Luca Teriaca

Context. Flare ribbons are footpoints of newly reconnected magnetic loops. They are commonly associated with strong plasma flows and a pronounced nonthermal broadening of transition-region lines resulting from impulsive energy deposition. However, the mechanism governing energy transport into flare ribbons remains uncertain, particularly under different magnetic field configurations. Aims. We aim to understand the energy deposition process at a flare ribbon located close to a sunspot by determining the physical nature of the small-scale downflows hosted by the ribbon. Methods. We studied small-scale downflows in a remote ribbon during the M7.7-class event on 30 September 2024 using coordinated observations from the Solar Dynamics Observatory, the Interface Region Imaging Spectrograph (IRIS), and Solar Orbiter. We used stereoscopic imaging from Solar Orbiter and IRIS to detect plane-of-sky motions on small scales (~200 km). We conducted spectroscopic analysis of IRIS Si IV profiles complemented by C II and Mg II diagnostics. Results. Apparent downflow speeds measured from Solar Orbiter imaging (limb view) are 85–130 km s ⁻¹ , while IRIS images (disk view) reveal plane-of-sky speeds of 70–85 km s ⁻¹ . In contrast, the IRIS Si IV profiles at the brightest ribbon pixels are well approximated by a single Gaussian, with minimal Doppler shifts and unexpectedly small nonthermal widths (~ 10 km s ⁻¹ ). Peak Si IV emission also coincides with a deeper Mg II central reversal, while the electron densities are one to two orders of magnitude higher than typical quiet-Sun values. Conclusions. The absence of significant line-of-sight velocity and nonthermal broadening differs from the typical behavior of flare ribbons. Our results are consistent with a scenario in which plasma motions are guided by highly inclined (nearly horizontal) magnetic fields in a chromospheric canopy and are not accompanied by strong turbulence or wave motions. This study suggests that flare ribbons can exhibit a wide diversity of lower-atmospheric responses and emphasizes the critical role of magnetic field geometry.

We present the point-spread function (PSF) of the Extreme Ultraviolet High-Resolution Imager (HRI _EUV ) on board Solar Orbiter, which observes the Sun at 174 Å. This PSF provides a quantitative description of light diffracted by the mesh and the mounting supporting the entrance filter, light diffracted by the mesh supporting the filter-wheel filter, as well as light that is diffusely scattered by the microroughness of the mirrors. Deconvolution with this PSF corrects the images for instrumental scattered light, substantially improving image quality and photometric accuracy. First, we determine the diffraction component of the PSF from mechanical drawings of the instrument. We find that 26% of the incoming light is diffracted, predominantly by the entrance-filter mounting and mesh. Second, we fit the diffuse scattered light using partial image occultations during the 2023 January 3 Mercury transit. We find that the diffuse scattered light is well described by a softened power law, which scatters 42% of light over the detector. Combined, 57% of the incoming light is redistributed over the detector by diffraction and scattering. Correcting for these effects markedly enhances the dynamic range and contrast of the observations. The intensity in bright structures intensifies by up to 40%, and the intensity in dark structures decreases by up to 85%. All image features become much clearer, facilitating a more precise scientific analysis of HRI _EUV observations.

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. Network loops are a common feature in the quiet Sun. The physical processes sustaining their energy budget are still discussed. Aims. We relied on a multi-instrumental (Solar Orbiter/EUI, Solar Orbiter/PHI, and IRIS) observation of a six-hour quiet-Sun region to measure the dynamics and the possible magnetic drivers of impulsive Extreme Ultraviolet (EUV) emission enhancements along network loops. Methods. We report the detection of small-scale impulsive EUV emission enhancements with EUI/HRIEUV in three network loops. We selected four EUV emission enhancements to measure their plane-of-sky velocities in HRIEUV, their Doppler velocities in the Si IV line (log T = 4.8) with IRIS, and their possible relation to small-scale flux emergence and fluctuation in one of the loop footpoints. Results. The plane-of-sky velocities of the four EUV emission enhancements have a component that seems to appear almost instantaneously along the loop (≥220 km s ⁻¹ ), and two of them had a co-temporal component with a plane-of-sky velocity of up to (77 ± 19) km s ⁻¹ , starting near one of the loop footpoints. In one case, we measured a co-temporal intensity increase in the Si IV line with IRIS that is associated with Doppler velocities down to −32 km s ⁻¹ and up to 18 km s ⁻¹ along the line of sight. Finally, we measured cases of small-scale (≈8 × 10 ¹⁶ Mx) mixed-polarity field emergence and fluctuation near one of the loop footpoints. Conclusions. We conclude that the fast components on the plane-of-sky are consistent with a thermal transfer or supersonic plasma flows, while the slower component is consistent with plasma flows. A possible physical origin for these EUV emission enhancements would be magnetic reconnection driven by either a photospheric motion of the loop footpoints or by the reconnection of the loop with small-scale magnetic bipoles.

Context. Largely radial ray-like structures, termed plumes, are often observed in coronal holes. Plumes have been proposed to channel magnetohydrodynamic (MHD) waves and the solar wind into the heliosphere. High-speed propagating disturbances (PDs), though well detected in plumes, cannot yet be clearly assigned to MHD waves or to mass flows. Additionally, plume bases as observed in the extreme ultraviolet are riddled with small-scale transients, which could be related to PDs. Aims. We studied three plumes within an equatorial coronal hole observed by the EUV High Resolution Imager (HRI _EUV ) of the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. The properties of small-scale brightenings at the plume bases were investigated to interpret their nature and the possible relation with PDs in plumes. Methods. A 30 Mm × 30 Mm sub-region is selected for each plume base. We processed the images with the difference of Gaussians (DoG) method to highlight the target brightenings, which were further identified with two different approaches. In the 30 minute observation, 50 brightenings were visually selected, which also helps to set thresholds for automatic detection, where we found 451 brightenings. Their properties, including apparent velocities on the plane of sky (PoS), were analyzed statistically. Potential field extrapolation based on the magnetic field data from the Polarimetric and Helioseismic Imager on board Solar Orbiter was used for correcting the PoS velocity to the real velocity along the magnetic field. Results. We observe that the majority of base brightenings are small scale (with an area of less than 1.3 Mm ² ), short lived (lasting less than five minutes), and slightly elongated at the plume bases. They display intricate movements, with most showing velocities less than 10 km s ⁻¹ on the PoS. The velocity distribution of the base brightenings selected via visual identification shows a peak at 6–7 km s ⁻¹ , while that of the automatically detected base brightenings peaks at a lower velocity of less than 5 km s ⁻¹ . Their 3D velocities are found to be substantially lower than (and difficult to reconcile with) the speeds detected at greater heights in the plumes. Conclusions. A direct link between base brightenings and PDs remains inconclusive due to the huge difference between their velocities. We propose two possibilities for base brightenings: they may be related to wave-driven Type I spicules or originate from interchange reconnections. Further investigation is required to validate these hypotheses.

Context. The HRI _EUV telescope was calibrated on ground at the Physikalisch-Technische Bundesanstalt (PTB), Germany’s national metrology institute, using the Metrology Light Source (MLS) synchrotron in April 2017 during the calibration campaign of the Extreme Ultraviolet Imager (EUI) instrument on board the Solar Orbiter mission. Aims. We used the pre-flight end-to-end calibration and component-level (mirror multilayer coatings, filters, detector) characterization results to establish the beginning-of-life performance of the HRI _EUV telescope, which will serve as a reference for radiometric analysis and monitoring of the telescope in-flight degradation. Methods. Calibration activities at component level and end-to-end calibration of the instrument were performed at PTB/MLS synchrotron light source (Berlin, Germany) and the SOLEIL synchrotron (Orsay, France). Each component optical property is measured and compared to its semi-empirical model. This pre-flight characterization is used to estimate the parameters of the semi-empirical models. The end-to-end response is measured and validated by comparison with calibration measurements, as well as with its main design performance requirements. Results. The telescope spectral response semi-empirical model is validated by the pre-flight end-to-end ground calibration of the instrument. It is found that HRI _EUV is a highly efficient solar EUV telescope with a peak efficiency superior to 1 e ⁻ .ph ⁻¹ , low detector noise (≈2.8 e rms), low dark current at operating temperature, and a pixel saturation above 120 ke- in low-gain or combined image mode. The ground calibration also confirms a well-modelled spectral selectivity and rejection, and low stray light. The EUI instrument achieves a state-of-the-art performance in terms of signal-to-noise and spatial resolution.

Solar flares are the most powerful, magnetically driven, explosions in the heliosphere. The nature of magnetic energy release in the solar corona that heats the plasma and accelerates particles in a flare, however, remains poorly understood. Here, we report high-resolution coronal observations of a flare by the Solar Orbiter mission that reveal initially weaker but rapid reconnection events, on timescales of a few seconds at most, leading to a more prominent activity of a similar nature that explosively causes a flare. Signatures of this process are further imprinted on the widespread raining plasma blobs with short lifetimes, giving rise to the characteristic ribbon-like emission pattern associated with the flare. Our observations unveil the central engine of a flare and emphasize the crucial role of an avalanche-like magnetic energy release mechanism at work.

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. 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.