Optical Spectral Observations of a Flickering White-Light Kernel in a C1 Solar Flare

TL;DR

This study analyzes optical spectra of a C1.1 solar flare, revealing a small, repeatedly brightening kernel with continuum and chromospheric emission, challenging existing models predicting a Balmer jump and suggesting complex opacity effects.

Contribution

First detailed optical spectral analysis of a small, flickering white-light kernel in a C1.1 solar flare, providing new observational constraints for flare modeling.

Findings

Detected a small, flickering brightening kernel in optical continuum.

Found no evidence of a Balmer jump in the excess emission.

Identified a possible 'blue continuum bump' near the Balmer jump.

Abstract

We analyze optical spectra of a two-ribbon, long duration C1.1 flare that occurred on 18 Aug 2011 within AR 11271 (SOL2011-08-18T15:15). The impulsive phase of the flare was observed with a comprehensive set of space-borne and ground-based instruments, which provide a range of unique diagnostics of the lower flaring atmosphere. Here we report the detection of enhanced continuum emission, observed in low-resolution spectra from 3600 \AA\ to 4550 \AA\ acquired with the Horizontal Spectrograph at the Dunn Solar Telescope. A small, $\le$0''.5 ($10^{15}$ cm$^2$) penumbral/umbral kernel brightens repeatedly in the optical continuum and chromospheric emission lines, similar to the temporal characteristics of the hard X-ray variation as detected by the Gamma-ray Burst Monitor (GBM) on the Fermi spacecraft. Radiative-hydrodynamic flare models that employ a nonthermal electron beam energy flux high enough to produce the optical contrast in our flare spectra would predict a large Balmer jump in emission, indicative of hydrogen recombination radiation from the upper flare chromosphere. However, we find no evidence of such a Balmer jump in the bluemost spectral region of the continuum excess. Just redward of the expected Balmer jump, we find evidence of a "blue continuum bump" in the excess emission which may be indicative of the merging of the higher order Balmer lines. The large number of observational constraints provides a springboard for modeling the blue/optical emission for this particular flare with radiative-hydrodynamic codes, which are necessary to understand the opacity effects for the continuum and emission line radiation at these wavelengths.