New Observations of Balmer Continuum Flux in Solar Flares, Instrument Description and First Results

TL;DR

This paper reports the development of a new instrument capable of measuring Balmer continuum flux in solar flares, confirming flux increases of up to 550% during an X-class flare and detecting early flux changes before H-alpha onset.

Contribution

Introduction of a novel instrument and methodology for measuring Balmer continuum flux from entire solar flare areas, enabling new observational insights.

Findings

Detected Balmer continuum flux increase of 230%-550% during an X-class flare.

Instrument proved sensitive enough to observe flux variations before H-alpha flare onset.

First ground-based confirmation of predicted Balmer continuum increase during solar flares.

Abstract

Increase in the Balmer continuum radiation during solar flares was predicted by various authors but never firmly confirmed observationally using ground-based slit spectrographs. Here we describe a new post-focal instrument - Image Selector - enabling to measure the Balmer continuum flux from the whole flare area, in analogy of successful detections of flaring dMe stars. The system was developed and put into operation at the horizontal solar telescope HSFA-2 of the Ond\v{r}ejov Observatory. We measure the total flux by a fast spectrometer from a limited but well defined region on the solar disk. Using a system of diaphragms, the disturbing contribution of a bright solar disk can be eliminated as much as possible. Light curves of the measured flux in the spectral range 350 - 440 nm are processed, together with the H{\alpha} images of the flaring area delimited by the appropriate diaphragm. The spectral flux data are flat-fielded, calibrated and processed to be compared with model predictions. Our analysis of the data proves that the described device is sufficiently sensitive to detect variations in the Balmer continuum during solar flares. Assuming that the Balmer-continuum kernels have at least a similar size as those visible in H\alpha, we find the flux increase in the Balmer continuum to reach 230 % - 550 % of the quiet continuum during the observed X-class flare. We also found temporal changes in the Balmer continuum flux starting well before the onset of the flare in H{\alpha}.