Collier, Hannah ; Krucker, Säm ; Hayes, Laura A. ; Kraaikamp, Emil ; Berghmans, David ; Ryan, Daniel F.

submitted to Astronomy & Astrophysics (2025)

Abstract: Aims. Flare ribbons form when energy released by coronal magnetic reconnection is deposited in the low layers of the solar atmosphere. Therefore, by studying the dynamics of flare ribbons, one obtains an indirect measurement of reconnection in the corona. The aim of this work is to quantify the spatial and temporal scales of Extreme Ultraviolet (EUV) flare ribbon kernels as a probe of the spatial extent and duration of energy injection in solar flares. Methods. To do this, unprecedented observations of an M2.5 GOES-class flare from the March 2024 major flare campaign of Solar Orbiter were used. These data were obtained at high-cadence in short-exposure mode with the Extreme Ultraviolet Imager’s high resolution telescope, HRIEUV. Individual kernels were automatically identified using a classical computer vision algorithm. From this, the size distributions of ribbon kernels for different cut-off thresholds were derived. In addition, an average light curve of individual kernels was extracted. Results. Our analysis shows that a significant fraction of EUV flare kernels were unresolved at a pixel scale of 135 km/pix. Furthermore, we derive incredibly short EUV kernel heating times of less than a few seconds. Conclusions. These findings indicate that despite the enhanced spatial resolution offered by Solar Orbiter’s proximity to the Sun at 0.38 AU here, the spatial extent of energy deposition in flares is still unresolved in several instances. These results emphasise the miniscule spatial and short temporal scales involved in fragmented flare energy release. This drives the need to adapt current flare models to incorporate highly transient energy injections over small cross-sectional areas. These findings necessitate an in-depth investigation into the implications of such small areas and short injections on the energy flux deposited in solar flares, and the response of the solar atmosphere.

Keyword(s): Solar Orbiter ; EUI ; flare