Fine-scale flare structures and their energetic implications from short-exposure extreme-ultraviolet imaging

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To capture the brightest and most rapidly evolving phases of solar flares, the Major Flare Solar Orbiter Observing Plan (SOOP) employs a dedicated short-exposure mode for the High Resolution Imager ( of the Extreme Ultraviolet Imager (EUI), providing high-cadence, high spatial-resolution extreme-ultraviolet (EUV) imaging. We investigated the spatial and temporal organisation of compact flare emission and its implications for local energy deposition, using high-cadence, short-exposure observations of the 2024 March 19 M2.1 flare. We combined the short-exposure (0.04 s) observations with hard X-ray (HXR) timing, imaging, and spectroscopic data from the Spectrometer Telescope for Imaging X-rays (STIX). We characterised impulsive ribbon kernels and later loop strands and compared footpoint areas measured with the Atmospheric Imaging Assembly (AIA), and STIX to constrain the local energy flux carried by flare-accelerated electrons. The short-exposure observations reveal compact flare emission largely obscured by saturation in normal-exposure $ s$^ and AIA imaging. The spatially integrated emission evolves co-temporally with the STIX 22--45 keV HXR emission, with no lag discernible beyond the 2 s sampling. The ribbons comprise repeatedly activated kernels with characteristic separations of ∼1.4--1.7 Mm, while the developing arcade shows a similar strand separation of ∼1.3 Mm. This result indicates that this ∼1--2 Mm spatial organisation persists into the newly formed flare loops. Measured kernel and strand widths of ∼0.4--0.5 Mm lie close to the instrumental resolution limit. The compact footpoint areas are approximately an order of magnitude smaller than those inferred from AIA or STIX, implying nominal, local non-thermal energy fluxes of the order of 10^11 erg cm -2 -1 at the HXR peaks. These estimates remain substantially higher across the tested intensity thresholds, although their absolute values depend on the area definition and spectral modelling The combination of high dynamic range, high spatial resolution, and high cadence in the short-exposure observations reveals a characteristic spatial organisation of the flare emission on scales of ∼1--2 Mm, as well as substantially higher local non-thermal energy fluxes than inferred from conventional EUV or HXR source areas. These results demonstrate the value of flare-optimised EUV imaging for future solar flare observations.

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