Marilena Mierla

Context. Constraining the plasma properties and energetics of coronal mass ejections (CMEs) in the inner corona is essential for understanding their early evolution, yet it remains challenging because of limited observations. Aims. We investigate the mass and density evolution as well as energy partitioning of two CMEs observed on 14 and 16 September 2025, and assess the potential of coordinated Proba-3/ASPIICS and Aditya-L1/VELC observations to constrain CME energetics in the low corona. Methods. We present the first coordinated observations of CMEs obtained simultaneously by Proba-3/ASPIICS and Aditya-L1/VELC. Using ASPIICS white-light observations, we estimated the CME mass, volume, number density, and the evolution of kinetic, thermal, and magnetic energies. Magnetic energies were estimated from observed CME properties using observationally constrained, physically motivated assumptions. Simultaneous VELC Fe XIV 5303 A observations provide independent estimates of the emission measure, electron number density, thermal energy, and CME lateral extent. Results. The electron number density evolves as n _e χ r ^-33 for the 14 September CME and as n _e ∝ r ⁻⁶ for the 16 September CME. The two events exhibit markedly different energy partitioning in the low corona. For the 14 September CME, the kinetic and magnetic energies are comparable, while the thermal energy remains nearly two orders of magnitude smaller, indicating limited plasma heating. In contrast, the 16 September CME exhibits a substantial thermal-energy enhancement, with thermal energy eventually becoming comparable to kinetic energy. For both CMEs, the estimated magnetic energy remains comparable to or exceeds the kinetic energy over the observed height. Conclusions. Our results demonstrate the scientific potential of synergetic ASPIICS and VELC observations for constraining the CME mass, density, and energetics in the inner corona, providing new observational constraints on the early evolution of CMEs.

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.

We describe the scientific objectives and instrument design of the ASPIICS coronagraph aboard the Proba-3 mission of the European Space Agency launched on 5 December 2024. Proba-3 consists of two spacecraft in a highly elliptical orbit around the Earth. One spacecraft carries the telescope, and the external occulter is mounted on the second spacecraft. The two spacecraft fly in a precise formation during 6 hours of a 19.63 hour orbit, together forming a giant solar coronagraph called ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun). The very long distance between the external occulter and the telescope (around 144 m) represents an increase of two orders of magnitude compared to classical externally occulted solar coronagraphs. This allows the inner corona to be observed in eclipse-like conditions, i.e., close to the solar limb (down to 1.099 R _⊙ ) and with very little stray light. ASPIICS provides a new perspective on the inner solar corona that will help solve several outstanding problems in solar physics, such as the origin of the slow solar wind and the physical mechanism of coronal mass ejections.

Context . ASPIICS is a giant-baseline visible light solar coronagraph, which relies on the millimetric positioning performance of the precision formation flying Proba-3 mission of the European Space Agency. Proba-3 was launched on 5 December 2024, and since then ASPIICS has observed the solar corona with the field of view (1.1–3) R _⊙ . Aims . Diffraction, in particular diffraction of solar disk light on the external occulter, is known to provide a major source of stray light in coronagraphs. Our aims were to analyze diffracted light visible in ASPIICS images, to compare it with the analytical-numerical diffraction model reported earlier, and to fine-tune the model. Methods . We compared the diffraction effects visible in the ASPIICS data with simulated diffraction images; in particular, we compared the geometrical properties and the radiometric signal. The properties of the diffraction described in previous works suggest how to fine-tune the model in order to achieve a better correspondence with the observations. Results . Early ASPIICS observations, where diffraction is pronounced, fully confirm all the qualitative properties of diffracted light suggested by the model. After fine-tuning the model we found a quantitative correspondence of the level of 30–50%, depending on the configuration. Conclusions . The performed analysis allows us to validate our analytical-numerical model and justify the assumptions, and to estimate the contribution of the diffracted light in the ASPIICS images. Over most of the field of view the diffracted light is two orders of magnitude below the coronal signal.

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 km s ⁻¹ for HRIEUV and AIA, respectively, with maximum speeds of about 40 km s ⁻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 km s ⁻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.

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.

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.

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 (HRI_EUV) 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 HRI_EUV 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/HRI_EUV 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.

Many operational space weather forecasting frameworks are based on the Potential Field Source Surface (PFSS) model of the magnetic field. The output of PFSS serves as input in many heliospheric models that provide solar wind velocity predictions at L1. Previous studies in the context of prediction of open magnetic flux observed at L1 have suggested different source surface heights (R_ss) for the PFSS model at different phases of the solar cycle (SC). We investigate the effects and necessity of optimizing the R_ss in the PFSS model in the context of its use in the popular Wang-Sheeley-Arge (WSA) model for solar wind velocity prediction. We used Heliospheric Upwind Extrapolation (HUX) to extrapolate solar wind velocity in the heliosphere. We performed a study of 16 Carrington Rotations (CR) at different phases of the SC24 and SC25, using different types of magnetograms and WSA model parameters. We combine the coronal models (PFSS+WSA) with the heliospheric model (HUX) to predict solar wind velocity at L1 in our framework, i.e., PFSS+WSA+HUX. Our study suggests that using a higher R_ss (3.0 R_⊙) compared to the conventional R_ss (2.5 R_⊙) near the solar minimum, results in an improvement in the average Pearson’s correlation coefficient (cc) from 0.61 to 0.75 between the observed and modeled values of solar wind velocity profile at L1. We found that the performance of the framework improved by using zero-point corrected (ZPC) maps in comparison to the standard (STD) Carrington maps from GONG, as demonstrated by an increase in the correlation coefficient from 0.31 to 0.51. We also found that the improved performance of the framework for ZPC maps as compared to the STD full Carrington maps, can be attributed to its capability to capture the global magnetic field. This was further confirmed by comparing the extrapolated global magnetic field structures with the large-scale corona observed in the extended field of view of the PROBA2/SWAP images. Our work is a first step in the direction of improving the WSA model and points out the potential ways to enhance the PFSS+WSA framework of solar wind forecasting at L1.