Where is the chromospheric response to conductive energy input from a hot pre-flare coronal loop?

TL;DR

This study investigates the pre-flare phase of a solar flare, finding evidence of energy release in the coronal loop without detectable chromospheric heating, challenging conduction-based energy transport models.

Contribution

It provides observational evidence that energy is released in the coronal loop before particle acceleration and questions the role of thermal conduction in chromospheric heating during pre-flare phases.

Findings

No detectable chromospheric emission increase during pre-heating

Energy supplied to the loop is not conducted to the chromosphere as expected

Pre-flare energy release occurs without particle acceleration evidence

Abstract

Before the onset of a flare is observed in hard X-rays there is often a prolonged pre-flare or pre-heating phase with no detectable hard X-ray emission but pronounced soft X-ray emission suggesting that energy is being released and deposited into the corona and chromosphere already at this stage. This work analyses the temporal evolution of coronal source heating and the chromospheric response during this pre-heating phase to investigate the origin and nature of early energy release and transport during a solar flare. Simultaneous X-ray, EUV, and microwave observations of a well observed flare with a prolonged pre-heating phase are analysed to study the time evolution of the thermal emission and to determine the onset of particle acceleration. During the 20 minutes duration of the pre-heating phase we find no hint of accelerated electrons, neither in hard X-rays nor in microwave emission. However, the total energy budget during the pre-heating phase suggests that energy must be supplied to the flaring loop to sustain the observed temperature and emission measure. Under the assumption of this energy being transported toward the chromosphere via thermal conduction, significant energy deposition at the chromosphere is expected. However, no detectable increase of the emission in the AIA wavelength channels sensitive to chromospheric temperatures is observed. The observations suggest energy release and deposition in the flaring loop before the onset of particle acceleration, yet a model in which energy is conducted to the chromosphere and subsequent heating of the chromosphere is not supported by the observations.