Statistical study of Balmer continuum enhancement in solar flares

Identifying the physical mechanisms of continuum emission in solar flares is important for improving our understanding of energy transport in the chromosphere. This requires reliable detection of enhanced continuum emission across different flare classes. This study aims to quantify the occurrence statistics and spatial and temporal characteristics of near-ultraviolet (NUV) continuum enhancements across various classes of solar flares. We analyzed 234 IRIS flare observations and developed two independent detection pipelines. Both pipelines initially extracted candidate enhancement events from pixel-level NUV time series and subsequently eliminated false positives by using the temporal and spatial correspondence between NUV and FUV continuum enhancement. For one of the pipelines, we used Gaussian process regression to quantify the uncertainty in the enhancement magnitudes. We detect NUV continuum enhancements in 80 out of 234 flares. These enhancements occurred predominantly on the flare ribbon edges and during the GOES impulsive phase, as well as after the GOES peak flux. In a few cases (4 pixels), NUV and FUV continuum enhancements were detected 7–15 minutes before the GOES start or more than 20 minutes after the peak, appearing as indistinct bright points in the active regions. Despite large uncertainties for C-class events, enhancement magnitudes increase with flare class, with X-class flares showing the strongest enhancement. Our analysis reveals that the enhancements are confined to localized regions on the flare ribbon edges. In terms of flare energetics, this suggests that enhancements may occur in the regions with freshly reconnected magnetic field lines, or in ribbon fronts with gradual and modest high-energy flux injection by non-thermal electrons. Enhancements found significantly after the flare peak in strong flares further suggest multiple heating episodes. The enhancement strengths of flare events as weak as C1.1 from this study serve as an important constraint for flare simulation models.