Context.
The Metis coronagraph is one of the remote sensing instruments of the ESA-NASA Solar Orbiter mission. The goal for the instrument is to enable the study of the solar atmosphere and solar ...wind by simultaneously acquiring images of the solar corona at two different wavelengths: visible light (VL), within a band ranging from 580 nm to 640 nm, and ultraviolet light, in the HI Ly
α
121.6 ± 10 nm. The visible-light channel of the coronagraph includes a polarimeter with electro-optically modulating liquid crystal variable retarders to measure the linearly polarized brightness of the K-corona and derive the electron density.
Aims.
In this paper, we present the first in-flight validation results of the Metis polarimetric channel together with a comparison to the on-ground calibrations. This paper seeks to validate the first use of an electro-optical device, the liquid crystal-based polarimeter, in deep space and within a hard radiation environment.
Methods.
We used the orientation of the K-corona’s linear polarization vector during the roll maneuvers of the Space Orbiter spacecraft for the in-flight calibration.
Results.
The Metis coronagraph on board the Solar Orbiter shows good agreement with the on-ground measurements. The in-flight validation confirms the expected performance of the visible-light channel polarimeter. Furthermore, a comparison between the first polarized brightness value obtained by Metis and the polarized brightness values obtained by the space-based coronagraph LASCO and the ground-based coronagraph K-Cor shows the consistency of the Metis calibrated results.
Context.
Galactic cosmic rays (GCRs) and solar particles with energies greater than tens of MeV penetrate spacecraft and instruments hosted aboard space missions. The Solar Orbiter Metis coronagraph ...is aimed at observing the solar corona in both visible (VL) and ultraviolet (UV) light. Particle tracks are observed in the Metis images of the corona. An algorithm has been implemented in the Metis processing electronics to detect the VL image pixels crossed by cosmic rays. This algorithm was initially enabled for the VL instrument only, since the process of separating the particle tracks in the UV images has proven to be very challenging.
Aims.
We study the impact of the overall bulk of particles of galactic and solar origin on the Metis coronagraph images. We discuss the effects of the increasing solar activity after the Solar Orbiter mission launch on the secondary particle production in the spacecraft.
Methods.
We compared Monte Carlo simulations of GCRs crossing or interacting in the Metis VL CMOS sensor to observations gathered in 2020 and 2022. We also evaluated the impact of solar energetic particle events of different intensities on the Metis images.
Results.
The study of the role of abundant and rare cosmic rays in firing pixels in the Metis VL images of the corona allows us to estimate the efficiency of the algorithm applied for cosmic-ray track removal from the images and to demonstrate that the instrument performance had remained unchanged during the first two years of the Solar Orbiter operations. The outcome of this work can be used to estimate the Solar Orbiter instrument’s deep charging and the order of magnitude for energetic particles crossing the images of Metis and other instruments such as STIX and EUI.
Context.
On 2021 February 12, two subsequent eruptions occurred above the western limb of the Sun, as seen along the Sun-Earth line. The first event was a typical slow coronal mass ejection (CME), ...followed ∼7 h later by a smaller and collimated prominence eruption, originating south of the CME, followed by a plasma blob. These events were observed not only by the SOHO and STEREO-A missions, but also by the suite of remote-sensing instruments on board Solar Orbiter.
Aims.
We show how data acquired by the Full Sun Imager (FSI), the Metis coronagraph, and the Heliospheric Imager (HI) from the Solar Orbiter perspective can be combined to study the eruptions and different source regions. Moreover, we show how Metis data can be analyzed to provide new information about solar eruptions.
Methods.
Different 3D reconstruction methods were applied to the data acquired by different spacecraft, including remote-sensing instruments on board Solar Orbiter. Images acquired by the two Metis channels in the visible light (VL) and H
I
Ly-
α
line (UV) were combined to derive physical information about the expanding plasma. The polarization ratio technique was also applied for the first time to Metis images acquired in the VL channel.
Results.
The two eruptions were followed in 3D from their source region to their expansion in the intermediate corona. By combining VL and UV Metis data, the formation of a post-CME current sheet (CS) was followed for the first time in the intermediate corona. The plasma temperature gradient across a post-CME blob propagating along the CS was also measured for the first time. Application of the polarization ratio technique to Metis data shows that by combining four different polarization measurements, the errors are reduced by ∼5 − 7%. This constrains the 3D plasma distribution better.
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.
We investigated the capability of mapping the solar wind outflow velocity of neutral hydrogen atoms by using synergistic visible-light and ultraviolet observations. We used polarised brightness ...images acquired by the LASCO/SOHO and Mk3/MLSO coronagraphs, and synoptic Lyα line observations of the UVCS/SOHO spectrometer to obtain daily maps of solar wind H I outflow velocity between 1.5 and 4.0 R⊙ on the SOHO plane of the sky during a complete solar rotation (from 1997 June 1 to 1997 June 28). The 28-days data sequence allows us to construct coronal off-limb Carrington maps of the resulting velocities at different heliocentric distances to investigate the space and time evolution of the outflowing solar plasma. In addition, we performed a parameter space exploration in order to study the dependence of the derived outflow velocities on the physical quantities characterising the Lyα emitting process in the corona. Our results are important in anticipation of the future science with the Metis instrument, selected to be part of the Solar Orbiter scientific payload. It was conceived to carry out near-sun coronagraphy, performing for the first time simultaneous imaging in polarised visible-light and ultraviolet H I Lyα line, so providing an unprecedented view of the solar wind acceleration region in the inner corona.
Context.
The determination of solar wind H
I
outflow velocity is fundamental to shedding light on the mechanisms of wind acceleration occurring in the corona. Moreover, it has implications in ...various astrophysical contexts, such as in the heliosphere and in cometary and planetary atmospheres.
Aims.
We aim to study the effects of the chromospheric Ly
α
line profile shape on the determination of the outflow speed of coronal H
I
atoms via the Doppler dimming technique. This is of particular interest in view of the upcoming measurements of the Metis coronagraph aboard the Solar Orbiter mission.
Methods.
The Doppler dimming technique exploits the decrease of coronal Ly
α
radiation in regions where H
I
atoms flow out in the solar wind. Starting from UV observations of the coronal Ly
α
line from the Solar and Heliospheric Observatory (SOHO), aboard the UltraViolet Coronagraph Spectrometer, and simultaneous measurements of coronal electron densities from
pB
coronagraphic observations, we explored the effect of the profile of the pumping chromospheric Ly
α
line. We used measurements from the Solar UV Measurement of Emitted Radiation, aboard SOHO, the Ultraviolet Spectrometer and Polarimeter, aboard the Solar Maximum Mission, and the Laboratoire de Physique Stellaire et Planetaire, aboard the Eight Orbiting Solar Observatory, both from representative on-disc regions, such as coronal holes and quiet Sun and active regions, and as a function of time during the solar activity cycle. In particular, we considered the effect of four chromospheric line parameters: line width, reversal depth, asymmetry, and distance of the peaks.
Results.
We find that the range of variability of the four line parameters is of about 50% for the width, 69% for the reversal depth, and 35% and 50% for the asymmetry and distance of the peaks, respectively. We then find that the variability of the pumping Ly
α
profile affects the estimates of the coronal H
I
velocity by about 9−12%. This uncertainty is smaller than the uncertainties due to variations of other physical quantities, such as electron density, electron temperature, H
I
temperature, and integrated chromospheric Ly
α
radiance.
Conclusions.
Our work suggests that the observed variations in the chromospheric Ly
α
line profile parameters along a cycle and in specific regions negligibly affect the determination of the solar wind speed of H
I
atoms. Due to this weak dependence, a unique shape of the Ly
α
profile over the solar disc that is constant in time can be adopted to obtain the values of the solar wind H
I
outflow velocity. Moreover, the use of an empirical analytical chromospheric profile of the Ly
α
, assumed uniform over the solar disc and constant in time, is justifiable in order to obtain a good estimate of the coronal wind H
I
outflow velocity using coronagraphic UV images.
Context.
The Metis coronagraph is one of the remote sensing instruments hosted on board the ESA/NASA Solar Orbiter mission. Metis is devoted to carry out the first simultaneous imaging of the solar ...corona in both visible light (VL) and ultraviolet (UV). High-energy particles can penetrate spacecraft materials and may limit the performance of the on-board instruments. A study of the galactic cosmic-ray (GCR) tracks observed in the first VL images gathered by Metis during the commissioning phase is presented here. A similar analysis is planned for the UV channel.
Aims.
We aim to formulate a prediction of the GCR flux up to hundreds of GeV for the first part of the Solar Orbiter mission to study the performance of the Metis coronagraph.
Methods.
The GCR model predictions are compared to observations gathered on board Solar Orbiter by the High-Energy Telescope in the range between 10 MeV and 100 MeV in the summer of 2020 as well as with the previous measurements. Estimated cosmic-ray fluxes above 70 MeV n
−1
have been also parameterized and used for Monte Carlo simulations aimed at reproducing the cosmic-ray track observations in the Metis coronagraph VL images. The same parameterizations can also be used to study the performance of other detectors.
Results.
By comparing observations of cosmic-ray tracks in the Metis VL images with FLUKA Monte Carlo simulations of cosmic-ray interactions in the VL detector, we find that cosmic rays fire only a fraction, on the order of 10
−4
, of the whole image pixel sample. We also find that the overall efficiency for cosmic-ray identification in the Metis VL images is approximately equal to the contribution of
Z
≥ 2 GCR particles. A similar study will be carried out during the whole of the Solar Orbiter’s mission duration for the purposes of instrument diagnostics and to verify whether the Metis data and Monte Carlo simulations would allow for a long-term monitoring of the GCR proton flux.
This paper describes the innovative optical design of the Metis coronagraph for the Solar Orbiter ESA-NASA mission. Metis is a multi-wavelength, externally occulted telescope for the imaging of the ...solar corona in both the visible and ultraviolet wavelength ranges. Metis adopts a novel occultation scheme for the solar disk, that we named “inverse external occulter”, for reducing the extremely high thermal load on the instrument at the spacecraft perihelion. The core of the Metis optical design is an aplanatic Gregorian telescope common to both the visible and ultraviolet channels. A suitable dichroic beam-splitter, optimized for transmitting a narrow-band in the ultraviolet (121.6 nm, HI Lyman-α) and reflecting a broadband in the visible (580–640 nm) spectral range, is used to separate the two optical paths. Along the visible light optical path, a liquid crystal electro-optical modulator, used for the first time in space, allows making polarimetric measurements.
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