Cold, tenuous solar flare: acceleration without heating

TL;DR

This paper reports a rare solar flare that exhibits strong non-thermal emissions and electron acceleration without significant thermal heating, challenging existing models of flare energy release.

Contribution

It provides detailed observations of a cold, tenuous flare with direct evidence of microwave emission originating in the acceleration region, a novel insight into flare physics.

Findings

High electron acceleration rate (~10^35 electrons/sec above 10 keV)

Electrons reach energies up to 100 keV

GOES temperatures remain below 6.1 MK

Abstract

We report the observation of an unusual cold, tenuous solar flare, which reveals itself via numerous and prominent non-thermal manifestations, while lacking any noticeable thermal emission signature. RHESSI hard X-rays and 0.1-18 GHz radio data from OVSA and Phoenix-2 show copious electron acceleration (10^35 electrons per second above 10 keV) typical for GOES M-class flares with electrons energies up to 100 keV, but GOES temperatures not exceeding 6.1 MK. The imaging, temporal, and spectral characteristics of the flare have led us to a firm conclusion that the bulk of the microwave continuum emission from this flare was produced directly in the acceleration region. The implications of this finding for the flaring energy release and particle acceleration are discussed.