The first solar flare to be formally documented was recorded by astronomer Richard Carrington, who observed a 'white light' flare in September 1859 - projecting the image produced by an optical telescope, without filters. See: Wikipedia
However, the mechanisms producing the white light flares and their emissions have yet to be fully explained. This extract is from a 2016 paper from the American Astronomical Society, SPD meeting #47, id.#6.17
“The white-light continuum of a solar flare was the first manifestation of a solar flare ever detected. Nevertheless, its mechanisms remain unknown, even today. Improved observations confirm the identification of white-light continuum emission and hard X-rays during the impulsive phase of a solar flare, both in space and in time, to within the observational limits. Two events observed near the limb, but not occulted by it (SOL2011-02-24 and SOL2012-02-18), show that these emissions appear to have physical heights lower than predicted by models by hundreds of kms, referring height to the location of optical-depth unity at disk center in the 500 nm continuum. We describe these results and place them in the context of the three extreme-limb events (within about 1o) reported by Krucker et al. (2015). The electrons responsible for hard X-ray bremsstrahlung coincide with the most intense flare energy release, but we do not presently understand the physics of energy transport nor the nature of particle acceleration apparently taking place at heights below the preflare temperature minimum.”
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