Eruption-Related Ultraviolet Irradiance Enhancements Associated with Flares

TL;DR

This study quantifies how eruption-related material contributes to ultraviolet flare irradiance, separating eruption emission from flare ribbons across nine eruptive M- and X-class flares (2022–2025) using masking in UV images and supplementary X-ray and photometric data. It finds sample-averaged eruption energy fractions during the impulsive phase of $10^{+4}_{-4}\%$ in 131 Å, $24^{+14}_{-14}\%$ in 171 Å, $21^{+14}_{-10}\%$ in 304 Å, and $13^{+6}_{-9}\%$ in 1600 Å, with individual events reaching much higher values (e.g., up to 100\% in 171 Å for an occulted flare). Three events with HXIs show limited or no spatial overlap between nonthermal heating and erupted material, suggesting that eruption brightenings are not universally driven by nonthermal electrons. The results imply that eruption-related UV emission can significantly influence flare spectra and light curves, affecting Sun-as-a-star and stellar flare interpretations and motivating future modeling to identify heating mechanisms such as nonthermal particle heating, Ohmic heating, or MHD wave dissipation.

Abstract

Large solar flares (GOES M-class or higher) are usually associated with eruptions of material. However, when considering flare irradiance enhancements and dynamics such as chromospheric evaporation, potential contributions from erupted material have historically been neglected. We analyse nine eruptive M- and X-class flares from 2024 to early 2025, quantifying the relative contributions of erupted material to irradiance enhancements during the events. SDO/AIA images from four different channels had ribbon and eruption irradiance contributions separated using a semi-automated masking method. The sample-averaged percentages of excess radiated energy by erupted material over the impulsive phase were $10^{+4}_{-4}\%$, $24^{+14}_{-14}\%$, $21^{+14}_{-10}\%$ and $13^{+6}_{-9}\%$ for the $131\,$Å, $171\,$Å, $304\,$Å and $1600\,$Å channels, respectively. For three events that were studied in further detail, HXR imaging showed little to no signatures of nonthermal heating within the eruptions. Our results suggest that erupted material can be a significant contributor to UV irradiance enhancements during flares, with possible heating mechanisms including nonthermal particle heating, Ohmic heating, or dissipation of MHD waves. Future work may clarify the heating mechanism and evaluate the impact of eruptions on spectral variability, particularly in Sun-as-a-star and stellar flare observations.

Figures (8)

• Figure 1: AIA $304\,\textrm{Å}$ images of M1.9 flare on 21 December 2024 with eruption and ribbon masks overlaid. Panel a.) shows a global view of the event. A close-up of the flare and eruption is shown in panel b.).
• Figure 2: X-ray and UV emission during M2.0 flare on 2 March 2022. Panel a.) shows lightcurves from STIX and XRS. Excess Ly$\alpha\;$ emission measured by EUVS-B is displayed in panel b.). Panels c.) and d.) show spatially-separated flare excesses from the flare's eruption and ribbons from four AIA channels (radiated power) and the two channels of HRI (count rate). Black dashed lines indicate GOES start, peak and end times from left to right.
• Figure 3: AIA $304\,\textrm{Å}$ images of M2.0 flare on 2 March 2022 with HXR image contours at 30%, 50%, 70% and 90% overlaid, shown in panels a.) through f.). Also shown in panel a.) is the mask used to spatially partition the eruption and ribbon contributions to overall flare irradiance enhancement. Panel b.) shows a closer view of the ribbon mask. In panel f.), yellow arrows indicate the locations of erupted material. Panel g.) shows X-ray lightcurves from STIX for emission from $20-32\,\textrm{keV}$ (pink) and $5-15\,\textrm{keV}$ (blue), with the time intervals over which each HXR image in panels a.)-f.) was generated indicated by vertical dashed lines.
• Figure 4: X-ray and UV emission during X9.0 flare on 3 October 2024. Panel a.) shows lightcurves from HXI and XRS. Excess Ly$\alpha\;$ emission measured by EUVS-B and LYRA is displayed in panel b.). Panels c.) and d.) show spatially-separated excess radiated power from the flare's eruption and ribbons from four AIA channels. Black dashed lines indicate GOES start, peak and end times from left to right.
• Figure 5: AIA $304\,\textrm{Å}$ images of X9.0 flare on 3 October 2024 with HXR image contours at 10%, 50% and 90%, and the mask used to spatially partition the eruption and ribbon contributions to overall flare irradiance enhancement overlaid. HXR images were generated for energies of $20-50\,\textrm{keV}$ and $15-20\,\textrm{keV}$ over a time interval of $30\,\textrm{s}$ (12:17:00-12:17:30 UT). Panel a.) shows a global view of the event. A close-up of the flare is shown in panel b.).