To capture the brightest and most rapidly evolving phases of solar flares, the Major Flare Solar Orbiter Observing Plan employed a dedicated short-exposure mode for the High Resolution Imager at 174 Angstrom (HRIEUV) of the Extreme Ultraviolet Imager. We investigate the spatial and temporal organisation of compact emission in the 19 March 2024 M2.1 flare using high-cadence, short-exposure HRIEUV observations. We combine these 0.04 s observations with hard X-ray timing, imaging, and spectroscopy from the Spectrometer Telescope for Imaging X-rays (STIX). We characterise impulsive ribbon kernels and later loop strands, and compare footpoint areas measured with HRIEUV, the Atmospheric Imaging Assembly, and STIX to constrain the local energy flux carried by flare-accelerated electrons. The short-exposure observations reveal compact emission largely obscured by saturation in normal-exposure EUV imaging. The integrated HRIEUV emission evolves co-temporally with the STIX 22-45 keV emission, with no lag discernible beyond the 2 s sampling. The ribbons comprise repeatedly activated kernels with characteristic separations of approximately 1.4-1.7 Mm, while the developing arcade shows a similar strand separation of approximately 1.3 Mm. Kernel and strand widths of approximately 0.4-0.5 Mm lie close to the instrumental resolution limit. The compact HRIEUV footpoint areas are approximately an order of magnitude smaller than those inferred from AIA or STIX, implying nominal local non-thermal energy fluxes on the order of 10^11 erg cm^-2 s^-1 at the hard X-ray peaks. These results reveal a characteristic 1-2 Mm spatial organisation of the flare emission and demonstrate the value of flare-optimised EUV imaging for future solar flare observations.

Small-scale propagating disturbances (PDs) are ubiquitous in the solar corona. The method called time-normalised optical flow (TNOF) was developed for mapping PDs velocity fields in time series of extreme-ultraviolet (EUV) images. We show PDs velocity fields of a quiet-Sun (QS) region containing a small coronal hole (CH) and filament channel (FC) that were jointly observed by Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter and Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). The QS observations acquired on 28 October 2023 in the 174 Å channel of High Resolution EUV Imager (HRIEUV) of EUI and 171 Å channel of AIA were used. During the time of the observations, the separation angle between Solar Orbiter and SDO was approximately 26°. A novel image-alignment analysis shows that the dominant formation heights are 11.4 Mm for HRIEUV and 4 Mm for AIA. Despite this height difference, the PDs velocity fields obtained from the observations from the two instruments agree well throughout the region. In the QS, the median PDs speed is about 6.7 and 7.4 kms−1 for HRIEUV and AIA, respectively, with maximum speeds of about 40 kms−1. The small equatorial CH region is dominated by a low temperature of ≈0.8 MK and is host to high PDs speeds, with a median speed of 17 kms−1. The velocity field bridges coherently across the CH from neighbouring QS regions from east to west, and the CH must therefore be overlaid by a system of long, low-lying closed magnetic loops. This unexpected configuration is supported by a potential field (PF) magnetic model and might be caused by the longevity of the CH, which allows time for interchange reconnection with neighbouring closed-field regions. The FC is observed to be multi-thermal, with a narrow central high-emission strip at low (0.8 MK) and high (2.5 MK) temperatures and low emission at a warm (1.2 MK) temperature. Despite this distinct temperature profile, the PDs speeds in the FC are similar to those of the QS. The TNOF velocity field shows that PDs tend to flow into the FC from neighbouring regions before they align along the FC in a coherent direction. This means that PDs within filaments are driven by external sources. The vector field is consistent with a highly non-potential barbs-and-spine tubular magnetic field; the PF model fails to replicate this configuration. We conclude that longer magnetic loops are required for higher PDs speeds, as observed for CH here, and that the smaller loop systems of the QS and FC generally lead to lower speeds. These multi-instrument results show that the TNOF method can confidently be used as a diagnostic tool for the kinematics of PDs, and it highlights its potential for probing the coronal magnetic field orientation, particularly in highly non-potential regions, where extrapolation models may fail.

Large solar flares (GOES M-class or higher) are usually associated with eruptions of material. However, when considering flare irradiance enhancements and dynamics such as chromospheric evaporation, potential contributions from erupted material have historically been neglected. We analyse nine eruptive M- and X-class flares from 2024 to early 2025, quantifying the relative contributions of erupted material to irradiance enhancements during the events. Atmospheric Imaging Assembly (AIA) images from four different channels had ribbon and eruption irradiance contributions separated using a semi-automated masking method. The sample-averaged percentages of excess radiated energy by erupted material over the impulsive phase were 10^+4_-4% , 24^+14_-14% , 21^+14_-10% and 13^+6_-9% for the 131 Å, 171 Å, 304 Å and 1600 Å channels, respectively. For three events that were studied in further detail, hard X-ray (HXR) imaging showed little to no signatures of nonthermal heating within the eruptions. Our results suggest that erupted material can be a significant contributor to UV irradiance enhancements during flares, with possible heating mechanisms including nonthermal particle heating, Ohmic heating, or dissipation of MHD waves. Future work may clarify the heating mechanism and evaluate the impact of eruptions on spectral variability, particularly in Sun-as-a-star and stellar flare observations.

Flare ribbons form when energy released by coronal magnetic reconnection is deposited in the low solar atmosphere, so by studying the dynamics of flare ribbons, one obtains an indirect measurement of reconnection. Our aim is to quantify the spatial and temporal scales of substructures in the Extreme Ultraviolet (EUV) flare ribbons, known as kernels, as a probe of the spatial extent and duration of energy injection during the impulsive phase of solar flares. 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, HRI_EUV. Individual kernels were automatically identified using a classical computer vision algorithm. Size distributions of ribbon kernels were derived, and an average light curve of individual kernels was extracted. The EUV flare kernels were small (≲ 60 pixels≈ 1 Mm^2) and a significant fraction were unresolved at a plate scale of 135 km/pix. Furthermore, we derived surprisingly short EUV kernel heating times of less than a few seconds. The average profile exhibits a sharp rise of 1.7±0.3 s from half-maximum, requiring an additional 2.3^+0.7_-0.4 s to return to its reference value. Our findings indicate that approximately half of the kernels were unresolved in this flare, despite the enhanced angular resolution offered by Solar Orbiter's proximity to the Sun at 0.38 AU here. Furthermore, we show that energy was only injected in a localised region (≲ 1 Mm^2) of flare ribbons for less than a few seconds. These results necessitate an in-depth investigation into the implications of such small-scale and transient injections on the energy flux deposited in solar flares, and the resulting response of the solar atmosphere.

We present the point-spread function (PSF) of the Extreme Ultraviolet High-Resolution Imager (HRIEUV) onboard Solar Orbiter, which observes the Sun at 174 Angstrom. This PSF provides a quantitative description of light diffracted by the mesh and 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

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.

Recent high spatial and temporal resolution extreme-ultraviolet (EUV) imaging observations have revealed that quasi-periodic pulsations (QPPs), a ubiquitous signature of impulsive energy release in solar and stellar flares, are also present in much smaller-scale coronal events known as EUV brightenings. Whether QPPs observed across such disparate spatial and energetic scales share a common physical origin remains an open question. Here we analyse 2,146 EUV brightenings observed with Solar Orbiter/EUI and 300 EUV solar flares observed with SDO/AIA, identifying 185 brightenings and 89 flares exhibiting statistically significant damped QPPs. We show that the relationship between damping time and oscillation period follows a common power-law scaling for EUV brightenings and EUV solar flares, consistent with previously reported X-ray QPPs spanning both solar and stellar flares. The persistence of this scaling over a wide range of energies and scales suggests that QPPs are governed by a common underlying physical mechanism.

Decayless kink oscillations, characterized by their lack of decay in amplitude, have been detected in coronal loops of varying scales in active regions, the quiet Sun, and coronal holes. Short-period (<50 s) decayless oscillations have been detected in short loops (< 50 Mm) within active regions. Nevertheless, long-period decayless oscillations in these loops remain relatively unexplored and crucial for understanding the wave modes and excitation mechanisms of decayless oscillations. We present the statistical analysis of decayless oscillations from two active regions observed by the Extreme Ultraviolet Imager onboard Solar Orbiter. The average loop length and period of the detected oscillations are 19 Mm and 151 s, respectively. We find 82 long-period and 23 short-period oscillations in these loops. We do not obtain a significant correlation between loop length and period. We discuss the possibility of different wave modes in short loops, although standing waves cannot be excluded from possible wave modes. Furthermore, a different branch exists for active region short loops in the loop length versus period relation, similar to decayless waves in short loops in the quiet Sun and coronal holes. The magnetic fields derived from MHD seismology, based on standing kink modes, show lower values for multiple oscillations compared to previous estimates for long loops in active regions. Additionally, the comparison of period distributions in short loops across different coronal regions indicates that different excitation mechanisms may trigger short-period kink oscillations in active regions compared to the quiet Sun and coronal holes.

Context. Extreme-ultraviolet (EUV) observations have revealed small-scale transient brightenings that may be caused by the same physical mechanisms as larger-scale solar flares. A notable feature of solar and stellar flares is the presence of quasi-periodic pulsations (QPPs), which are a potentially intrinsic characteristic. Aims. We investigated the properties of QPPs detected in EUV brightenings, which are considered to be small-scale flares, and compared their statistical properties with those observed in solar and stellar flares. Methods. We extracted integrated light curves of 22 623 EUV brightenings in two quiet Sun regions observed by the Solar Orbiter/Extreme Ultraviolet Imager and identified QPPs in their light curves using Fourier analysis. Results. Approximately 2.7% of the EUV brightenings exhibited stationary QPPs. The QPP occurrence rate increased with the surface area, lifetime, and peak brightness of the EUV brightenings. The detected QPP periods ranged from approximately 15 to 260 seconds, which is comparable to the periods observed in solar and stellar flares. Consistent with observations of QPPs in solar and stellar flares, no correlation was found between the QPP period and peak brightness. However, unlike the trend observed in solar flares, no correlation was found between the QPP period and the lifetime or length scale. Conclusions. The presence of QPPs in EUV brightenings supports the interpretation that these events may be small-scale manifestations of flares, and the absence of period scaling with loop length further suggests that standing waves may not be the primary driver of QPPs in these events.

We present fast-cadence and high-resolution observations of flare ribbons from the Solar Orbiter Extreme Ultraviolet Imager (EUI). Utilizing the short-exposure observations from the EUI High Resolution Imager in the extreme-ultraviolet, we find small-scale blob/bead-like kernel structures propagating within a hook at the end of a flare ribbon, during the impulsive phase of a C9.9-class solar flare. These bead structures are dynamic, with well-resolved spatial separations as low as ≈420–840 km (3–6 pixels) below the observable limit of full-disk solar imagers. We analyze the evolution of the plane-of-sky apparent velocity and separation of the flare ribbon structures, finding evidence for multiple processes occurring simultaneously within the flare ribbon. These processes include quasiperiodic pulsation–like brightenings, slow back-and-forth zigzag motions along the ribbon, rapid apparent motions along the ribbon (600+ km s ^−1 ), and stationary blob-like structures. Finally, we conduct fast Fourier transform analysis and analyze the start times of exponential growth in the power spectrum at different spatial scales across the flare ribbon. Our analysis reveals that the ribbon beads form with a key spatial separation of 1.7–1.9 Mm before developing into more complex structures at progressively larger and smaller spatial scales. This observation is consistent with predictions of the tearing mode instability.

Impulsive solar energetic particle (SEP) events are typically associated with solar flares but the related particle injection and acceleration processes are still not well understood. We use in-situ and remote-sensing data from Solar Orbiter to establish a plausible link between a series of eruptions in a flaring region and a sequence of four SEP events measured at 0.5 AU between 5 and 6 March 2022. The direct comparison between these four events from the same source region allows to study the variability of the injected SEPs during an extended period of magnetic connectivity between Solar Orbiter and the flaring active region. In this study we analyze energetic electron, proton, and heavy ion data provided by the Energetic Particle Detector (EPD) suite onboard Solar Orbiter. Via a velocity dispersion analysis (VDA) of all measured particle species we estimate the solar event onset times which coincide with a series of solar eruptions that is observed by the Extreme Ultraviolet Imager (EUI) and the Spectrometer Telescope for Imaging X-rays (STIX) onboard Solar Orbiter. Further high-time-resolution EUV images and photospheric magnetic field information of the related active region is given by the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory. Solar Orbiter and Earth were nearly perfectly radially aligned at this time which enabled this additional remote sensing by SDO. We find that the energy spectra of the heavy ion in-situ measurements show significant differences between the four investigated SEP events in terms of overall particle intensity, spectral slope, and 3He / 4He abundances. By comparison with the remote-sensing observations we find that the two stronger SEP events (with higher 3He / 4He ratios) are related to solar eruptions with a more complex eruption pattern leading to extended brightening and restructuring of coronal loop structures. These new detailed observations can be used as starting point for quantitative modelling of flare-associated energetic particle acceleration and release in active regions.This work has been funded by the Spanish Ministerio de Ciencia, Innovación y Universidades project PID2019-104863RBI00/AEI/10.13039/501100011033.

Solar wind, classified by its bulk speed and the Alfvénic nature of its fluctuations, generates the heliosphere. The elusive physical processes responsible for the generation of the different types of this wind are a topic of active debate. Recent observations reveal intermittent jets, with kinetic energy in the picoflare range, emerging from dark areas of a polar coronal hole threaded by open magnetic field lines. These could substantially contribute to solar wind. However, their ubiquity and direct links to solar wind have not been established. Here, we report a unique set of remote-sensing and in situ observations from the Solar Orbiter spacecraft that establish a unified picture of fast and Alfvénic slow wind, connected to the similar widespread picoflare jet activity in two coronal holes. Radial expansion of coronal holes ultimately regulates the speed of the emerging wind.

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 (HRIEUV) 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 HRIEUV 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/HRIEUV 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.

Nanojets are small-scale jets generated by component reconnection, characterized by their motion perpendicular to the reconnecting magnetic field lines. As an indicator of nanoflare events, they are believed to play a significant role in coronal heating. Using high-resolution EUV imaging observations from the Solar Orbiter/Extreme Ultraviolet Imager (EUI), we identified 27 nanojets during an M7.6 flare on September 30, 2024. Most nanojets exhibit velocities around 1000 km/s, comparable to the typical coronal Alfvén speed. To our knowledge, these speeds are the highest ever reported for small-scale jets. The average kinetic energy of the nanojets is estimated to be 2.3×1025 erg, with events of higher speeds typically displaying greater kinetic energy and longer durations.

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/HRIEUV), 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/HRIEUV, 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.

Context. Coronal jets are ubiquitous, collimated million-degree ejections that contribute to the energy and mass supply of the upper solar atmosphere and the solar wind. Solar Orbiter observations provide an unprecedented opportunity to study fine-scale jets from a unique vantage point close to the Sun. Aims. We aim to uncover thin jets originating from coronal bright points (CBPs) and investigate observable features of plasmoid-mediated reconnection. Methods. We analyzed eleven datasets from the High Resolution Imager 174 & Aring; of the Extreme Ultraviolet Imager (HRIEUV) on board Solar Orbiter, focusing on narrow jets from CBPs and signatures of magnetic reconnection within current sheets and outflow regions. To aid in the interpretation, we compared the observations with radiative-magnetohydrodynamic simulations of a CBP conducted with the Bifrost code. Results. We identified thin coronal jets originating from CBPs with widths ranging from 253 km to 706 km. These are scales that could not be resolved with previous EUV imaging instruments. Remarkably, these jets are 30-85% brighter than their surroundings and can extend up to 22 Mm, while maintaining their narrow form. For one of the datasets, we directly identified plasmoid-mediated reconnection through the development within the current sheet of a small-scale plasmoid that reaches a length of 332 km and propagates at 40 km s(-1). For another dataset, we inferred indirect traces of plasmoid-mediated reconnection through the intermittent boomerang-like pattern that appears in the outflow region. The simulation self-consistently produces a current sheet and small-scale plasmoids similar to those observed, whose synthetic HRIEUV emission reproduces both direct imprints within the current sheet and intermittent patterns in the outflow region associated with their ejection. Conclusions. Our findings highlight Solar Orbiter's unique capability to capture narrow jets and sub-megameter-scale plasmoid-mediated reconnection signatures in the corona. These results motivate future statistical studies aimed at assessing the role of such fine-scale phenomena in coronal dynamics and solar wind formation.

Transverse oscillations in solar coronal loops that do not exhibit significant damping have been found to be ubiquitous. The high spatial and temporal resolution of Solar Orbiter/EUI has revealed the presence of higher-frequency transverse waves, which were previously unobservable due to cadence limitations. Through meta-analysis, we have established that the relationship between transverse wave frequency and their spectral energy flux is well described by a power law. We propose that the slope (δ) of this power-law distribution can determine whether high-frequency transverse oscillations make a dominant contribution to coronal heating, with δ < 1 indicating a stronger role. Since the slope of the observed power law is less than 1, our results suggest that high-frequency waves contribute significantly to coronal heating in both quiet Sun (QS) and active regions (ARs). Our statistical analysis demonstrates that high-frequency transverse oscillations can effectively heat the QS. However, for ARs, the total energy flux appears insufficient unless oscillations with frequencies up to 0.17 Hz, which remain unobserved, are present. Future EUI campaigns with higher cadence observations could confirm the existence of such high-frequency oscillations.

Solar nanojets are small-scale jets generated by component magnetic reconnection, characterized by collimated plasma motion perpendicular to the reconnecting magnetic field lines. As an indicator of nanoflare events, they are believed to play a significant role in coronal heating. Using high-resolution extreme-ultraviolet imaging observations from the Extreme Ultraviolet Imager on board the Solar Orbiter mission, we identified 27 nanojets in an erupting filament on 2024 September 30. They are potentially associated with the untwisting of magnetic field lines of the filament. Most nanojets exhibit velocities around 450 km s ^−1 , with the fastest reaching approximately 800 km s ^−1 , significantly higher than previously reported but comparable to the typical coronal Alfvén speed. To our knowledge, these are the highest speeds ever reported for small-scale jets (less than ∼1 Mm wide) in the solar atmosphere. Our findings suggest that these nanoflare-type phenomena can be more dynamic than previously recognized and may contribute to the energy release process of solar eruptions and the heating of coronal active regions.

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. We investigate 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. We perform a statistical analysis of the EUV brightenings by using quiet-Sun observations at the highest possible spatial resolution ever obtained by EUI. We use observations in the 17.4 nm passband of the High Resolution EUV Imager (HRIEUV) 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 use a wavelet-based automatic detection algorithm to detect and characterise the events of interest, and study their morphological and photometrical properties. We report the detection of 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 Mm^2 and a lifetime of 3 seconds. In general their sizes lie in the range of 0.01 Mm^2 to 50 Mm^2 , and their lifetimes vary between 3 seconds and 40 minutes. We find an increasingly high number of EUV brightenings at smaller spatial and temporal scales. We estimate that about 3600 EUV brightenings appear per second on the whole Sun. The HRIEUV brightenings thus represent the most prevalent, localised and finest-scale transient EUV brightenings in the quiet regions of the solar corona.