Statistical Study of Balmer Continuum Enhancement in Solar Flares

TL;DR

This study analyzes 234 solar flare observations to statistically characterize near-ultraviolet continuum enhancements, revealing their occurrence patterns, spatial localization, and relation to flare energetics.

Contribution

It provides the first comprehensive statistical analysis of NUV continuum enhancements across various flare classes using IRIS data.

Findings

NUV enhancements detected in 80 out of 234 flares.

Enhancements mainly occur on flare ribbon edges during the impulsive phase.

Enhancement magnitude increases with flare class, strongest in X-class flares.

Abstract

Identifying the physical mechanisms of continuum emission in solar flares is important to improve our understanding of the transport of energy in the chromosphere. 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 extract candidate enhancement events from pixel-level NUV time series and subsequently eliminate false positives by making use of the temporal and spatial correspondence between NUV and FUV continuum enhancement. We detected NUV continuum enhancements in 80 out of 234 flares. The enhancements occurred predominantly on the flare ribbon edges and during the GOES impulsive phase but also after the GOES peak flux. In a few cases (4 pixels), NUV and FUV continuum enhancement was 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 magnitude 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 the possibility of enhancement occurring preferably in the regions with freshly reconnected magnetic field lines, or the ribbon fronts with gradual and modest high-energy flux injection of the 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.