array (EOVSA). Similar to the case of the ELP, we find a gradual microwave enhancement that occurs about three minutes after the main impulsive phase microwave peaks. Radio sources coincide with both footpoints of the ELP loops and spectral fits on the time-varying microwave spectra demonstrate a clear deviation of the electron distribution from the Maxwellian case, which could result from injected nonthermal electrons or nonuniform heating to the footpoint plasma. We further point out that the delayed microwave enhancement suggests the presence of an additional heating process, which could be responsible for the evaporation of heated plasma that fills the ELP loops, producing the prolonged ELP emission.