Finding Missing Flares

Solar flares are one of the most common solar phenomena, but there’s still much to learn about them. Until now, researchers suspected that the high-energy radiation of a solar flare is only occasionally accompanied by a flare at visible wavelengths, but new work suggests that white-light flares aren’t so uncommon after all.

Classes of Solar Flares

Flares are bright, localized flashes of radiation powered by the release of pent-up magnetic energy in a star’s atmosphere. The Sun launches anywhere from tens to hundreds of solar flares each year, and flares are common on other stars as well. While the radiation from a flare can span the entire electromagnetic spectrum, flares are categorized by the strength of their soft (i.e., lower energy) X-ray emission, ranging from relatively mild A-class flares to powerful X-class flares.

In addition to strong X-ray and ultraviolet emission, some flares also brighten across the visible portion of the spectrum. These white-light flares make up just a small fraction of all solar flares, though they appear to be more common on other stars. (The flare associated with the 1859 Carrington event — arguably the most famous solar flare — was the first white-light flare to be recorded.) To understand the origins of white-light flares, it’s critical to determine whether these events are truly as scarce as they seem.

New Search Methods

Recently, Yingjie Cai (Chinese Academy of Sciences; University of Chinese Academy of Sciences) and collaborators demonstrated a new way to find white-light solar flares. First, the team considered where current search methods might be failing. Typically, white-light flares are identified by searching for differences between optical images of the Sun taken at different times. The team’s new method, which was developed by analyzing the properties of previously discovered white-light flares, improves upon the existing method in two key ways: 1) searching for spatial and temporal clusters of brightening at visible wavelengths, which are characteristic of white-light flares, and 2) searching for increased emission relative to the local background level of emission, rather than an arbitrary threshold brightness. This second tactic may enable the discovery of faint white-light flares, which seem to be especially rare.

Cai and coauthors applied their method to a sample of 90 solar flares evenly split between C, M, and X-class flares. Ultimately, they found that 9 of 30 C-class flares, 18 of 30 M-class flares, and 28 of 30 X-class flares could be classified as white-light flares. For a given flare energy class, white-light flares are more common among confined flares — those that are not accompanied by an explosion of plasma into space — than eruptive flares. The team also found that the duration of a white-light flare is related to its energy, with less energetic flares being more fleeting than more energetic flares.

Promising Proportions

At first glance, these results suggest that more powerful flares are more likely to emit strongly at visible wavelengths, in line with previous research. However, the fact that 30% of the C-class flares in this study were white-light flares — the largest proportion of C-class white-light flares found by any study so far — suggests that white-light flares may be common even among weaker flares, just more difficult to detect with current instruments. If this is the case, the development of increasingly sensitive solar telescopes should boost the number of known C-class white-light flares.

Looking forward, the authors plan to refine their identification model, helping to amass a sample of white-light flares that can be used for statistical studies and comparisons with flares on other stars. This should also help to disambiguate the origins of white-light flares; its not yet clear what powers the visible-light emission of a solar flare, nor is it clear if the visible-light emission always arises from the same source.

Citation

“Statistics of Solar White-Light Flares. I. Optimization and Application of Identification Methods,” Yingjie Cai et al 2024 ApJ 975 69. doi:10.3847/1538-4357/ad793b