Observations of Electron-driven Evaporation during a Flare Precursor

TL;DR

This study observes electron-driven explosive chromospheric evaporation during a solar flare precursor, linking spectral line shifts with nonthermal emissions and confirming electron-driven evaporation as a key process.

Contribution

It provides direct spectroscopic evidence of electron-driven evaporation during the flare precursor, highlighting the correlation between spectral shifts and nonthermal emissions.

Findings

Fe XXI shows blue shifts indicating hot plasma upflows.

Si IV exhibits red shifts indicating cooler plasma downflows.

Blue shifts correlate with microwave and HXR emissions, confirming electron-driven evaporation.

Abstract

We investigate the relationship between the blue shifts of a hot emission line and the nonthermal emissions in microwave and hard X-ray (HXR) wavelengths during the precursor of a solar flare on 2014 October 27. The flare precursor is identified as a small but well-developed peak in soft X-ray and extreme-ultraviolet passbands before the GOES flare onset, which is accompanied by a pronounced burst in microwave 17 & 34 GHz and HXR 25-50 keV. The slit of Interface Region Imaging Spectrograph (IRIS) stays at one ribbon-like transient during the flare precursor, where shows visible nonthermal emissions in NoRH and RHESSI images. The IRIS spectroscopic observations show that the hot line of Fe XXI 1354.09 A (logT ~ 7.05) displays blue shifts, while the cool line of Si IV 1402.77 A (logT ~ 4.8) exhibits red shifts. The blue shifts and red shifts are well correlated to each other, indicative of an explosive chromospheric evaporation during the flare precursor particularly combining with a high nonthermal energy flux and a short characteristic timescale. In addition, the blue shifts of Fe XXI 1354.09 A are well correlated with the microwave and HXR emissions, implying that the explosive chromospheric evaporation during the flare precursor is driven by nonthermal electrons.