Silvano Fineschi

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.

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.

Context. Coronal mass ejections (CMEs) are complex transient solar phenomena often associated with solar flares, large-scale current sheets (CS), and the formation of dense plasma structures, known as plasmoids or plasma blobs. Multiwavelength observations are required to study the properties, onset, and evolution of these structures. Aims. We studied the properties of multiple plasma blobs that are likely related with magnetic reconnection that continues after the transit of a CME over the solar west limb on November 9, 2021, using the observations acquired by the Solar Orbiter/Metis coronagraph. Metis visible-light (VL) images show the propagation of plasma structures around the main CME propagation latitude, as also observed by the LASCO coronagraphs on board SOHO. The main goal was to study the properties of these plasmoids, using the two Metis observation channels, VL broadband and the ultraviolet (UV) H I Lyman- α line, to estimate the energies involved and infer the formation heights of the blobs under the hypothesis that they might be blobs that formed through a tearing instability along the post-CME CS. We then identified observational signatures of their formation in the images. Methods. We identified and tracked five blobs using VL images and measured their propagation speed. For the three brightest, we derived the electron densities and then calculated the kinetic, potential, and thermal energy densities that were dragged by the blobs. The five blobs were not visible in the UV images. A comparison between the intensities observed by Metis and the theoretical intensities expected in the UV channel enabled us to constrain the plasma temperature and the associated thermal energy density of the blobs, however. These estimates were used to infer the most probable heliocentric distances and times of the blob formation. Results. The blob speeds were between 550 and 770 km s ⁻¹ and densities between 8.2 ⋅ 10 ⁴ and 3.6 ⋅ 10 ⁵ cm ⁻³ . We estimated that the blob formation region ranged from 3.5 to 5.0 R _⊙ , that is, outside the Metis field of view, which extended between 5.3 and 9.6 R _⊙ on that day, but inside the LASCO-C2 field of view, as supported by the observed morphology. Conclusions. We characterize the evolution of post-CME blobs observed in the VL channel of Metis in terms of their energetics and formation heights, and, using simultaneous observations in the UV channel, in which the blobs were not visible, we also infer a constraint on the plasma temperature inside them. Our results suggest that some blobs formed inside the LASCO-C2 field of view (from 2 to 6 R _⊙ ), while other blobs likely formed in the inner corona (below 2 R _⊙ ) and were observed higher up by the LASCO and Metis coronagraphs.

Context. Comprehensive solar observations from the limb to the extended corona are essential to study the main processes that connect coronal sources of outflows with the heliosphere. In particular, inferring the temperature structure of the solar corona is important to constrain coronal models and to characterise the mechanisms responsible for the plasma heating and acceleration. However, electron temperature is a parameter that is difficult to obtain from direct measurements in the heliocentric range between 3 and 8 R _⊙ . Aims. The aim of this work is to show the potentiality of a method of inferring the coronal temperature by exploiting unprecedented combined visible light and extreme-ultraviolet (EUV) observations acquired by Metis and by the Full Sun Imager (FSI) telescope of the Extreme Ultraviolet Imager (EUI) on Solar Orbiter. Methods. We analysed coordinated observations performed by the two instruments on March 21, 2021. We combined the first image acquired by FSI in the EUV channel at 17.4 nm using its coronagraphic mode with the visible light polarized brightness (pB) Metis data. The intensities measured by Metis and EUI/FSI originate from physical processes that depend differently on electron density and temperature. We propose a method of combining them, allowing us to place constraints on the electron temperature. The electron density, derived from the inversion of the polarized brightness, was used to calculate the expected counts in the FSI passband based on the instrument response function, which is mainly a function of the electron temperature. From the comparison with the measured counts, we were able to infer two different temperature values, corresponding to the two possible solutions, given the analytical shape of the response function. Results. The electron temperature results at a heliocentric distance of 4.25 R _⊙ (i.e. the average height of the Metis/FSI superposition region of the analysed dataset) are (5.3 _−1.5 ^+2.0 ) · 10 ⁵ K and (1.4 _−0.2 ^+0.3 ) · 10 ⁶ K for the east streamer and (5.7 _−1.4 ^+1.9 ) · 10 ⁵ K and (1.4 _−0.3 ^+0.2 ) · 10 ⁶ K for the west streamer. The values derived from the proposed method are consistent with previous estimates in coronal streamers. Conclusions. For the first time, we have analysed combined coronal observations from EUI and Metis, which has given us a unique opportunity to infer, from their measurements, the physical parameters of the streamer belt. The electron temperature results derived in the present work can be considered as a range of possible values that can constrain this parameter at a coronal height of 4.25 R _⊙ . The proposed method is reasonable within the limits of the validity of the assumptions considered in this work.

Context. Metis is the solar coronagraph on board the ESA/NASA mission Solar Orbiter (SolO), launched in 2020. It is designed to acquire simultaneous images of the solar corona in visible polarized light (580−640 nm), and in the narrowband H I Lyman-α line (i.e., 121.6 nm). The instrument visible-light channel includes a polarimeter composed of two liquid crystal variable retarder (LCVR) cells, a quarter-wave plate, and a linear polarizer, with the LCVR cells acting as the modulating element. By applying a specific voltage to the cells, it is possible to modify the incoming polarized light by changing the direction of the associated vector by a known angle, i.e., the angle of retardance. Aims. The polarimetric characterization of Metis is needed to correctly derive the properties of the plasma in the observed solar corona. This work describes the steps we took to fully characterize the visible channel of the Metis coronagraph, i.e., by deriving the modulation and demodulation matrices for each pixel, the latter being the key element for deriving the Stokes vector of the observed K corona. We completed the characterization by deriving the error associated with the derived values. Methods. The first step is to fully characterize the Metis polarimeter. This is needed to derive the relation between the voltage applied to the liquid crystal cells and the angle of retardance imposed on the incoming polarized light. This step represents the starting point for the calibration of the full instrument. Results. We derived (i) the voltage versus retardance curve associated with the polarimeter and (ii) the demodulation tensor of the Metis coronagraph, which is now used to retrieve the Stokes vector that describes the coronal light polarization state. Conclusions. This calibration is fundamental to disentangle the coronal light from the instrumental effects, such as disuniformity and instrumental polarization.

Context. The solar wind plays a crucial role in shaping the heliosphere and influencing space weather. Understanding its origin and acceleration requires measurements of coronal dynamics. The Metis coronagraph on board of Solar Orbiter provides high-resolution simultaneous imaging of the middle solar corona in the polarized visible light and ultraviolet H I Lyα, which allows us to derive solar wind speed maps. Aims. We determine solar wind speed maps by applying the Doppler dimming technique to Metis observations in the distance range from about 3.0 to 7.6 R_⊙. The goal is to present the detailed algorithm, investigate the dependence of the speed on the parameters of the coronal model, and provide maps of the solar wind speed at the minimum of the solar activity. This is useful to improve our understanding of the physical processes that accelerate the wind. Methods. Solar wind speeds are inferred by analyzing H I Lyα intensities in combination with electron density maps derived from visible-light polarized-brightness data. The Doppler dimming effect is used to estimate outflow velocities with different coronal model parameters, such as the electron temperature, the kinetic temperature, and the helium abundance, which are tested to assess their effect on the results. Results. The wind speed maps confirm the bimodal distribution of the solar wind outflow velocities that characterize the near-minimum phases of solar activity. The slow wind (100−200 km s⁻¹) confined to the equatorial streamer belt and fast wind (250−400 km s⁻¹) originates from the polar coronal holes. The transition between these regions is sharp, with a steep velocity gradient at mid-latitudes. Variations in the coronal model parameters significantly affect the inferred speeds. This highlights the need for precise constraints on the coronal conditions. Conclusions. Our method allows a systematic mapping of the solar wind speed and can be applied to data that are daily acquired by Metis throughout the current solar cycle. This provides new information on regions in which the wind is accelerated and on their evolution. These results provide valuable constraints for heliospheric models and theoretical studies of the formation of the solar wind. Future observations, in particular, during closer Solar Orbiter perihelia, will refine these measurements and improve our understanding of the solar corona and solar wind dynamics.

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.