The Project for On-Board Autonomy – 3 (PROBA-3) of the European Space Agency (ESA) is a technological demonstration mission for precise formation flying of two spacecrafts in a Highly Elliptical Earth Orbit (HEO). The mission’s main objective is to verify and validate high precision formation flying techniques, performing formation control, collision avoidance, and rendezvous operations; this goal can be achieved by operating a suite of different metrology systems. In addition to the technological goals, PROBA-3 accommodates the Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun (ASPIICS) scientific payload that takes advantage of the high precision formation flying capabilities to create a 144-meter externally occulted Coronagraph Instrument (CI), with the occulter on one satellite and the telescope on the other one. The metrology systems play the critical role for the success of the mission, and among these, the Shadow Position Sensors (SPS) has been designed and engineered to return the formation position with the highest accuracy. The SPS system consists of a set of 8 silicon photo-multipliers (SiPM) mounted in front of the ASPIICS telescope monitoring the symmetry of the penumbra projected by the external occulter on the coronagraph’s entrance pupil plane. A dedicated metrology algorithm converts the SiPM readout currents in relative and absolute lateral and longitudinal position coordinates that are provided to the GNC for fine formation maintenance manoeuvring. The flight algorithm has been developed by the team of the Astrophysical Observatory of Turin (OATo), part of the Italian National Institute for Astrophysics (INAF), and is based on a pseudo-paraboloidal description of the expected penumbra profile. The algorithm has been successfully tested during the ASPIICS calibration campaign held in 2021 in Turin, Italy. The same algorithm has been implemented in the data reduction pipeline of the Science Operation Center (SOC) together with an alternative algorithm, proposed by ESA, in order to have a cross-validation and, when possible, a refinement of the SPS metrology performances. In this work, we followed the approach proposed by ESA to implement and validate this alternative algorithm. The suggested approach uses the Cardano’s method to determine the radial position of each sensor with respect to the centre of the projected umbra and to calculate the coordinates of the umbra centre in the reference system centred on the telescope’s entrance pupil. This procedure is computationally costly with respect to the flight algorithm; however, it can achieve higher accuracy in those regions where the pseudo-paraboloid returns greater positioning errors, while guaranteeing an axis-symmetric modeling of the penumbra profile. In this paper, the implementation of this alternative algorithm is described and the performance of the two algorithms are compared and discussed in order to determine their advantages and disadvantages when executed in the SOC pipeline.

We present a comprehensive investigation of the fast coronal mass ejection (CME) event of 2022 September 5 by combining remote sensing observations, in situ measurements, and numerical magnetohydrodynamic (MHD) simulations. The CME, one of the most energetic of the recent solar cycle, was observed by SOHO and STEREO-A in the corona and intercepted in situ by the Parker Solar Probe (PSP) at similar to 0.07 au and Solar Orbiter at similar to 0.71 au. Using multiviewpoint coronagraphic data, we reconstruct the 3D geometry and kinematics of the CME-driven shock, deriving its propagation speed, direction, and initial size. These results are used to constrain a data-driven MHD simulation based on the RIMAP framework, which incorporates realistic solar wind conditions reconstructed from PSP and WIND measurements. The simulation reproduces key features of the shock and CME evolution detected at PSP, with a good quantitative agreement. For the Solar Orbiter, the model captures qualitative features of the shock passage, but does not fully reproduce the detailed temporal evolution of the event. The analysis highlights the importance of ambient solar wind structure and CME geometry in shaping shock propagation and evolution. We find that a simple cone model is insufficient to explain the observed duration of the event, requiring the inclusion of a velocity "tail" in the CME profile. Our results demonstrate the capability of combining multispacecraft observations with data-constrained MHD modeling to investigate CME evolution across the inner heliosphere and provide critical inputs for understanding shock-driven particle acceleration in extreme solar events.

Context. Magnetic reconnection events are frequently observed in the solar wind. Understanding the patterns and structures within the solar wind is crucial to put observed magnetic reconnection events into context, since their occurrence rate and properties are likely influenced by solar wind conditions. Aims. We employed unsupervised learning techniques such as self-organizing maps (SOM) and K-Means to cluster and interpret solar wind data at 1 AU for an improved understanding of the conditions that lead to magnetic reconnection in the solar wind. Methods. We collected magnetic field data and proton density, proton temperature, and solar wind speed measurements taken by the Wind spacecraft. After preprocessing the data, we trained a SOM to visualize the high-dimensional data in a lower-dimensional space and applied K-Means clustering to identify distinct clusters within the solar wind data. We then compare the results with the Xu, F., & Borovsky, J. E. (2015, J. Geophys. Res. Space Phys., 120, 70) classification of the solar wind. Results. Our analysis revealed that the reconnection events are distributed across five different clusters: (a) slow solar wind, (b) compressed slow wind, (c) highly Alfv & eacute;nic wind, (d) compressed fast wind, and (e) ejecta. Compressed slow and fast wind and ejecta are clusters associated with solar wind transients such as stream interaction regions and interplanetary coronal mass ejections. The majority of the reconnection events are associated with the slow solar wind, followed by the highly Alfv & eacute;nic wind, compressed slow wind, and compressed fast wind, and a small fraction of the reconnection events are associated with ejecta. Conclusions. Unsupervised learning approaches with SOM and K-Means lead to physically interpretable solar wind clusters based on their transients and allow for the contextualization of magnetic reconnection exhausts' occurrence in the solar wind.

On 2022 March 26, the ESA Solar Orbiter mission observed the early evolution of a coronal mass ejection (CME). On that day, the spacecraft was at a heliocentric distance of 0.32 au and a longitude separation from Earth of 74 . ° 5. The CME source region, observed with the Solar Orbiter Polarimetric and Helioseismic Imager magnetometer, shows no preexisting filament or flux rope. The event was first observed in the inner corona by the Solar Orbiter Extreme Ultraviolet Imager instrument with the Full Sun Imager telescope, showing the initial propagation of a flux rope seen face-on in the EUV, and formed during the eruption. Higher up, the event was observed by Metis with the Visible Light channel with an unprecedented time cadence of 20 s and a spatial resolution of 20” corresponding to about 4600 km bin −1 . The sequence of total brightness images shows the existence of small-scale circular flows inside the expanding flux rope, surrounded by multiple nested arch-shaped features. These motions, never reported so far, occur inside the void of the CME, with projected speed ∼40% higher than the CME propagation speed. The formation of the flux rope during the eruption suggests that these motions can be interpreted as a signature of conversion of magnetic writhe into twist, starting from the shearing of a preexisting arcade.

The study of space weather impacts of coronal mass ejections (CMEs) requires the formulation, implementation and validation of predictive approaches to address issues such as the arrival of CMEs to Earth and, if so, its arrival time and speed. The problem of predicting the CMEs’ travel times has been addressed by means of empirical models, physics-based models, and artificial intelligence techniques. In this talk, we propose a physics-driven artificial intelligence (AI) method, in which we encode physical information into the process of neural network training. Specifically we include the drag-based model in the definition of the loss functions to minimize during the training process of a cascade of two neural networks fed with both remote sensing and in situ data. We show that including physical information in the AI architecture improves its predictive capabilities and the proposed physics-driven AI method leads to more accurate and robust results for the CMEs’ travel time prediction with respect to the purely-data driven AI approach.

The identification and characterization of the coronal mass ejections (CMEs) and fast solar wind flows in the in situ data are important for understanding dynamics of these phenomena and consequently for space weather forecasting. In this study, we apply Self-Organizing Maps (SOMs) and clustering techniques to analyze in situ solar wind observations. SOMs (Kohonen, T, 1982) [1] an unsupervised learning technique, is employed to project high-dimensional interplanetary plasma parameters such as velocity, density, temperature, and magnetic field onto a lower-dimensional representation, preserving the topological structure of the data. Clustering algorithms, such as k-means, are then applied to the SOM output to distinguish between ICME events, fast and slow solar wind flows. Our approach is validated using a few months long interval of the ACE and Wind in situ observations, with labeled CME intervals from Richardson and Cane [2] as a benchmark. This combination of SOMs and clustering provides a framework for automated identification of interplanetary plasma structures, important for space weather studies but also for operational services. [1] T. Kohonen, ‘Self-organized formation of topologically correct feature maps’, Biol. Cybern., vol. 43, no. 1, pp. 59–69, Jan. 1982, doi: 10.1007/BF00337288[2] Richardson, Ian; Cane, Hilary, 2024, "Near-Earth Interplanetary Coronal Mass Ejections Since January 1996"https://doi.org/10.7910/DVN/C2MHTH

One of the most important objectives of solar physics is the physical understanding of the solar atmosphere, the structure of which is also described in terms of the density (N) and temperature (T) distributions of the atmospheric matter. Several multi-frequency analyses show that the characteristics of these distributions are still debated, especially for the outer coronal emission. We aim to constrain the T and N distributions of the solar atmosphere through observations in the centimetric radio domain. We employ single-dish observations from two of the INAF radio telescopes at the K-band frequencies (18 - 26 GHz). We investigate the origin of the significant brightness temperature ($T_B$) level that we detected up to the upper corona ($\sim 800$ Mm of altitude with respect to the photospheric solar surface). To probe the physical origin of the atmospheric emission and to constrain instrumental biases, we reproduced the solar signal by convolving specific 2D antenna beam models. The analysis of the solar atmosphere is performed by adopting a physical model that assumes the thermal bremsstrahlung as the emission mechanism, with specific T and N distributions. The modelled $T_B$ profiles are compared with those observed by averaging solar maps obtained during the minimum of solar activity (2018 - 2020). The T and N distributions are compatible (within $25\%$ of uncertainty) with the model up to $\sim 60$ Mm and $\sim 100$ Mm of altitude, respectively. The analysis of the role of the antenna beam pattern on our solar maps proves the physical nature of the atmospheric emission in our images up to the coronal tails seen in our $T_B$ profiles. The challenging analysis of the coronal radio emission at higher altitudes, together with the data from satellite instruments will require further multi-frequency measurements.

Throughout the initial four years of the operative mission, METIS coronagraph [1] carried out numerous scientific observations, including some focused-on comets. Among the observed cometary targets, there are periodic comets, like 2P/Encke, sunskirters, such as 96P/Machholz (see Figure 1), some sungrazers [2] and even a long period comet, the C/2021 A1 (Leonard) [3], having an orbital period of approximately 80,000 years. Although many of these observations, especially of periodic comets, were specifically planned, some comets were also identified a posteriori on images collected for solar corona studies.Figure 1: UV channel observation for the 96P/Machholz sunskirter comet during its transit in the METIS Field of View on January 30, 2023.Metis is the coronagraph onboard SolO and it has been conceived to acquire images of the solar corona both in linearly polarized visible light (VL, 580–640 nm) and narrow-band (±10 nm) ultraviolet (UV) around the HI Lyman-a (121.6 nm) spectral line. Metis is the first coronagraph able to perform such simultaneous observations.The instrument is designed to image the structure and dynamics of the full solar corona in an annular FoV covering the range from 1.6° to 2.9°, with a plate scale up to 10 “/px in VL channel and up to 20”/px in UV. Owing to the eccentricity of the spacecraft orbit, the heliocentric distances imaged are from 1.6 to 3.1 solar radii at minimum perihelion distance (0.28 au), up to the range from 6.0 to 12.0 solar radii when the spacecraft is around 1.0 au. A sketch of the raytrace of the two channels of the Metis coronagraph, i.e. the UV and VL, is given in Figure 2.Figure 2: Metis layout. On the top: the UV path. On the bottom: the VL path [4]. The ability of METIS to perform simultaneous imaging in a narrow UV band around HI Ly-alpha and in the visible wavelength range can be highly impactful in cometary studies. UV images enable the study of neutral hydrogen coma morphology and the estimation of the water outgassing rate from the comet nucleus. Conversely, visible polarization images allow the derivation of comet parameters correlated with the physical properties (distribution, density, size, ...) of the dust grains in the coma.In this work, a summary of the activities and main results obtained so far is presented, highlighting some original results obtained from METIS comet observations and sharing some valuable “lessons learned" from these four years of activity. AcknowledgementsSolar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. Metis was built and operated with funding from the Italian Space Agency (ASI), under contracts to the National Institute of Astrophysics (INAF) and industrial partners. Metis was built with hardware contributions from Germany (Bundesministerium für Wirtschaft und Energie through DLR), from the Czech Republic (PRODEX) and from ESA. References[1] Antonucci et al, A&A 642, A10 (2020).[2] Bemporad et al, A&A 680, A90 (2023).[3] Corso et al, EPSC2022-901 (2022)[4] Fineschi, S. et al., Exp. Astron. 49, 239-263 (2020).

We carried out tomographic reconstructions of the three-dimensional distribution of the electron density of the solar corona based on white light polarized brightness (pB) images taken by the Metis coronagraph on board the Solar Orbiter (SolO) mission. We selected three different time intervals during 2022, and further implemented independent synchronous reconstructions based on LASCO-C2 pB images for comparison purposes. The range of elongations covered by the field-of-view (FoV) of Metis considerably varies as SolO describes its highly eccentric orbit, whereas that of LASCO-C2 remains almost constant. During the selected time intervals, their FoVs partially overlap, allowing a comparison of the reconstructions within the regions in common. The shape and size of the reconstructed coronal structures, streamers and coronal holes, are consistent, demonstrating the suitability of the images of the synoptic program of Metis for tomographic reconstruction of the coronal electron density over its varying FoV. A comparison between the two tomographic reconstructions for each analyzed time interval, shows that the Metis-to-C2 ratio of reconstructed electron density has a median value of ≈ 1.7 . This is consistent with the observed ratio of the pB measurements of the two instruments. Our analysis thus also illustrates the value of tomography as a tool for intercalibrating solar coronagraphs irrespective of their spatial location, as long as their FoV partially overlap.

Context. Ultraviolet (UV) emission from coronal mass ejections can provide information on the evolution of plasma dynamics, temperature, and elemental composition, as demonstrated by the UV Coronagraph Spectrometer (UVCS) on board the SOlar and Heliospheric Observatory (SOHO). Metis, the coronagraph on board Solar Orbiter, provides for the first time coronagraphic imaging in the UV H I Ly- α line and, simultaneously, in polarized visible light, thus providing a host of information on the properties of coronal mass ejections and solar eruptions such as their overall dynamics, time evolution, mass content, and outflow propagation velocity in the expanding corona. Aims. For this work, we analyzed six coronal mass ejections observed by Metis between April and October 2021, which are characterized by a very strong H I Ly- α emission. We studied in particular the morphology, kinematics, and the temporal and radial evolution of the emission of such events, focusing on the brightest UV features. Methods. The kinematics of the eruptive events under consideration were studied by determining the height-time profiles of the brightest parts on the Metis plane of the sky. Furthermore, the 3D positions in the heliosphere of the coronal mass ejections were determined by employing co-temporal images, when available, from two other coronagraphs: LASCO/C2 on board SOHO, and COR2 on board STEREO-A. In three cases, the most likely source region on the solar surface could be identified. Finally, the radiometrically calibrated Metis images of the bright UV features were analyzed to provide estimates of their volume and density. From the kinematics and radiometric analysis, we obtained indications of the temperatures of the bright UV cores of these events. These results were then compared with previous studies with the UVCS spectrocoronagraph. Results. The analysis of these strong UV-emitting features associated with coronal mass ejections demonstrates the capabilities of the current constellation of space coronagraphs, Metis, LASCO/C2, and COR2, in providing a complete characterization of the structure and dynamics of eruptive events in their propagation phase from their inception up to several solar radii. Furthermore, we show how the unique capabilities of the Metis instrument to observe these events in both the H I Ly- α line and polarized VL radiation allow plasma diagnostics on the thermal state of these events.

On 2021 September 28, a C1.6 class flare occurred in active region NOAA 12871, located approximately at 27°S and 51°W on the solar disk with respect to Earth’s point of view. This event was followed by a partial halo coronal mass ejection (CME) that caused the deflection of preexisting coronal streamer structures, as observed in visible-light coronagraphic images. An associated type II radio burst was also detected by both space- and ground-based instruments, indicating the presence of a coronal shock propagating into interplanetary space. By using H i Ly α (121.6 nm) observations from the Metis coronagraph on board the Solar Orbiter mission, we demonstrate for the first time the capability of UV imaging to provide, via a Doppler dimming technique, an upper limit estimate of the evolution of the 2D proton kinetic temperature in the CME-driven shock sheath as it passes through the field of view of the instrument. Our results suggest that over the 22 minutes of observations, the shock propagated with a speed decreasing from about 740 ± 110 km s ^−1 to 400 ± 60 km s ^−1 . At the same time, the postshock proton temperatures peaked at latitudes around the shock nose and decreased with time from about 6.8 ± 1.01 MK to 3.1 ± 0.47 MK. The application of the Rankine–Hugoniot jump conditions demonstrates that these temperatures are higher by a factor of about 2–5 than those expected from simple adiabatic compression, implying that significant shock heating is still going on at these distances.

Aiming to assess the progress and current challenges on the formidable problem of the prediction of solar energetic events since the COSPAR / International Living With a Star (ILWS) Roadmap paper of Schrijver et al. (2015), we attempt an overview of the current status of global research efforts. By solar energetic events we refer to flares, coronal mass ejections (CMEs), and solar energetic particle (SEP) events. The emphasis, therefore, is on the prediction methods of solar flares and eruptions, as well as their associated SEP manifestations. This work complements the COSPAR International Space Weather Action Teams (ISWAT) review paper on the understanding of solar eruptions by Linton et al. (2023) (hereafter, ISWAT review papers are conventionally referred to as ’Cluster’ papers, given the ISWAT structure). Understanding solar flares and eruptions as instabilities occurring above the nominal background of solar activity is a core solar physics problem. We show that effectively predicting them stands on two pillars: physics and statistics. With statistical methods appearing at an increasing pace over the last 40 years, the last two decades have brought the critical realization that data science needs to be involved, as well, as volumes of diverse ground- and space-based data give rise to a Big Data landscape that cannot be handled, let alone processed, with conventional statistics. Dimensionality reduction in immense parameter spaces with the dual aim of both interpreting and forecasting solar energetic events has brought artificial intelligence (AI) methodologies, in variants of machine and deep learning, developed particularly for tackling Big Data problems. With interdisciplinarity firmly present, we outline an envisioned framework on which statistical and AI methodologies should be verified in terms of performance and validated against each other. We emphasize that a homogenized and streamlined (i.e., readily performed) method validation is another open challenge. The performance of the plethora of methods is typically far from perfect, with physical reasons to blame, besides practical shortcomings: imperfect data, data gaps and a lack of multiple, and meaningful, vantage points of solar observations. We briefly discuss these issues, too, that shape our desired short- and long-term objectives for an efficient future predictive capability. A central aim of this article is to trigger meaningful, targeted discussions that will compel the community to adopt standards for performance verification and validation, which could be maintained and enriched by institutions such as NASA’s Community Coordinated Modeling Center (CCMC) and the community-driven COSPAR/ISWAT initiative.

For the first time the evolution of the coronal reconfiguration after a Coronal Mass Ejection (CME) was observed by the multi-channel Metis Coronagraph on-board the ESA - Solar Orbiter mission. The images acquired in the Visible Light (VL) between 3.0 and 5.4 Rsun show the formation after a CME of a bright elongated radial feature interpreted as a post-CME Current Sheet (CS). This interpretation is supported by the appearance of the same feature as an intensity decrease in the UV Lyman-alpha images. The unique combination of VL and UV images allowed for the first time to map in 2D the time evolution of multiple plasma physical parameters inside and outside the CS region. In particular, the CS electron temperature reached peak values higher than 1 MK, more than 2 times larger than the surrounding corona in the covered altitude range. An elongated vertical diffusion region (DR), characterized as a region of much higher thermal pressure and lower magnetic pressure, is observed to slowly propagate outward during 13 hours of observations. Inside this region the Alfvènic Mach number is on the order of MA ≃ 0.02 - 0.11, the plasma β is close to unity, and the level of turbulence is higher than the surrounding corona, but decreases slowly with time. All these results provide one of the most complete pictures of these features, and support the idea of a magnetic reconnection coupled with turbulence and occurring in multiple smaller scale CSs resulting in the much broader macroscopic feature observed with the Metis coronagraph. This allows magnetic reconnection to provide a significant heating of the local plasma, despite the weakness of involved coronal magnetic fields in the considered altitude range.

Coronal Mass Ejections (CMEs) correspond to dramatic expulsions of plasma and magnetic field from the solar corona into the heliosphere. CMEs are scientifically relevant because they are involved in the physical mechanisms characterizing the active Sun. However, more recently, CMEs have attracted attention for their impact on space weather, as they are correlated to geomagnetic storms and may induce the generation of solar energetic particle streams. In this space weather framework, the present paper introduces a physics-driven artificial intelligence (AI) approach to the prediction of CMEs’ travel time, in which the deterministic drag-based model is exploited to improve the training phase of a cascade of two neural networks fed with both remote sensing and in situ data. This study shows that the use of physical information in the AI architecture significantly improves both the accuracy and the robustness of the travel time prediction.

Context. We present the results for the in-flight radiometric calibration performed for the Visible Light (VL) channel of the Metis coronagraph on board Solar Orbiter. Aims. The radiometric calibration is a fundamental step in building the official pipeline of the instrument, devoted to producing the calibrated data in physical units (L2 data). Methods. To obtain the radiometric calibration factor ( ϵ VL ), we used stellar targets transiting the Metis field of view. We derived ϵ VL by determining the signal of each calibration star by means of the aperture photometry and calculating its expected flux in the Metis band pass. The analyzed data set covers the time range from the beginning of the Cruise Phase of the mission (June 2020) until March 2021. Results. Considering the uncertainties, the estimated factor ϵ VL is in a good agreement with that obtained during the on-ground calibration campaign. This implies that up to March 2021 there was no measurable reduction in the VL channel throughput. Finally, we compared the total and polarized brightness visible light images of the solar corona acquired with Metis and STEREO-A/COR2 during the November 2020 superior conjunction of these instruments. A general good agreement was obtained between the images of these instruments for both the total and polarized brightness.

On February 21 and March 21 – 22, 2021, the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter observed three prominence eruptions. The eruptions were associated with coronal mass ejections (CMEs) observed by Metis, Solar Orbiter’s coronagraph. All three eruptions were also observed by instruments onboard the Solar–TErrestrial RElations Observatory (Ahead; STEREO-A), the Solar Dynamics Observatory (SDO), and the Solar and Heliospheric Observatory (SOHO). Here we present an analysis of these eruptions. We investigate their morphology, direction of propagation, and 3D properties. We demonstrate the success of applying two 3D reconstruction methods to three CMEs and their corresponding prominences observed from three perspectives and different distances from the Sun. This allows us to analyze the evolution of the events, from the erupting prominences low in the corona to the corresponding CMEs high in the corona. We also study the changes in the global magnetic field before and after the eruptions and the magnetic field configuration at the site of the eruptions using magnetic field extrapolation methods. This work highlights the importance of multi-perspective observations in studying the morphology of the erupting prominences, their source regions, and associated CMEs. The upcoming Solar Orbiter observations from higher latitudes will help to constrain this kind of study better.

On-board the Solar Orbiter ESA/NASA mission there is Metis, a coronagraph designed to study the solar corona by providing an artificial solar eclipse.Metis features two channels working at the ultraviolet Lyman-α (121.6 nm) and in the visible light (580-640 nm).On-ground, the Metis radiometric performance has been tested using a flat-field panel (uniform illumination); the stability of the performance can be verified in-flight through the analysis of the stars passing in the Metis Field of View.Care must be taken to ensure the quality of the calibration, both before launch and for the long period associated with the space mission lifetime.For this reason, we are carrying out long period research of stars that cross the Field of View of Metis.In this paper, we describe the vignetting function acquired: on-ground, simulated via a raytracing code and in-flight derived from on-ground measurements (performing some adjustments to account for the real Metis flight configuration).These vignetting functions are then compared with the vignetting data derived from the passage of the star Theta Ophiuchi in