How important are electron beams in driving chromospheric evaporation in the 2014 March 29 flare?

TL;DR

This study uses high-resolution IRIS and RHESSI observations to investigate the role of electron beams and conduction in driving chromospheric evaporation during the 2014 March 29 solar flare, revealing conduction's significant role throughout the flare.

Contribution

The paper provides observational evidence that conduction can drive chromospheric evaporation during both the peak and decay phases of a solar flare, challenging the exclusive role of electron beams.

Findings

Conduction likely drives evaporation during the decay phase.

Electron beams may initiate evaporation during the flare's early phase.

Upflows are sometimes observed without coincident HXR sources.

Abstract

We present high spatial resolution observations of chromospheric evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging Spectrograph (IRIS) observations of the FeXXI 1354.1 A line indicate evaporating plasma at a temperature of 10 MK along the flare ribbon during the flare peak and several minutes into the decay phase with upflow velocities between 30 km s$^{-1}$ and 200 km s$^{-1}$. Hard X-ray (HXR) footpoints were observed by RHESSI for two minutes during the peak of the flare. Their locations coincided with the locations of the upflows in parts of the southern flare ribbon but the HXR footpoint source preceded the observation of upflows in FeXXI by 30-75 seconds. However, in other parts of the southern ribbon and in the northern ribbon the observed upflows were not coincident with a HXR source in time nor space, most prominently during the decay phase. In this case evaporation is likely caused by energy input via a conductive flux that is established between the hot (25 MK) coronal source, which is present during the whole observed time-interval, and the chromosphere. The presented observations suggest that conduction may drive evaporation not only during the decay phase but also during the flare peak. Electron beam heating may only play a role in driving evaporation during the initial phases of the flare.