Recovering the Hidden Core of Solar Flares in Saturated Extreme-Ultraviolet Images: First Validation with EUI Data

(Science nugget #98 by S. Guastavino^1,2, D. Berghmans³, C. Verbeeck³, S. J. Hofmeister⁴, E. Kraaikamp³, E. C. M. Dickson³, F. Benvenuto¹, P. Massa⁵, A. M. Massone^1,2, and M. Piana^1,2)

Introduction

Extreme-ultraviolet (EUV) observations provide essential information on the thermal structure and evolution of active regions, flares, and eruptive phenomena. Since 2010, the Atmospheric Imaging Assembly (AIA; [1]) onboard NASA's Solar Dynamics Observatory [2] has produced an unparalleled archive of multiwavelength EUV observations from an Earth orbit, while the Extreme Ultraviolet Imager (EUI; [3]) onboard ESA's Solar Orbiter [4] now complements these measurements with high-resolution observations from a different heliocentric orbit. Although, in principle, the AIA archive is an extraordinary resource for statistical studies of solar activity at EUV wavelengths, observations of the brightest flares are affected by severe CCD primary saturation. Saturation causes blooming, while the diffraction patterns introduced by the instrument's point spread function become particularly visible around the saturated region. As a result, the flare core is corrupted in the most energetic and thus scientifically most interesting events, and prevents systematic quantitative analysis. An example of a saturated image is shown in the right panel of Figure 1.

Figure 1. Left panel: relative Sun-spacecraft geometry at 23:30 UT on March 19, 2024. Right panel: AIA saturated image at 23:24:57 UT on March 19, 2024.

Inverse Diffraction and Validation

In a series of papers dating back to the late 2010's [5,6,7], an inverse-diffraction approach has been introduced to recover the signal lost in the primary saturation region by exploiting the diffraction component of the instrument point spread function. The current methodology, called Adaptive Sparsity-Enhancing DESAT (SE-DESAT [7,8]) combines adaptive sparsity-promoting regularization [9] with expectation-maximization to identify saturated pixels, estimate the background, and recover the missing EUV emission [8]. However, until now, validation relied only on simulations or internal consistency since no observational ground truth existed. Solar Orbiter/EUI observes the Sun in a passband closely matching AIA 171 Å while avoiding saturation thanks to its ability to take short exposure images. Therefore, when the same flare is observed simultaneously by both spacecraft, the desaturated AIA image can be directly compared with an independent unsaturated EUI observation after standard co-alignment and radiometric corrections. This provides the first observational validation of the method.

Figure 2. Zoomed-in crop of the flaring site on March 19, 2024 at a solar-referenced time of 23:16:39 UT. Left panel: original saturated image; right panel: desaturated image with the inverse-diffraction Adaptive SE-DESAT method.

Results

The M2.1 flare of March 19, 2024 offers an ideal testbed for validating inversion-based desaturation because of the radial alignment between SDO and Solar Orbiter, as shown in the left panel of Figure 1. Figure 2 shows a zoomed crop of the flaring region of interest: the left panel shows the original AIA image, severely affected by saturation, while the right panel shows the corresponding desaturated image (note the different intensity scales). As shown in the top panels of Figure 3, the desaturated AIA image shows excellent morphological agreement with the corresponding EUI observation, while the original saturated image strongly underestimates the flare emission. The agreement is confirmed photometrically by the temporal evolution of the brightest pixels as shown in the bottom panels of Figure 3, demonstrating that the reconstructed signal reliably reproduces the flare dynamics.

Figure 3. Top-left: Solar Orbiter/EUI/HRI_EUV short-exposure image of the flaring site on March 19, 2024 at the solar-referenced time of 23:16:39 UT. Top-right: overlay of the reprojected EUI/HRI_EUV image and the desaturated AIA frame, illustrating close spatial and morphological correspondence. The original AIA and EUI frames were acquired at 23:24:57 UT and 23:20:16 UT, respectively, and have been corrected for light-travel time and reprojected to a common helioprojective grid. Bottom panels represent time series of the sum of, respectively, the three and eight brightest pixels for EUI (graphically rescaled by 10^-2, red), AIA original (green), and AIA desaturated (blue).

Movie 1. An animation showing the comparison between the desaturated AIA images and the corresponding Solar Orbiter/EUI/HRIEUV short-exposure images of the flaring site on March 19, 2024.

Conclusions

The comparison with Solar Orbiter/EUI demonstrates that inverse-diffraction desaturation reliably restores both the intensity and morphology of saturated AIA flare cores. Apart from a modest discrepancy during the earliest impulsive phase, the restored signal closely follows the independent EUI observations. This validation opens the possibility of exploiting the full AIA archive, including the most intense saturated flares, for quantitative statistical studies of flare energetics, timing, and morphology.

Acknowledgements

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The Royal Observatory of Belgium team thanks the Belgian Federal Science Policy Office (BELSPO) for the provision of financial support in the framework of the PRODEX program of ESA under contract number 4000143743. The EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB, LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA 4000134088, 4000112292, 4000117262, and 4000134474); the Centre National d'Études Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through Deutsches Zentrum für Luft- und Raumfahrt (DLR); and the Swiss Space Office (SSO). "EUI Data Release 6.0 2023-01" is public and can be freely downloaded from the EUI website (https://www.sidc.be/EUI/data/) and from the Solar Orbiter Archive (http://soar.esac.esa.int/soar/). S.G. acknowledges the EUI Guest Investigator Call 2025.

Publication

This nugget is based on the following work: Guastavino, S., Berghmans, D., Verbeeck, C., Hofmeister, S. J., Kraaikamp, E., Dickson, E. C. M., Benvenuto, F., Massa, P., Massone, A. M., & Piana, M., "Validation of an Extreme-ultraviolet Desaturation Technique for the Atmospheric Imaging Assembly on Board the Solar Dynamics Observatory Using Observations from the Extreme Ultraviolet Imager on Board Solar Orbiter", The Astrophysical Journal Letters, 1005, L50 (2026). https://doi.org/10.3847/2041-8213/ae7d2e

Affiliations

(1) MIDA, Dipartimento di Matematica, Università di Genova, Via Dodecaneso 35, I-16146 Genoa, Italy

(2) Osservatorio Astrofisico di Torino, Istituto Nazionale di Astrofisica, via Osservatorio 20, I-10025 Pino Torinese, Italy

(3) Solar-Terrestrial Centre of Excellence - SIDC, Royal Observatory of Belgium, Ringlaan 3 Avenue Circulaire, B-1180 Brussels, Belgium

(4) Columbia Astrophysics Laboratory, Columbia University, New York, USA

(5) University of Applied Sciences and Arts Northwest Switzerland (FHNW), School of Computer Science, Bahnhofstrasse 6, Windisch 5210, Switzerland

References

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