Eruption-Related Ultraviolet Irradiance Enhancements Associated with Flares

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.

💡 Research Summary

This paper investigates the contribution of eruptive material associated with large solar flares to ultraviolet (UV) irradiance enhancements, a component that has traditionally been neglected in flare energy budgets. The authors selected nine eruptive M‑ and X‑class flares that occurred between 2024 and early 2025, using strict criteria to ensure that each event featured a clearly visible eruption distinct from the flare ribbons and that the eruption lay within 50″–200″ of the flare site. The primary data set consists of high‑cadence (12 s) images from the Solar Dynamics Observatory’s Atmospheric Imaging Assembly (SDO/AIA) in four channels: 131 Å (hot coronal plasma), 171 Å (mid‑temperature corona), 304 Å (transition‑region He II), and 1600 Å (continuum and C IV). Complementary observations from Solar‑Orbiter/EUI, STEREO/EUVI, and hard X‑ray imagers (STIX and ASO‑S/HXI) were used for three events to probe non‑thermal signatures and to provide additional spatial context.

The core methodology involves constructing two masks for each flare: a “ribbon mask” that encloses the brightest 20 % contour of the 304 Å image and an “eruption mask” defined as a concentric sector extending outward from the ribbon edge to a manually chosen radius. The masks are applied to every frame in each AIA channel; pixel‑wise digital numbers (DN) within each mask are summed, then converted to physical radiated power using the AIA response functions provided in SolarSoft. By integrating the power over the impulsive phase (GOES start to peak) and, for the 304 Å and 1600 Å channels, over the full flare duration, the authors obtain the total radiated energy contributed by ribbons and by the eruption separately.

The statistical results show that eruptive material contributes a non‑negligible fraction of the UV energy budget: on average 10 % ± 4 % in the 131 Å channel, 24 % ± 14 % in 171 Å, 21 % ± 12 % in 304 Å, and 13 % ± 7 % in 1600 Å. The larger percentages in the mid‑temperature (171 Å) and transition‑region (304 Å) channels indicate that the erupted plasma is heated to temperatures of 10⁴–10⁵ K, consistent with the expected thermodynamic state of prominence or filament material that has been energized during the flare.

Hard X‑ray imaging for three events reveals little or no non‑thermal HXR emission co‑spatial with the eruptive masks, suggesting that the heating of the erupted plasma is not primarily driven by precipitating electron beams. The authors discuss alternative heating mechanisms, including Ohmic dissipation of currents generated during magnetic reconnection, damping of magnetohydrodynamic (MHD) waves, and collisional excitation within the dense prominence material. The fact that the erupted material often covers a much larger area than the ribbons, despite having lower surface brightness, explains how it can still contribute a sizable fraction of the total UV output.

The paper also compares disk‑integrated UV measurements from PROBA‑2/LyRA and GOES/EXIS EUVS‑B with the spatially resolved AIA results. The authors find that the excess irradiance associated with eruptions is detectable in full‑Sun irradiance records, implying that Sun‑as‑a‑star observations of stellar flares could misinterpret such enhancements as purely chromospheric signatures. This has ramifications for interpreting blue asymmetries, line broadening, and overall UV variability in stellar flare spectra.

Limitations acknowledged by the authors include the subjectivity of the manual masking procedure, the lack of absolute radiometric calibration for the auxiliary instruments (EUI, EUVI), the modest sample size, and the possibility of residual overlap between ribbon and eruption emission. They propose future work involving automated, machine‑learning‑based segmentation, multi‑wavelength (radio, NUV, EUV) coordinated campaigns, and three‑dimensional MHD simulations to quantify the heating processes more rigorously.

In conclusion, the study provides robust evidence that eruptive material can account for up to roughly a quarter of the UV radiated energy during large solar flares. This finding challenges the conventional flare energy partition that attributes UV emission almost exclusively to ribbons and coronal loops. Incorporating eruptive contributions is essential for accurate flare energy budgets, for interpreting Sun‑as‑a‑star observations, and for improving models of flare‑driven ionospheric disturbances on Earth.