Frederic Auchere

Context . Recent high-resolution observations from the Solar Orbiter mission can help detect the indirect signatures of heating at the smallest scales. Aims . In this work, we measure the properties and investigate the physical origin of propagating disturbances (PDs) in the intensity at the smallest resolvable scales with Solar Orbiter/EUI along coronal loops. Methods . We used two sequences of EUI/HRIEUV at high spatial (down to 125 per pixel) and temporal resolutions (5 of cadence). We placed slits along 13 active region (AR) coronal loops. We measured the plane-of-sky (PoS) velocities and the intensity perturbation damping along the slits of PDs. We also measured the periodicity of PDs in one slit by using a Fourier analysis. Results . We report the detection of PDs that have high PoS velocities ranging between 500 and 2000 (which we call “fast” PDs). They are only visible in the upper part of the coronal loops. The intensity increase associated with these fast PDs is on the order of 4–8% of the HRIEUV intensity, and we measured little to no damping of their intensity with distance. We also measured a peak in the Fourier power spectra at 2 min above the 95% confidence limit that is associated with fast PDs. The fast PDs are detected in the same coronal loops as PDs with a lower PoS velocity (70–90), which we refer to as “slow” PDs. Unlike the fast PDs, these slow PDs are only visible in the lower part of the coronal loop, and they show clear intensity damping. Conclusions . Slow PDs show properties consistent with slow magneto-acoustic modes or upflows. On the other hand, fast PDs cannot be explained by slow magneto-acoustic modes. Instead, they show properties consistent with fast flows induced by magnetic reconnection, current sheet generated by propagating transverse oscillations, and fast magnetohydrodynamics modes or Alfvén waves.

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. 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.

This paper presents a follow-up to previous work on correcting point-spread-function (PSF)-induced Doppler artifacts in observations by the SPICE spectrograph on Solar Orbiter. In a previous paper, we demonstrated the correction of these artifacts in the y − λ plane with PSF regularization, treating the forward problem with a method based on large sparse matrix inversion. It has since been found that similar apparent artifacts are also present in the x − λ direction, i.e., across adjacent slit positions. Correcting this is difficult (although not impossible) with the previous matrix inversion method due to the time variation between slit positions. We have therefore devised a new method that addresses both x − λ and y − λ artifacts simultaneously by applying wavelength-dependent shifts at each x − y plane of the spectral cube. This paper demonstrates the SPICE data issue, describes the new method, and shows a comparison with the previous one. We explore the time variation of the correction parameters for the SPICE data and show a clear orbit dependence. The results of the method are significantly higher-quality Doppler signals, which we estimate at less than ∼5 km/s uncertainty for brighter lines in the absence of other systematics. Furthermore, we show the new SPICE polar observation results as a demonstration. The correction codes are written in Python, publicly available on GitHub, and can be directly applied to SPICE level 2 datasets.

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.

The Chromospheric LAyer SpectroPolarimeter missions, CLASP2 and CLASP2.1, demonstrated that the near-UV spectral region between 279.30 and 280.68 nm is suitable for studying the magnetism of the solar chromosphere. In particular, the spectropolarimetric observations in the Mg ii h and k resonant doublet, Mn i 279.91 and 280.19 nm resonant lines, and Fe ii 279.79 and 280.66 nm lines acquired by these suborbital space experiments have been proven useful for inferring the magnetic field stratification in the solar chromosphere. However, several lines of the CLASP2/2.1 spectral region with significant circular polarization signals had remained unexplored. After identifying two Ni i (279.95 and 280.59 nm) lines, one Mn ii (280.62 nm) line, and one Fe i (280.53 nm) line, here we apply the weak-field approximation to the spectropolarimetric observations of active region plages by CLASP2 and CLASP2.1. By comparing the results with previous studies, we are able to estimate the formation heights of these CLASP2/2.1 additional spectral lines and to demonstrate their suitability to determine the magnetic field stratification from the photosphere to the upper chromosphere.

Context. Plasma upflows with a Doppler shift exceeding −10 km s⁻¹ at active region (AR) boundaries are considered potential sources of the nascent slow solar wind. These upflows are often located at the footpoints of large-scale fan-like loops and show temperature-dependent Doppler shifts with redshifts in the transition region and blueshifts in the lower corona. Aims. We investigate the driving mechanisms of a pair of coronal upflow regions on the western and eastern peripheries of an AR, which have different magnetic topologies and surroundings. It is aimed to explore how these upflows couple to the lower atmosphere. Methods. Using observations of the Fe XII 19.51 nm line from Hinode, we identified two upflow regions at the western and eastern boundaries of a decaying AR. Context images for the two regions were obtained by the High Resolution Imager (HRI) telescope of the Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter mission. Other instruments on Solar Orbiter and other observatories provide diagnostics to the lower atmosphere. Potential Field Source Surface (PFSS) extrapolations were used to examine the magnetic field configuration associated with the AR upflows. Results. The eastern upflow region, located over the AR moss, displays small-scale dynamic fibril structures, whereas the western region hosts fan-like loops. We found blueshifted Ne VIII emission at the eastern site, in contrast to redshifted Ne VIII profiles in the west. Magnetic field extrapolations reveal a pseudostreamer topology connecting both these regions. Moreover, low transition-region lines show systematically reduced redshift below the eastern footpoint. Conclusions. The observations support the scenario in which both upflows are driven by pressure imbalances created by coronal reconnection, leading to a continuous upflow above approximately 0.6 MK (i.e., Ne VIII line formation temperature). Meanwhile, mass flows in the lower transition region beneath the eastern upflow region appear to respond passively to the pressure-driven coronal upflows.

We determine magnetic fields from the photosphere to the upper chromosphere combining data from the Hinode satellite and the CLASP2.1 sounding rocket experiment. CLASP2.1 provided polarization profiles of the Mg ii h and k lines, as well as of the Mn i lines around 2800 Å, across various magnetic structures in an active region, containing a plage, a pore, and the edges of a sunspot penumbra. By applying the weak-field approximation to the circular polarization profiles of these spectral lines, we obtain a longitudinal magnetic field map at three different heights in the chromosphere (lower, middle, and upper). This is complemented by data from Hinode (photospheric magnetic field), the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory (high spatial resolution observations of the chromosphere and corona). We quantify the height expansion of the plage magnetic fields and find that the magnetic fields expand significantly in the middle chromosphere, shaping the moss observed above in the transition region and corona. We identified an area with polarity reversal at the upper chromosphere around the edge of the pore, suggesting the presence of a magnetic discontinuity in the upper chromosphere. Transient and recurrent jetlike events are observed in this region, likely driven by magnetic reconnection. Around the penumbral edge, we find large-scale magnetic fields corresponding to the superpenumbral fibrils seen in the upper chromosphere. In the superpenumbral fibrils, we find Zeeman-induced linear polarization signals, suggesting the presence of a significantly inclined magnetic field, as strong as 1000 G in the upper chromosphere.