Submillimeter radiation as the thermal component of the Neupert Effect

TL;DR

This paper investigates the Neupert effect in solar flares, demonstrating that submillimeter radiation at 212 GHz acts as the thermal component, with timing and emission characteristics supporting a thermal bremsstrahlung origin.

Contribution

It provides observational evidence linking 212 GHz emission to the thermal component of the Neupert effect, proposing a thermal bremsstrahlung origin shifting from chromosphere to corona.

Findings

212 GHz emission correlates with the thermal component of the Neupert effect.

Maximum hard X-ray flux coincides with the derivative of 212 GHz flux.

Delay at 12 GHz cannot be explained by magnetic trapping.

Abstract

The Neupert effect is the empirical observation that the time evolution of non-thermal emission (e.g. hard X-rays) is frequently proportional to the time derivative of the thermal emission flux (soft X-rays), or, vice versa, that time integrated non-thermal flux is proportional to thermal flux. We analyzed the GOES M2.2 event SOL2011-02-14T17:25, and found that the 212 GHz emission plays quite well the role of the thermal component of the Neupert effect. We show that the maximum of the hard X-ray flux for energies above 50 keV is coincident in time with the time-derivative of the 212 GHz flux, within the uncertainties. The microwave flux density at 15.4 GHz, produced by optically thin gyrosynchrotron mechanism, and hard-X rays above 25 keV mark the typical impulsive phase, and have similar time evolution. On the other hand, the 12 GHz emission is delayed by about 25 seconds with respect of the microwave and hard X-ray peak. We argue that this delay cannot be explained by magnetic trapping of non-thermal electrons. With all the observational evidence, we suggest that the 212 GHz emission is produced by thermal bremsstrahlung, initially in the chromosphere, and shifting to optically thin emission from thehot coronal loops at the end of the gradual phase.