Cis Verbeeck

Context. Small-scale impulsive energy-release events are widely considered a key ingredient of quiet-Sun coronal heating. The discovery of extreme ultraviolet (EUV) campfires by Solar Orbiter has opened a new observational window at unprecedented spatial scales. Aims. We quantified the thermal-energy content of EUV campfires, examined their impulsive heating characteristics, and investigated how their properties vary with atmospheric height. Methods. We analysed the complete set of campfires obtained in a single observing sequence on 30 May 2020 with Solar Orbiter/EUI HRI _EUV at 174 Å. Temperatures and emission measures were adopted from SDO/AIA diagnostics, and geometric uncertainties were explored using multiple volume models. Statistical properties were characterised using cumulative distribution functions and maximum-likelihood techniques, and results were compared with Solar Orbiter/STIX microflares and SDO/AIA nanoflare-like events for context. Results. Campfire durations show a clear dependence on height: short-duration events occur at all heights, whereas longer-lived brightenings are preferentially associated with chromospheric layers. Emitting volumes and thermal energies, by contrast, show no comparable height dependence. The background-subtracted thermal energies span ~10 ²⁰ –10 ²⁴ erg, with rare events reaching ~5 × 10 ⁵ erg. The statistically robust power-law regime extends over ~10 ²² –10 ²⁴ erg, while non-background-subtracted estimates are systematically higher by approximately one order of magnitude. Directly detected campfires contribute a sub-percent fraction of the canonical quiet-Sun heating requirement. Accounting approximately for the detection threshold and differential emission measure completeness could raise this contribution to a value of order a few percent, though this remains a single-epoch, order-of-magnitude estimate. Conclusions. Extreme ultraviolet campfires extend the flare energy distribution towards lower energies and provide height-resolved constraints on impulsive heating in the quiet Sun. The height dependence of event durations – driven mainly by an excess of long-lived events in the lower chromosphere – combined with broadly invariant energy and volume distributions, indicates that atmospheric stratification primarily modulates cooling and dissipation timescales rather than the characteristic energy scale of the heating process.

Context . Recent high-resolution observations from the Solar Orbiter mission can help detect the indirect signatures of heating at the smallest scales. Aims . In this work, we measure the properties and investigate the physical origin of propagating disturbances (PDs) in the intensity at the smallest resolvable scales with Solar Orbiter/EUI along coronal loops. Methods . We used two sequences of EUI/HRIEUV at high spatial (down to 125 per pixel) and temporal resolutions (5 of cadence). We placed slits along 13 active region (AR) coronal loops. We measured the plane-of-sky (PoS) velocities and the intensity perturbation damping along the slits of PDs. We also measured the periodicity of PDs in one slit by using a Fourier analysis. Results . We report the detection of PDs that have high PoS velocities ranging between 500 and 2000 (which we call “fast” PDs). They are only visible in the upper part of the coronal loops. The intensity increase associated with these fast PDs is on the order of 4–8% of the HRIEUV intensity, and we measured little to no damping of their intensity with distance. We also measured a peak in the Fourier power spectra at 2 min above the 95% confidence limit that is associated with fast PDs. The fast PDs are detected in the same coronal loops as PDs with a lower PoS velocity (70–90), which we refer to as “slow” PDs. Unlike the fast PDs, these slow PDs are only visible in the lower part of the coronal loop, and they show clear intensity damping. Conclusions . Slow PDs show properties consistent with slow magneto-acoustic modes or upflows. On the other hand, fast PDs cannot be explained by slow magneto-acoustic modes. Instead, they show properties consistent with fast flows induced by magnetic reconnection, current sheet generated by propagating transverse oscillations, and fast magnetohydrodynamics modes or Alfvén waves.

We present the point-spread function (PSF) of the Extreme Ultraviolet High-Resolution Imager (HRI _EUV ) on board Solar Orbiter, which observes the Sun at 174 Å. This PSF provides a quantitative description of light diffracted by the mesh and the mounting supporting the entrance filter, light diffracted by the mesh supporting the filter-wheel filter, as well as light that is diffusely scattered by the microroughness of the mirrors. Deconvolution with this PSF corrects the images for instrumental scattered light, substantially improving image quality and photometric accuracy. First, we determine the diffraction component of the PSF from mechanical drawings of the instrument. We find that 26% of the incoming light is diffracted, predominantly by the entrance-filter mounting and mesh. Second, we fit the diffuse scattered light using partial image occultations during the 2023 January 3 Mercury transit. We find that the diffuse scattered light is well described by a softened power law, which scatters 42% of light over the detector. Combined, 57% of the incoming light is redistributed over the detector by diffraction and scattering. Correcting for these effects markedly enhances the dynamic range and contrast of the observations. The intensity in bright structures intensifies by up to 40%, and the intensity in dark structures decreases by up to 85%. All image features become much clearer, facilitating a more precise scientific analysis of HRI _EUV observations.

High-dynamic-range imaging in the extreme-ultraviolet regime is frequently compromised by detector saturation and charge blooming during intense solar flares, preventing reliable photometry of flare cores. This study presents, for the first time, a direct validation of a sparsity-regularized inverse-diffraction desaturation algorithm applied to saturated images from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory, using contemporaneous short-exposure, unsaturated observations from the Extreme Ultraviolet Imager on Solar Orbiter. The validation exploits a near-radial spacecraft alignment on 2024 March 19, when an M2.1 flare erupted in active region 13615. To obtain coregistered intensity maps, the data were processed with reprojection onto a common helioprojective grid, exposure-time normalization, light-travel-time correction, and field-of-view pointing refinement. Photometric comparisons were performed using peak-based statistics within a rigorously defined region of interest. Results show close morphological agreement and a temporally stable ratio between the desaturated and reference intensities through the impulsive and early decay phases. An early-phase discrepancy is observed and attributed to rapid subexposure evolution, sampling effects, and bandpass/response differences. These findings demonstrate that the desaturation procedure successfully recovers physically meaningful time-dependent core fluxes and, for the first time, provide independent empirical support for the use of desaturated archive frames in quantitative studies of large flares.

Solar flares are the most powerful, magnetically driven, explosions in the heliosphere. The nature of magnetic energy release in the solar corona that heats the plasma and accelerates particles in a flare, however, remains poorly understood. Here, we report high-resolution coronal observations of a flare by the Solar Orbiter mission that reveal initially weaker but rapid reconnection events, on timescales of a few seconds at most, leading to a more prominent activity of a similar nature that explosively causes a flare. Signatures of this process are further imprinted on the widespread raining plasma blobs with short lifetimes, giving rise to the characteristic ribbon-like emission pattern associated with the flare. Our observations unveil the central engine of a flare and emphasize the crucial role of an avalanche-like magnetic energy release mechanism at work.

Context. Small-scale extreme-ultraviolet (EUV) transient brightenings are observationally abundant and critically important to investigate. Determining whether they share the same physical mechanisms as larger-scale flares would have significant implications for the coronal heating problem. A recent study has revealed that quasi-periodic pulsations (QPPs), a common feature in both solar and stellar flares, could also be present in EUV brightenings in the quiet Sun (QS). Aims. We aim to characterise the properties of EUV brightenings and their associated QPPs in both QS and active regions (ARs) using an unprecedented 1 s cadence observations from Solar Orbiter’s Extreme Ultraviolet Imager (Solar Orbiter/EUI). Methods. We applied an automated detection algorithm to analyse statistical properties of EUV brightenings. The QPPs were identified using complementary techniques optimised for both stationary and non-stationary signals, including a Fourier-based method, ensemble empirical mode decomposition, and wavelet analysis. Results. Over 500 000 and 300 000 brightenings were detected, respectively, in ARs and QS regions. Brightenings with lifetimes shorter than 3 s were detected, demonstrating the importance of high temporal resolution. The QPP occurrence rates were approximately 11% in AR brightenings and 9% in QS brightenings, with non-stationary QPPs being more common than stationary ones. The QPP periods range from 5 to over 500 s and display similar distributions between the ARs and QS regions. Moderate linear correlations were found between QPP periods and the lifetime and spatial scale of the associated brightenings, while no significant correlation was found with peak brightness. We found a consistent power-law scaling, with a weak correlation and a large spread, between QPP period and lifetime in EUV brightenings, solar, and stellar flares. Conclusions. The results support the interpretation that EUV brightenings may represent a small-scale manifestation of the same physical mechanisms driving larger solar and stellar flares. Furthermore, the similarity in the statistical properties of EUV brightenings and their associated QPPs between ARs and QS regions suggests that the underlying generation mechanisms might not strongly depend on the large-scale magnetic environment.

We present detailed analysis of an active region coronal jet accompanying a minifilament eruption that is fully captured and well resolved in high spatial resolution 174 Å coronal images from Solar Orbiter’s Extreme Ultraviolet Imager (EUI). The active region jet is simultaneously observed by the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory. An erupting minifilament is rooted at the edge of an active region where mixed-polarity magnetic flux is present. Minority-polarity positive flux merges and cancels with the active region’s dominant negative flux at an average rate of 10 ¹⁹ Mx hr ⁻¹ , building a minifilament-holding flux rope and triggering its eruption. The eruption shows a slow rise followed by a fast rise, akin to large-scale filament eruptions. EUI images and Mg II k spectra, displaying simultaneously blueshift and redshift at the opposite edges of the spire, indicate counterclockwise untwisting of the jet spire. This jet is the clearest, most comprehensively observed active-region jet with this instrument set, displaying striking similarities with quiet Sun and coronal hole jets. Its magnetic (≤10 ²⁸ erg), thermal (10 ²⁵ erg), and kinetic (10 ²⁵ erg) energies suggest a significant contribution to local coronal heating. We conclude that magnetic flux cancelation builds a minifilament-carrying twisted flux rope and also eventually triggers the flux rope’s eruption that makes the coronal jet, in line with our recent results on the buildup and explosion of solar coronal jets in quiet Sun and coronal holes. That is, this active region jet clearly works the same way as the vast majority of quiet Sun and coronal hole jets.

Solar Orbiter conducted a series of flare-optimised observing campaigns in 2024 using the Major Flare Solar Orbiter Observing Plan (SOOP). Dedicated observations were performed during two distinct perihelia intervals in March/April and October, during which over 22 flares were observed, ranging from B- to M-class. These campaigns leveraged high-resolution and high-cadence observations from the mission’s remote-sensing suite, including the High-Resolution EUV Imager (EUI/HRI _EUV ), the Spectrometer/Telescope for Imaging X-rays (STIX), the Spectral Imaging of the Coronal Environment (SPICE) spectrometer, and the High Resolution Telescope of the Polarimetric and Helioseismic Imager (PHI/HRT), as well as coordinated ground-based and Earth-orbiting observations. EUI/HRI _EUV , operating in short-exposure modes, provided two-second-cadence, non-saturated EUV images, revealing structures and dynamics on scales not previously observed. Simultaneously, STIX captured hard X-ray imaging and spectroscopy of accelerated electrons, while SPICE acquired EUV slit spectroscopy to probe chromospheric and coronal responses. Together, these observations offer an unprecedented view of magnetic reconnection, energy release, particle acceleration, and plasma heating across a broad range of temperatures and spatial scales. These campaigns have generated a rich dataset that will be the subject of numerous future studies addressing Solar Orbiter’s top-level science goal: “How do solar eruptions produce energetic particle radiation that fills the heliosphere?” . This paper presents the scientific motivations, operational planning, and observational strategies behind the 2024 flare campaigns, along with initial insights into the observed flares. We also discuss lessons learned for optimizing future Solar Orbiter Major Flare campaigns and provide a resource for researchers aiming to utilize these unique observations.

Magnetoconvection at the solar surface governs the dynamics in the upper solar atmosphere and sustains the heliosphere. Properties of this fundamental process are poorly described near the solar poles. Here we report the first out-of-ecliptic remote-sensing observations of the south pole of the Sun from a high-latitude campaign of the Solar Orbiter spacecraft, which reveal spatial and temporal evolution of supergranular convective cells. The supergranular cells have spatial scales of 20–40 Mm. From 8 days of observations starting on 2025 March 16, our analysis shows that the magnetic network migrates poleward, on average, at high latitudes (above 60°), with speeds in the range of 10–20 m s ⁻¹ , depending on the structures being tracked. These results shed light on the buildup of the polar magnetic field that is central to our understanding of the solar cycle and the heliospheric magnetic field.

We present detailed analysis of an active region coronal jet accompanying a minifilament eruption that is fully captured and well-resolved in high spatial resolution 174A coronal images from Solar Orbiters Extreme Ultraviolet Imager (EUI). The active region jet is simultaneously observed by the Interface Region Imaging Spectrograph (IRIS) and the Solar Dynamics Observatory (SDO). An erupting minifilament is rooted at the edge of an active region where mixed-polarity magnetic flux is present. Minority-polarity positive flux merges and cancels with the active regions dominant negative flux at an average rate of 1019 Mx/hr, building a minifilament-holding flux rope and triggering its eruption. The eruption shows a slow rise followed by a fast rise, akin to large-scale filament eruptions. EUI images and Mg II k spectra, displaying simultaneously blueshifts and redshifts at the opposite edges of the spire, indicate counterclockwise untwisting of the jet spire. This jet is the clearest, most comprehensively observed active-region jet with this instrument set, displaying striking similarities with quiet Sun and coronal hole jets. Its magnetic, thermal, and kinetic energies suggest a significant contribution to local coronal heating. We conclude that magnetic flux cancelation builds a minifilament-carrying twisted flux rope and also eventually triggers the flux ropes eruption that makes the coronal jet, in line with our recent results on the buildup and explosion of solar coronal jets in quiet Sun and coronal holes. That is, this active region jet clearly works the same way as the vast majority of quiet Sun and coronal hole jets.