Extreme-ultraviolet transient brightenings in the quiet-Sun corona: Closest-perihelion observations with Solar Orbiter/EUI

(Solar Orbiter Nugget #67 by N. Narang¹, C. Verbeeck¹, M. Mierla¹, D. Berghmans¹, F. Auchère², L. P. Chitta³, E. Priest⁴, and EUI Team)

1. Introduction

The extreme-ultraviolet (EUV) brightenings identified by Extreme Ultraviolet Imager (EUI) [1] aboard Solar Orbiter [2], commonly known as campfires, are the small-scale transient brightenings detected in the solar corona. The ubiquitous presence of localised transient brightenings throughout the solar atmosphere is generally attributed to an impulsive release of energy by the process of magnetic reconnection [3,4]. In the solar transition region and lower corona, a plethora of small-scale localised brightening events in the quiet Sun, with typical length scales of a few Megameters, has been documented in the literature [5,6]. Despite the detections of a variety of EUV transients in the micro-to-nano-flare energy range, their estimated energy-flux rates have been reported to be insufficient to balance the coronal quiet-Sun energy losses [7,8].

Recently, the EUI/HRIEUV observations of quiet Sun have revealed the presence of new members of the small-scale EUV transient brightenings in the solar corona [9,10]. Commonly known as campfires, these HRIEUV brightenings appear in the lower corona, and above the locations of supergranular network boundaries. They have been reported to have sizes between 0.08 Mm² and 5 Mm² and lifetimes between 10 seconds and 200 seconds. These brightenings are proposed to be the newest members of the nanoflare family, and can possibly play significant role in heating the solar corona.

In this study [11], we took advantage of the unique EUV observations of the quiet-Sun corona with unprecedented resolution provided by HRIEUV in close proximity to the Sun. These HRIEUV observations have spatial and temporal resolutions about two times better than those used earlier to study the HRIEUV brightenings. With such unique set of observations we perform a statistical study of the properties of smallest-scale HRIEUV brightenings which were previously inaccessible.

2. Solar Orbiter/EUI perihelion observations

Solar Orbiter has a unique orbit, and, by virtue of this, it can achieve the closest proximity to the Sun (Solar Orbiter perihelion); that is, just 0.293 AU away from the Sun. In this study, we used the quiet-Sun observations at the Solar Orbiter perihelion of HRIEUV in the 17.4 nm passband taken on 12 October 2022 from 05:25:00 UTC to 06:09:30 UTC and on 6 October 2023 from 10:00:00 UTC to 11:00:00 UTC.

An example of the HRIEUV field-of-view of the two datasets is shown in Figure 1 with context images of the full-disc of the Sun as captured by the Full Sun Imager (FSI; aboard EUI). Both the datasets were obtained as part of the Solar Orbiter Observing Plan named R_SMALL_HRES_HCAD_RS-burst. 

Figure 1: Representative images of the EUI quiet-Sun observations used in this article. Panels (a) and (b) show the visualisation from JHelioviewer, where the EUI/FSI 17.4 nm images overlaid with the near-simultaneous HRIEUV images are shown. Panels (c) and (d) show the HRIEUV images, with the detected EUV brightenings at the specific instance marked in cyan.

The HRIEUV images have a pixel scale of 0.492′′ which for these observations correspond to approximately 105 km on the solar surface. These observations have the highest possible spatial resolution that can be achieved by HRIEUV. They have an extremely high cadence of 3 seconds, which is the best suitable fast cadence with which HRIEUV can obtain good-quality quiet-Sun observations. These unique HRIEUV datasets thus constitute the best resolved, spatially and temporally, observations of the quiet Sun ever obtained in EUV passbands. The observations used here are the best of their kind to date to study the EUV brightenings occurring on the finest spatial and temporal scales.

3. Properties of the finest-scale EUV brightenings

Figure 2. Illustrative examples of close-up view of six EUV brightenings. The FoV of each panel is 4 Mm×4 Mm.

Using the above mentioned unique observations of the quiet solar corona, we have detected the smallest and shortest-lived EUV transient brightenings reported till now. Figure 2 shows six illustrative examples of a close-up view of the finest-scale EUV brightenings. The size and lifetime of these prevalent and ubiquitous 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. The distributions of various morphological and photometrical properties of the EUV brightenings are shown in Figure 3.

Figure 3. Probability density distributions of properties of EUV brightenings. The probability density function (PDF) and power-law fit are shown for the distributions of surface area, lifetime, volume, and total intensity.

We find an increasingly high number of EUV brightenings on smaller spatial and temporal scales. Their spatial area coverage is about 0.1% with a high occurrence rate of about 6.1 × 10⁻¹⁶ m−2 s−1 over quiet regions of the Sun. This gives an average occurrence rate over the whole surface area of the Sun of approximately 3600 EUV brightenings per second. This occurrence rate is more than one order of magnitude greater than that for the previously reported values for the similar events [12]. This is primarily by virtue of high spatial and temporal resolution observations of HRIEUV obtained at the closest-perihelia of the Solar Orbiter. The HRIEUV brightenings thus represent the most prevalent, localised, and finest-scale transient EUV brightenings in the quiet regions of the solar corona.

4. Conclusions

We find that EUV brightenings with a surface area smaller than 0.2 Mm² and lifetimes shorter than 2 minutes represent the dominant population group within the detected EUV brightenings. Thus, the high spatial and temporal resolution of HRIEUV with its high radiometric sensitivity is indispensable to study the smallest-scale EUV brightenings in details. However, our understanding of the thermal properties of the HRIEUV transient brightenings remains elusive due to the current absence of high-resolution multi-wavelength imaging and spectral observations. Such high-resolution observations coordinated with HRIEUV that can simultaneously map transition region and corona are necessary in this context. Future solar missions such as MUSE and Solar–C/EUVST, in combination with Solar Orbiter/EUI observations, will provide unique opportunities to gain better insights into the finest-scale events in the solar corona.

• This study has been published in Narang, N., Verbeeck, C., Mierla, M., et al. 2025, A&A, 699, A138, https://doi.org/10.1051/0004-6361/202554650

Affiliations

1 Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Brussels, Belgium

2 Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, Orsay, France

3 Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany

4 Mathematics Institute, St Andrews University, St Andrews, UK

Acknowledgements

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. 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 4000106864, 4000112292 and 4000134088); the Centre National d’Etudes Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through the Deutsches Zentrum für Luftund Raumfahrt (DLR); and the Swiss Space Office (SSO). The research that led to these results was subsidized by the Belgian Federal Science Policy Office through the contract B2/223/P1/CLOSE-UP.

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