Energy budget in the 2017-09-07 "cold" solar flare

TL;DR

This study analyzes a cold solar flare observed with advanced instruments, measuring magnetic fields and energy partitioning, revealing that nonthermal electrons likely drive the thermal response without significant magnetic field variation.

Contribution

First observation of a cold flare with the Expanded Owens Valley Solar Array, providing new magnetic field measurements and energy analysis in such flares.

Findings

Magnetic field measurements showed no significant variation during the flare.

Nonthermal energy deposition is sufficient to explain the thermal response.

Spectral evolution of nonthermal electrons exhibits a prominent soft-hard-soft pattern.

Abstract

A subclass of early impulsive solar flares, cold flares, was proposed to represent a clean case, where the release of the free magnetic energy (almost) entirely goes to acceleration of the nonthermal electrons, while the observed thermal response is entirely driven by the nonthermal energy deposition to the ambient plasma. This paper studies one more example of a cold flare, which was observed by a unique combination of instruments. In particular, this is the first cold flare observed with the Expanded Owens Valley Solar Array and, thus, for which the dynamical measurement of the coronal magnetic field and other parameters at the flare site is possible. With these new data, we quantified the coronal magnetic field at the flare site, but did not find statistically significant variations of the magnetic field within the measurement uncertainties. We estimated that the uncertainty in the corresponding magnetic energy exceeds the thermal and nonthermal energies by an order of magnitude; thus, there should be sufficient free energy to drive the flare. We discovered a very prominent soft-hard-soft spectral evolution of the microwave-producing nonthermal electrons. We computed energy partitions and concluded that the nonthermal energy deposition is likely sufficient to drive the flare thermal response similarly to other cold flares.