Extreme-ultraviolet brightenings in the quiet Sun: Signatures in spectral and imaging data from the Interface Region Imaging Spectrograph^⋆

¹ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
e-mail: chris.nelson@esa.int
² Université Paris-Saclay, CNRS, Institut d’astrophysique spatiale, 91405 Orsay, France
³ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁴ ETH-Zurich, Wolfgang-Pauli-Str. 27, 8093 Zurich, Switzerland
⁵ Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, 7260 Davos Dorf, Switzerland
⁶ UCL-Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁷ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
⁸ Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Ringlaan 3 Av. Circulaire, 1180 Brussels, Belgium
⁹ Skobeltsyn Institute of Nuclear Physics, Moscow State University, 11999 Moscow, Russia

Received: 14 February 2023
Accepted: 8 June 2023

Abstract

Context. Localised transient EUV brightenings, sometimes named ‘campfires’, occur throughout the quiet Sun. However, there are still many open questions about these events, in particular regarding their temperature range and dynamics.

Aims. We aim to determine whether any transition region response can be detected for small-scale extreme-ultraviolet (EUV) brightenings and, if so, to identify whether the measured spectra correspond to any previously reported bursts in the transition region, such as explosive events (EEs).

Methods. EUV brightenings were detected in a ∼29.4 min dataset sampled by the Solar Orbiter Extreme Ultraviolet Imager (EUI) on 8 March 2022 using an automated detection algorithm. Any potential transition region response was inferred through analysis of imaging and spectral data sampled through coordinated observations conducted by the Interface Region Imaging Spectrograph (IRIS).

Results. EUV brightenings display a range of responses in IRIS slit-jaw imager (SJI) data. Some events have clear signatures in the Mg II and Si IV SJI filters, whilst others have no discernible counterpart. Both extended and more complex EUV brightenings are sometimes found to have responses in IRIS SJI data. Examples of EUI intensities peaking before, during, and after their IRIS counterparts were found in light curves constructed co-spatial to EUV brightenings. Importantly, therefore, it is likely that not all EUV brightenings are driven in the same way, with some events seemingly being magnetic reconnection driven and others not. A single EUV brightening occurred co-spatial to the IRIS slit, with the returned spectra matching the properties of EEs.

Conclusions. EUV brightening is a term used to describe a range of small-scale events in the solar corona. The physics behind all EUV brightenings is likely not the same. More research is therefore required to assess their importance for global questions in the field, such as coronal heating.