Context. Galactic cosmic rays (GCRs) and solar energetic particles (SEPs) with energies greater than tens of megaelectron volts are at the origin of spacecraft deep charging. The High Energy Telescope of the Energetic Particle Detector instrument measures the particle flux incident on the Solar Orbiter spacecraft. An algorithm implemented in the processing electronics of the visible light (VL) instrument of the Metis coronagraph generates cosmic-ray matrices containing the pixels fired by high-energy particles. These independent observations allow us to monitor the secondary particle production into the spacecraft. Aims. We studied the GCR flux long-term variations during the ascending phase of solar cycle 25 and the evolution of two SEP events observed on July 24–26, 2023, and on February 9–14, 2024, above 80 MeV with the aim of evaluating the impact of galactic and solar high-energy particles on Metis. Methods. A Python tool named REBECCA has been developed for the automated analysis of the Metis cosmic-ray matrices. The number of observed particle tracks is compared to Monte Carlo simulations of the Metis VL bidimensional CMOS sensor used as a particle detector. Results. We present the modulation of the GCR energy spectrum from 2020 through 2024 above 100 MeV. The dynamics of two intense SEP events is also reported. Monte Carlo simulations indicate that the composition of particles in the cosmic-ray matrices is dominated by protons. Going from solar minimum to maximum, an increase in particles produced by cosmic rays in the spacecraft material surrounding Metis was observed. Conversely, during the whole evolution of SEP events, protons made up more than 90% of the particles. These observations were gathered near the ecliptic during a positive polarity epoch of the global solar magnetic field. Analogous studies will be conducted during the negative polarity epoch, within 1 au, both above and below the ecliptic plane, throughout the remaining duration of the mission.

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-alpha 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. We present the results of the in-flight radiometric calibration performed for the ultraviolet (UV) H I Ly-alpha channel of the Metis coronagraph on board Solar Orbiter. Aims. The radiometric calibration is a fundamental procedure required to produce data in physical units. The quantity that allows us to pass from raw data into calibrated data is the radiometric calibration factor, & varepsilon;(UV). Methods. To obtain the & varepsilon;(UV) results, we used observations of stellar targets transiting the Metis field of view. We derived & varepsilon;(UV) by determining the signal of each calibration star by means of the aperture photometry and evaluating its expected flux in the Metis narrow bandpass (121.6 +/- 10 nm). The analyzed data cover a time range from the beginning of the Cruise Phase in June 2020, up until August 2021. Results. We find that the UV channel requires a significant additional correction of the response across the field of view, compared to that provided by the vignetting function measured on the ground and refined in flight, specifically tailored to the UV channel. This correction is provided by the ratio of images of the back-illumination of the closed door. Here, we use the stellar measurements to refine and improve such a correction map. After correcting for the spatial disuniformity, a radiometric calibration factor & varepsilon;(UV) = 0.20 +/- 0.03 DN/photon was found. No significant changes in the UV channel throughput were observed during the period from June 2020 to August 2021. In addition, the analysis of a smaller number of stars observed in 2022 and 2023 enabled us to extend the validity of the radiometric calibration to that period, after considering a suitable scaling factor due to the change of operating voltages occurred in April 2022. From this second analysis, the value of the radiometric calibration factor is & varepsilon;(UV) = 0.11 +/- 0.03 DN/photon. In order to support the radiometric calibration results, we performed a comparison between average radial profiles of the H I Ly-alpha intensity obtained from Metis UV images acquired in 2020-2021 and those measured with the Ultra-Violet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO) during the period of the activity minimum of solar cycle 22 in 1996. We found that intensity profiles of these instruments are consistent with each other.

Context. Waves are thought to play a significant role in the heating of the solar atmosphere and the acceleration of the wind. Among the many types of waves observed in the Sun, the so-called p modes with a 3 mHz frequency peak dominate the lower atmosphere. In the presence of magnetic fields, these waves can be converted into magnetohydrodynamic modes, which then leak into the corona through magnetic conduits. High-resolution off-limb observations have revealed signatures of ubiquitous and global 3 mHz oscillations in the corona, although they are limited to low heights and to incompressible modes. Aims. We present high-cadence, high-resolution observations of the corona in the range 1.7-3.6 R-circle dot taken in broad-band 580-640 nm visible light by the Metis coronagraph on board Solar Orbiter. These observations were designed to investigate density fluctuations in the middle corona. Methods. The data were acquired over several days in March 2022, October 2022, and for two days in April 2023. We selected representative regions of the corona on three sample dates. Analysis of the data in those regions revealed the presence of periodic density fluctuations. By examining several time-distance diagrams, we determined the main properties (apparent propagation speed, amplitude) of those fluctuations. We also show power spectra in selected locations in order to determine the dominant frequencies. Results. We found wave-like, compressible fluctuations of low amplitude - on the order of 0.1% of the background - in several large-scale regions in the corona at least up to 2.5 R-circle dot. We also found that the apparent propagation speeds of these perturbations typically fall in the range 150-450 km s(-1). A power spectrum analysis of the time series revealed an excess power in the range 2-7 mHz, often with peaks at 3 or 5 mHz, i.e. in a range consistent with p-mode frequencies of the lower solar atmosphere.

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-circle dot. 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-circle dot (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-circle dot. The proposed method is reasonable within the limits of the validity of the assumptions considered in this work.

This study presents observations of a large pseudostreamer solar eruption and, in particular, the post-eruption relaxation phase, as captured by Metis, on board the Solar Orbiter, on 2022 October 12, during its perihelion passage. Utilizing total-brightness data, we observe the outward propagation of helical features up to 3 solar radii along a radial column that appears to correspond to the stalk of the pseudostreamer. The helical structures persisted for more than 3 hr following a jet-like coronal mass ejection associated with a polar crown prominence eruption. A notable trend is revealed: the inclination of these features decreases as their polar angle and height increase. Additionally, we measured their helix pitch. Despite the 2 minute time cadence limiting direct correspondence among filamentary structures in consecutive frames, we find that the Metis helical structure may be interpreted as a consequence of twist (nonlinear torsional Alfvén waves) and plasma liberated by interchange reconnection. A comparison was performed between the helix parameters as outlined by fine-scale outflow features and those obtained from synthetic white-light images derived from the high-resolution magnetohydrodynamics simulation of interchange reconnection in a pseudostreamer topology by P. F. Wyper et al. A remarkable similarity between the simulation-derived images and the observations was found. We conjecture that these Metis observations may represent the upper ends of the spatial and energy scales of the interchange reconnection process that has been proposed recently as the origin of the Alfvénic solar wind.

The Comet Interceptor mission has been adopted by the European Space Agency (ESA) Science Programme Committee in June 2022 as the first "F" mission in the Science Programme. The aim of the mission is to increase the knowledge on comets and on the Solar System formation by encountering and exploring a Dynamically New Comet (DNC) or an Interstellar Object (ISO) originating at another star. EnVisS (Entire Visible Sky) is an all-sky camera designed to fly on Comet Interceptor and whose scientific task is to study the radiance and the polarization properties of the comet coma in the visible spectrum. The camera is composed of an optical head, a filter strip assembly and a detector. The Institute for Photonics and Nanotechnologies (CNR-IFN) of Padova and Leonardo SpA (Campi Bisenzio-Florence) are in charge of the design of the filter package, which currently consists of three filter strips glued side by side. The central strip is a high transmission broadband (BB) filter in the range 550-800 nm with no polarization properties, while the side ones are linear polarization filters with the same transmission bandpass as the BB and with polarization axis at 45 degrees from one another. In the CNR-IFN laboratories different types of polarizers have been tested to establish which one has the most fitting properties for EnVisS's purposes. The analyzed filters are Moxtek Visible Light Polarizer RCV8N2EC and Ultra BroadBand Polarizer UBB01A, and Polarcor Wide Band Polarizer. For each type of polarizing filter, both transmissivity and reflectivity have been measured and compared both with those of the other filters as well as data provided by the manufacturer. Overall, measurements of the filters' transmissivity and reflectivity agree with those provided by the supplier and mostly fit EnVisS' purposes. Thanks to its optimal performance and the fused silica substrate, Moxtek UBB01A is considered the best candidate filter for the instrument between the polarizers that have been characterized.

Comet Interceptor (CI) is an ESA mission in cooperation with the Japanese Aerospace Exploration Agency (JAXA). It aims to provide the first-ever in-situ (as opposed to ground-based observation) characterisation of a long period comet, which could be a dynamically-new comet or an interstellar object, and to perform the first simultaneous multi-point exploration of a cometary coma and nucleus.The science of the mission encompasses two main themes: Comet Nucleus Science and Comet Environment Science. More specifically, the key questions to address are:1) Comet Nucleus Science - What is the surface composition, shape, morphology, and structure ofthe target object?2) Comet Environment Science - What is the composition of the coma, its connection to the nucleus(activity) and the nature of its interaction with the solar wind?The CI mission consists of the main spacecraft (S/C A) and the two probes (named Probe B1 and Probe B2, respectively). The Comet Interceptor payload complement includes remote sensing and in situ measurement instruments accommodated on-board the main S/C and the two probes, so as to enable performing multi-point observations of the selected object during the fly-by. Comet Interceptor will be launched together with the Ariel mission, to the Sun-Earth second Lagrange point, L2. Following a waiting phase at L2, used to select the actual target object and to optimise the related transfer orbit, CI will cruise to the encounter and release the two probes shortly before performing the fly-by. The duration of the waiting phase depends on the actual target and its maximum length is estimated to be four years. After their release, the two probes will perform autonomous operations, relaying the scientific data back to the main spacecraft. The maximum duration of the CI mission, from launch to the end of the post-encounter phase, is six years. The payload for spacecraft A and B2 is provided by ESA Member States. Among these, on the B2 probe, the full sky camera EnVisS (Entire Visible Sky) will study and map the comet dust coma environment from an advantageous point inside the coma itself. EnVisS has been designed with a very wide 180° Field of View (FoV), which, by exploiting the B2 spacecraft rotation, enables the reconstruction of full-sky images. EnVisS adopts a flexible push-broom/push-frame imaging technique, wherein slices of the sky will be acquired while the probe rotates. Subsequently, on-ground, the acquired slices will be stitched together to form a full-sky image. The camera will be equipped with a 3-strip filter assembly transmitting the visible wavelength range 550-800 nm. One of the strips is a high-transmission broadband filter, while the others are linear polarization filters. Thanks to these filters, EnVisS will measure the intensity, the degree of linear polarization and polarization angle orientation of the light scattered by the dust particles in the comet coma, providing an unprecedented extended coverage of the phase angles.

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).

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum V capability of 600 ms^-1 . Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

Stellar in-flight calibrations have a relevant impact on the capability of space optical instruments, such as telescopes or cameras, to provide reliable scientific products, i.e., accurately calibrated data. Indeed, by using the in-flight star images, instrument optical performance can be checked and compared with the on-ground measurements. The analysis of star images carried out throughout the entire lifetime of the instrument in space will enable tracking changes in instrument performance and sensitivity due to degradation or misalignment of the optical components. In this paper, we present the concept, the necessary input and the available outputs of the simulations performed to predict the stars visible in the field of view (FoV) of a specific space instrument. As an example of the method, its application to two specific cases, the Metis coronagraph onboard Solar Orbiter and the stereo camera STereo Channel (STC) onboard BepiColombo, are given. Due to their proximity to the Sun, and to Mercury for STC, both instruments operate under harsh environmental conditions in terms of radiation exposure ((e.g., cosmic rays and SEP), high temperatures and significant temperature variations. Therefore, it is crucial to monitor their optical performances.

AbstractThe Metis coronagraph on-board the Solar Orbiter ESA mission is devoted to the study of the solar corona, but will also have the chance to observe near-Sun comets. Starting from November 2021 the scientific phase of the Solar Orbiter mission will take place.Metis can observe comets passing into its Field of View (FoV) covering an annulus between 1.6° and about 3° centered in the Sun direction.For the first time, simultaneous observation of comets both in the visible and in the UV at the Lyman-alpha can be performed.Comets with known trajectories passing in the Metis FoV will be planned to be observed; all the other comets, likely to be the majority, will be observed by serendipity.1. IntroductionA comet passing through the solar corona provides a means to estimate the plasma parameters at each point along its path, which is a useful complement to remote sensing.In addition to the clear value for solar physics, heliophysics, and space weather application, continuous coverage of the region surrounding the Sun by a coronagraph is important for the advancement of our understanding of near-Sun comets [1].SOHO-LASCO C2 and C3 have discovered about 4 thousand Sun-grazing and near-Sun comets; STEREO-SECCHI has also discovered a number of comets.SOHO-UVCS had the chance to study about 20 comets in the UV [2].Figure 1 SOHO-LASCO and STEREO-SECCHI bandpasses overlaid on a typical comet spectrum expressed in arbitrary units [1].As for the future solar missions, Solar Orbiter has been launched in February 2020 and, after a commissioning phase, its nominal scientific mission will start in November 2021. The mission is foreseen to last for about 10 years with the spacecraft orbit slowly moving out of the ecliptic plane to allow the study of the Sun’s polar regions.On board the Solar Orbiter spacecraft the Metis coronagraph is mounted [3].2. The Metis coronagraphMetis has been conceived to acquire both visible and UV images of the solar corona. It is the first coronagraph that will allow simultaneous observation in visible light (VL) and UV.The Metis instrument is conceived to image the solar corona from a near-Sun orbit in the UV narrow band HI Lyman-alpha at 121.6 nm, and in the polarized broad-band visible light (580 – 640 nm).The instrument is designed to image the structure and dynamics of the full corona in the range from 1.6 to 3.1 solar radii (Rs), at minimum perihelion distance (0.28 AU), and from 2.8 Rs to 5.5 Rs, at 0.5 AU. The annular FoV covered by the instrument is between 1.6° and 2.9°, and the attained spatial resolution is 20" [4].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 VL channel bandpass of Metis includes the NaI doublet, i.e. at about 589 nm, and this feature should help comet observation since the NaI emissions may make near-Sun comets very bright. As for the UV channel, strong Lyman-alpha emissions are expected to be observed in comets [5].The scientific highlights of Metis coronagraph observation of comets can be summarized as follows:- VL observations near comet perihelion are useful to study scattering, side distribution, and quantity of dust.- HI Lyman-alpha observation allows to infer solar wind direction, speed and density.- Analyze comet tail direction and its dynamics [6].3. Planning for observing cometsIn order to predict and allow planning for the observation of known comets, a tool to calculate the “transit” of the comet in the FoV of Metis is being devised by the team.An example of the foreseen passage of the comet C/2021 A1 Leonard in December 2021 can be seen in Figure 3.Figure 3 Expected “transit” of the comet Leonard in the Metis FoV both for the VL and the UV channel.4. ConclusionsThe Metis coronagraph on-board the Solar Orbiter mission will have the chance to observe some comets.Future observation campaigns are foreseen and will be devoted to study some of the possible comets passing inside the instrument FoV both in the VL, with the possibility of polarization imaging, and at the same time in the UV (Lyman-alpha).Some other comets will be imaged by chance during the nominal observation foreseen for Metis.Both the populations “foreseen” and ‘by chance’ comets can give some insights to probe the near environment of the Sun but also help in studying the physics and processes occurring in the tail and nucleus of the comet itself.For the first time and of extreme importance, with the Metis coronagraph, comets are going to be observed simultaneously both in the visible and in the UV (Lyman-alpha).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] Jones, G.H. et al., "The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets", Space Sci Rev 214(20) (2018).[2] Bemporad, A. et al., "A review of SOHO/UVCS observations of sungrazing comets", Planetary and Space Science 55(9), 1021-1030 (2007).[3] Antonucci, E. et al., "Metis: the Solar Orbiter visible light and ultraviolet coronal imager,” A&A 642, A10 (2020).[4] Fineschi, S. et al., "Optical design of the multi-wavelength imaging coronagraph Metis for the solar orbiter mission," Exp. Astron. 49, 239-263 (2020).[5] Bertaux, J. L. et al. "Interpretation of Hydrogen Lyman-Alpha Observations of Comets Bennett and Encke", A&A 25, 415-430 (1973)[6] Nisticò, G. et al., "Oscillations of cometary tails: a vortex shedding phenomenon?", A&A 615, A143 (2018).

Stellar in-flight calibration plays a pivotal role in improving the reliability of scientific data acquired by space optical instruments. Changes in sensitivity and performance of the image quality, caused by factors such as optical component degradation or misalignment, can be discovered and tracked by employing in-flight star images and comparing them with on-ground measurements. In this work, we introduce two simulation processes useful for this purpose and apply them to the Metis coronagraph aboard the ESA/NASA Solar Orbiter spacecraft. The first simulation process is a methodology for predicting star visibility in the Field of View (FoV) of the instrument. The second one improves the former code, integrating characteristics on the source, such as star magnitude, and the instrumental features, including reflectivity/transmission of the optical elements, and detector characteristics, e.g. bias, dark current, ... The ultimate aim of the simulation is to generate an estimation of the intensity, in Digital Numbers, expected for each pixel of the detector, thus offering valuable insights into the instrument's response to varying input flux conditions. This innovative approach will provide a comprehensive tool to anticipate and understand the coronagraph's behavior in response to different celestial scenarios (e.g. from minimum to solar maximum conditions), contributing to more effective in-flight calibrations. Indeed, Metis operates in proximity to the Sun, in a challenging environment marked by high temperatures and significant temperature variations. Although the current results are preliminary, further work is needed to refine and fully understand the simulation outcomes.

Context. The Solar Orbiter Metis coronagraph captures images of the solar corona in both visible (VL) and ultraviolet (UV) light. Tracks ascribable to the passage of galactic and solar particles appear in the Metis images. An algorithm implemented in the Metis processing electronics allows us to separate the pixels fired by VL photons from those crossed by high-energy particles. These spurious pixels are stored in cosmic-ray matrices that can be visually analyzed for particle monitoring deep into the spacecraft’s interior. This algorithm has been enabled for the VL instrument only, since the process of separating the particle tracks from pixels fired by photons in the UV images was shown to be quite challenging with respect to a quantitative analysis. Aims. This work is aimed at studying galactic cosmic rays (GCRs) and solar energetic particles (SEPs) with the Metis cosmic-ray matrices in February 2023. Methods. We compared a visual analysis of Metis cosmic-ray matrices gathered on February 22, 2023, with GCRs only, and on February 25, 2023 with both GCRs and SEPs, to Monte Carlo simulations of the VL instrument during the same days. Results. We estimated the solar modulation parameter associated with the GCR proton energy spectrum in February 2023. We show that Metis plays the role of monitoring galactic and solar protons. The Metis particle observations are used for the diagnostics of the VL instrument performance and to study the spacecraft inner charging from solar minimum towards the next solar maximum. These achievements have been attained with the benefit of the joint observations of Metis, the Energetic Particle Detector/High Energy Telescope, and near-Earth and Earth-based instruments.

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.

This Letter reports the first observation of the onset of fully developed turbulence in the solar corona. Long time series of white-light coronal images, acquired by Metis aboard Solar Orbiter at 2 minutes cadence and spanning about 10 hr, were studied to gain insight into the statistical properties of fluctuations in the density of the coronal plasma in the time domain. From pixel-by-pixel spectral frequency analysis in the whole Metis field of view, the scaling exponents of plasma fluctuations were derived. The results show that, over timescales ranging from 1 to 10 hr and corresponding to the photospheric mesogranulation-driven dynamics, the density spectra become shallower moving away from the Sun, resembling a Kolmogorov-like spectrum at 3 R circle dot. According to the latest observation and interpretive work, the observed 5/3 scaling law for density fluctuations is indicative of the onset of fully developed turbulence in the corona. Metis observation-based evidence for a Kolmogorov turbulent form of the fluctuating density spectrum casts light on the evolution of 2D turbulence in the early stages of its upward transport from the low corona.

The work will describe the activities performed in the framework of the realization of a laboratory set-up for the integration and testing of a prototype of the EnVisS fish-eye camera. The EnVisS instrument is an all-sky camera conceived, and specifically designed, for Comet Interceptor, an ESA Fast mission foreseen to launch in 2029 to study a dynamically new comet. EnVisS will be mounted on a spinning stabilized probe performing a fast, about 20 hours, fly-by of the comet; the instrument task is to image the full coma of the comet in the 550-800 nm wavelength range to study the dust properties and its distribution. At the CNR-IFN premises in Padova-Italy, an ad-hoc laboratory test bench has been devised and set-up to integrate the EnVisS prototype and allow the verification of its optical performance. The final goal of the set-up will be twofold. At first, the EnVisS breadboard optical head developed by Leonardo S.p.A. (Florence-Italy) will be assembled with a dummy filter and a COTS detector package. After, together with the verification of the prototype optical performance, carry on a simulation of the acquisition scheme foreseen for the camera in flight. In this paper, the requirements for the set-up and the solutions adopted for its realization will be presented. An overview of the results obtained during the commissioning of the lab set-up, performed with some commercial elements (i.e. a fish-eye lens coupled to a camera), will be given.

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.

Comet Interceptor is the first Fast (F-class) mission in the European Space Agency (ESA) Cosmic Vision program and has been conceived to study a long-period comet. The mission concept includes a spin-stabilized probe venturing close to a yet-to-be selected, and possibly dynamically new, comet. On this probe, the Entire Visible Sky (EnVisS) camera will be hosted. EnVisS will address several fields of cometary science by carrying out observations, close to, and within, a comet’s coma. Both intensity and polarimetric measurements are foreseen. In this work, the up-to-date instrument concept, design and scientific capabilities of EnVisS will be presented. 1) IntroductionThe Comet Interceptor spacecraft mission configuration includes a spacecraft (called A) and two probes (called PB1 and PB2). Spacecraft A will carry on remote and in-situ observations of the target from afar; while PB1, by the Japan Aerospace Space Agency (JAXA), and PB2, by ESA, will perform close fly-bys of the target [1].PB2 spin-stabilized solution allows EnVisS to adopt a rotational push-broom or push-frame imaging technique to scan and image the whole environment around the probe. The filter strip assembly [2] mounted as close as possible to the detector grants the possibility to observe and perform polarimetric imaging in the visible range.EnVisS will map the intensity and the degree of linear polarization and polarization angle orientation of the light scattered by the dust particles in the comet coma with an extended phase angle coverage. Linear polarization is directly linked with dust size distribution, morphology, porosity and composition [3]. Monitoring linear polarization will provide insights into how these parameters correlate. 2) Instrument conceptThe EnVisS instrument works in the visible wavelength range from 550-800 nm. To acquire the full sky, it features an extremely wide Field of View (FoV).EnVisS adopts a flexible push-broom/push-frame imaging technique: as the probe rotates (see Figure 1 ), slices of the sky are acquired and later stitched together on-ground to reconstruct a full-sky image.Figure 1: In (a) placement of the EnVisS camera on the B2 probe. In (b) illustration of EnVisS full sky imaging scanning concept. In (c) schematics of the filter strips images on the 2k x 2k detector. The probe spin-axis will point to the comet nucleus for most of the time, except at the closest approach when the comet nucleus will fall inside the camera FoV (see Figure 1 (a) for the EnVisS placement on PB2).3) EnVisS: Instrument DesignThe EnVisS instrument consists of different parts (see Figure 2 ):a fish-eye optical head [4]; a commercial space-qualified detector package from 3D-Plus [5] equipped with an ad hoc filter strip package (FSA) [6]; ad-hoc electronics (power and data handling units); software. Figure 2: In (a) EnVisS CAD model. In (b) Optical head and camera components are highlighted [4]. The FSA contains three broad-band filters, all working in the same wavelength range (i.e., 550-800 nm):one broadband intensity filter centred on the detector (see Figure 1 (c) blue central strip I); two linear polarizing filters with transmission axis angles oriented at 45° one to the other; positioned on the sides (see Figure 1 (c) the red and yellow strips P1 and P2). A flexible approach is considered to achieve the required SNR. Depending on the target object activity, the map of the coma will be taken with different spatial resolutions, i.e. smearing and pixel binning.Along track, the signal from the coma is not expected to vary too much, high spatial resolution is not required and smearing can be tolerated. The spatial resolution is retained in the across-track direction and ensures a sampling of the comet phase function every 0.2°. This strategy will also allow for an adjustment of the exposure time if the radiance of the coma differs from expected.Further pixel binning on-board, or co-adding, on-ground, of the images over different rotations, could be considered if the signal is extremely low. AcknowledgementsThis work has been supported by: the Italian Space Agency (ASI) through contracts to the Istituto Nazionale di Astrofisica (2020-4-HH and 2023-14-HH.0), and the European Space Agency (ESA) through a Contract to the Italian National Research Council (CNR) (Contract n. 4000136673/21/NL/IB/ig); and Instituto de Astrofísica de Andalucía (IAA-CSIC, Granada, Spain) with SENER (Barcelona, Spain) being supported by the Spanish Ministerio de Ciencia e Innovación (MCIN) through ESA PRODEX and the Spanish National Plan Ref PID2021-126365NB-C21 respectively. References[1] Jones, et al., “The Comet Interceptor Mission”, Space Sci Rev 220, 9 (2024).[2] Naletto et al., “Characterization of the polarizing filters for the EnVisS camera”. Proc. SPIE paper 13092-225 (2024).[3] Fulle, A.C. Levasseur-Regourd, N. McBride, E. Hadamcik "In situ dust measurements from within the coma of 1P/Halley, The Astronomical Journal, 119:1968-1977, (2000).[4] Tofani, et al., “Design of the EnVisS instrument optical head”, SPIE Proc. 12777, International Conference on Space Optics — ICSO 2022; 127772P (2023).[5] https://www.3d-plus.com/[6] Nordera, et al., “Ghost analysis of the EnVisS camera for the Comet Interceptor ESA mission”, SPIE Proc. 12180, 1218036 (2022).