Context. The heating of the solar corona by small heating events requires an increasing number of such events at progressively smaller scales, with the bulk of the heating occurring at scales that are currently unresolved. Aims. The goal of this work is to study the smallest brightening events observed in the extreme-UV quiet Sun. Methods. We used commissioning data taken by the Extreme Ultraviolet Imager (EUI) on board the recently launched Solar Orbiter mission. On 30 May 2020, the EUI was situated at 0.556 AU from the Sun. Its High Resolution EUV telescope (HRI EUV , 17.4 nm passband) reached an exceptionally high two-pixel spatial resolution of 400 km. The size and duration of small-scale structures was determined by the HRI EUV data, while their height was estimated from triangulation with simultaneous images from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) mission. This is the first stereoscopy of small-scale brightenings at high resolution. Results. We observed small localised brightenings, also known as 'campfires', in a quiet Sun region with length scales between 400 km and 4000 km and durations between 10 sec and 200 sec. The smallest and weakest of these HRI EUV brightenings have not been previously observed. Simultaneous observations from the EUI High-resolution Lyman-α telescope (HRI Lya ) do not show localised brightening events, but the locations of the HRI EUV events clearly correspond to the chromospheric network. Comparisons with simultaneous AIA images shows that most events can also be identified in the 17.1 nm, 19.3 nm, 21.1 nm, and 30.4 nm pass-bands of AIA, although they appear weaker and blurred. Our differential emission measure (DEM) analysis indicated coronal temperatures peaking at log T ≈ 6.1 − 6.15. We determined the height for a few of these campfires to be between 1000 and 5000 km above the photosphere. Conclusions. We find that 'campfires' are mostly coronal in nature and rooted in the magnetic flux concentrations of the chromospheric network. We interpret these events as a new extension to the flare-microflare-nanoflare family. Given their low height, the EUI 'campfires' could stand as a new element of the fine structure of the transition region-low corona, that is, as apexes of small-scale loops that undergo internal heating all the way up to coronal temperatures.

Studies of the onset of Earth-directed coronal mass ejections (CMEs) rely on solar disk observations where CME structures are difficult to disentangle because of the diversity and transient character of the phenomena involved. Dimmings and coronal waves are among the best evidence of the large-scale reorganization of coronal magnetic fields associated with the onset of CMEs. The physical mechanism behind EIT waves is still unclear: they are considered as MHD waves and/or as a consequence of plasma compression on the extending border of a dimming. In this paper, we address the problem of automatically detecting and analyzing EIT waves and dimmings in EUV images. This paper presents a "proof of principle" that automated detection of EIT wave and dimmings is indeed possible. At the current stage of work, the method can unambiguously detect dimmings and EIT waves when applied on a typical test-case event. Moreover, we propose a way to extract these events from the data, and determine such parameters as life time, depth, area and volume of dimmings for future catalogs. For EIT waves we unambiguously define, in near solar minimum conditions, the eruption center, the front of EIT wave and its propagation velocity. In addition, we show that the presented methods yield new insights about the geometrical shape of dimmings and the connection with the EIT wave front properties, and the apparent angular rotation of the EIT wave under study.

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Abstract. Recent observations of give strong support to Parker's hypothesis (1988) that small scale dissipative events make the main contribution to quiet Sun coronal heating. They also showed that these small scale events are associated not only with the magnetic network, but also with the cell interiors . Taking into account in addition the results of the analysis performed by Priest with co-authors (2000) who demonstrated that the heating is quasi-homogeneous along the arcs we come to the conclusion that the sources driving these dissipative events are also small scale sources. Typically they are of the order of or smaller than the linear scale of the events observed, that is < 2000 km. To describe statistical properties of quiet Sun corona heating by microflares, nanoflares, and even smaller events, we consider a cellular automata model subject to uniform small scale driving and dissipation. The model consists of two elements, the magnetic field source supposed to be associated with the small scale hydrodynamic turbulence convected from the photosphere and local dissipation of small scale currents. The dissipation is assumed to be provided by either anomalous resistivity, when the current density exceeds a certain threshold value, or by the magnetic reconnection. The main problem considered is how the statistical characteristics of dissipated energy flow depend upon characteristics of the magnetic field source and on physical mechanism responsible for the magnetic field dissipation. As the threshold value of current is increased, we observe the transition from Gaussian statistics to power-law type. In addition, we find that the dissipation provided by reconnection results in stronger deviations from Gaussian distribution.

Key Points:• Long-lasting multi-MeV electron enhancement during a period of a relatively weak geomagnetic storm not recorded in GEO. • Electron seed population was accelerated to relativistic energies by the enhanced chorus waves. • Relativistic electrons were further accelerated up to 10 MeV by inward diffusion Corresponding author: Ch. Katsavrias, ckatsavrias@phys.uoa.grWe report observations of energetic electron flux and Phase Space Density (PSD) to show that a relatively weak magnetic storm with Sym−H min ≈ −50nT, resulted in a relativistic and ultra-relativistic electron enhancement of two orders of magnitude similar to the St. Patrick's event of 2015, an extreme storm with Sym − H min ≈ −235nT. This enhancement appeared at energies up to ≈ 10 MeV, lasted for at least 24 days and was not recorded in geosynchronous orbit where most space weather alert data are collected. By combined analysis of PSD radial profiles and Fokker-Planck simulation, we show that the enhancement of relativistic and ultra-relativistic electrons is caused by different mechanisms: first, chorus waves during the intense substorm injections of April 21-25 accelerate the seed electron population to relativistic energies and redistribute them while inward diffusion driven by Pc5 ULF waves further accelerates them to ultra-relativistic energies.

Context. The three-dimensional fine structure of the solar atmosphere is still not fully understood as most of the available observations are taken from a single vantage point. Aims. The goal of the paper is to study the three-dimensional distribution of the small-scale brightening events (“campfires”) discovered in the extreme-UV quiet Sun by the Extreme Ultraviolet Imager (EUI) aboard Solar Orbiter. Methods. We used a first commissioning data set acquired by the EUI’s High Resolution EUV telescope on 30 May 2020 in the 174 Å passband and we combined it with simultaneous data taken by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory in a similar 171 Å passband. The two-pixel spatial resolution of the two telescopes is 400 km and 880 km, respectively, which is sufficient to identify the campfires in both data sets. The two spacecraft had an angular separation of around 31.5° (essentially in heliographic longitude), which allowed for the three-dimensional reconstruction of the campfire position. These observations represent the first time that stereoscopy was achieved for brightenings at such a small scale. Manual and automatic triangulation methods were used to characterize the campfire data. Results. The height of the campfires is located between 1000 km and 5000 km above the photosphere and we find a good agreement between the manual and automatic methods. The internal structure of campfires is mostly unresolved by AIA; however, for a particularly large campfire, we were able to triangulate a few pixels, which are all in a narrow range between 2500 and 4500 km. Conclusions. We conclude that the low height of EUI campfires suggests that they belong to the previously unresolved fine structure of the transition region and low corona of the quiet Sun. They are probably apexes of small-scale dynamic loops heated internally to coronal temperatures. This work demonstrates that high-resolution stereoscopy of structures in the solar atmosphere has become feasible.

On 2017 April 1 and 3, two large eruptions on the western solar limb, which were associated with M4.4- and M5.8-class flares, respectively, were observed with the Sun Watcher with Active Pixels and Image Processing (SWAP) Extreme Ultraviolet (EUV) solar telescope on board the Project for On Board Autonomy 2 (PROBA2) spacecraft. The large field-of-view (FOV) of SWAP, combined with an advantageous off-point, allows us to study the eruptions up to approximately 2 solar radii (Rs), where space-based coronagraph observations begin. These measurements provide us with some of the highest EUV observations of an eruption, giving crucial additional data points to track the early evolution of Coronal Mass Ejections. In SWAP observations, we track the evolution of off-limb erupting features as well as associated on-disk EUV waves, and the kinematics of both are calculated. The first eruption shows a clear deceleration throughout the lower corona into coronagraph observations, whereas the second eruption, which had a lower initial velocity, shows no obvious acceleration or deceleration profile. This paper presents a unique set of observations, allowing features observed in EUV to be traced to greater heights in the solar atmosphere, helping to bridge the gap to the FOV of white-light coronagraphs. Even with these favorable data sets, it remains a challenging task to associate features observed in EUV with those observed in white light, highlighting our urgent need for single-instrument observations of the combined lower and middle corona.

EUV waves are large-scale disturbances in the solar corona initiated by coronal mass ejections. However, solar EUV images show only the wave fronts projections along the line-of-sight of the spacecraft. We perform 3D reconstructions of EUV wave front heights using multi-point observations from STEREO-A and STEREO-B, and study their evolution to properly estimate the EUV wave kinematics. We develop two different methods to solve the matching problem of the EUV wave crest on pairs of STEREO-A/-B images by combining epipolar geometry with the investigation of perturbation profiles. The proposed approaches are applicable at the early and maximum stage of the event when STEREO-A/-B see different facets of the EUV wave, but also at the later stage when the wave front becomes diffusive and faint. The techniques developed are demonstrated on two events observed at different separation of the STEREO spacecraft (42 • and 91 • ). For the 7 December 2007 event, we find that the emission of the EUV wave front mainly comes from a height range up to 90-104 Mm, decreasing later to 7-35 Mm. Including the varying height of the EUV wave front allows us to correct the wave kinematics for the projection effects, resulting in velocities in the range 217-266 km/s. For the 13 February 2009 event, the wave front height doubled from 54 to 93 Mm over 10 min, and the velocity derived is 205-208 km/s. In the two events under study, the corrected speeds differ by up to 25% from the uncorrected ones, depending on the wave front height evolution.