Extreme-ultraviolet transient brightenings in the quiet-Sun corona

Closest perihelion observations with Solar Orbiter/EUI

¹ Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, 1180 Brussels, Belgium
² Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
³ Centre Spatial de Liège, Université de Liège, Av. du Pré-Aily B29, 4031 Angleur, Belgium
⁴ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁵ Mathematics Institute, St Andrews University, KY16 9SS St Andrews, UK
⁶ Centre for mathematical Plasma Astrophysics (CmPA), Department of Mathematics, KU Leuven, 3001 Leuven, Belgium
⁷ Institute of Geodynamics of the Romanian Academy, Bucharest, Romania

^⋆ Corresponding author: nancy.narang@oma.be

Received: 19 March 2025
Accepted: 6 May 2025

Abstract

Context. The extreme-ultraviolet (EUV) brightenings identified by Solar Orbiter, commonly known as campfires, are the smallest transient brightenings detected to date outside active regions in the solar corona.

Aims. In order to understand their possible contribution to quiet-Sun heating, we investigated the spatio-temporal distribution of a large ensemble of the finest scale EUV transient brightenings observed by the Extreme Ultraviolet Imager (EUI) aboard Solar Orbiter.

Methods. We performed a statistical analysis of the EUV brightenings by using quiet-Sun observations at the highest possible spatial resolution ever obtained by the EUI. We used observations in the 17.4 nm passband of the High Resolution EUV Imager (HRI_EUV) of EUI acquired during the closest perihelia of Solar Orbiter in 2022 and 2023. Solar Orbiter being at a distance 0.293 AU from the Sun, these observations have an exceptionally high image scale of 105 km, recorded at a fast cadence of 3 seconds. We used a wavelet-based automatic detection algorithm to detect and characterise the events of interest, and we studied their morphological and photometrical properties.

Results. We report the detection of the smallest and shortest lived EUV brightenings to date in the quiet Sun. The size and lifetime of the detected EUV brightenings appear power-law distributed down to a size of 0.01 Mm² and a lifetime of 3 seconds. In general, their sizes lie in the range of 0.01 Mm² to 50 Mm², and their lifetimes vary between 3 seconds and 40 minutes. We find an increasingly high number of EUV brightenings on smaller spatial and temporal scales. We estimate that about 3600 EUV brightenings appear per second on the whole Sun. The HRI_EUV brightenings thus represent the most prevalent, localised, and finest scale transient EUV brightenings in the quiet regions of the solar corona.

Conclusions. Using observations from EUI/HRI_EUV at the highest possible achievable spatial resolution with the fastest cadence ever attained for quiet-Sun EUV observations, we detect the smallest and shortest lived EUV brightenings to date. Future studies that can provide estimates of the thermal energy content of the smallest-scale EUV brightenings will help to provide better insights into their role in the coronal heating.