Context. The Extreme Ultraviolet Imager (EUI), onboard Solar Orbiter consists of three telescopes: the two High Resolution Imagers in EUV (HRIEUV) and in Lyman-{\alpha} (HRILya), and the Full Sun Imager (FSI). Solar Orbiter/EUI started its Nominal Mission Phase on 2021 November 27. Aims. EUI images from the largest scales in the extended corona off limb, down to the smallest features at the base of the corona and chromosphere. EUI is therefore a key instrument for the connection science that is at the heart of the Solar Orbiter mission science goals. Methods. The highest resolution on the Sun is achieved when Solar Orbiter passes through the perihelion part of its orbit. On 2022 March 26, Solar Orbiter reached for the first time a distance to the Sun close to 0.3 au. No other coronal EUV imager has been this close to the Sun. Results. We review the EUI data sets obtained during the period 2022 March-April, when Solar Orbiter quickly moved from alignment with the Earth (2022 March 6), to perihelion (2022 March 26), to quadrature with the Earth (2022 March 29). We highlight the first observational results in these unique data sets and we report on the in-flight instrument performance. Conclusions. EUI has obtained the highest resolution images ever of the solar corona in the quiet Sun and polar coronal holes. Several active regions were imaged at unprecedented cadences and sequence durations. We identify in this paper a broad range of features that require deeper studies. Both FSI and HRIEUV operate at design specifications but HRILya suffered from performance issues near perihelion. We conclude emphasising the EUI open data policy and encouraging further detailed analysis of the events highlighted in this paper.

First perihelion of EUI on the Solar Orbiter mission

pdf/the-swap-filter-a-simple-azimuthally-varying-radial-filter-2hqsj8za.pdf

The SWAP Filter: A Simple Azimuthally Varying Radial Filter for Wide-Field EUV Solar Images

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. 

Combined coronal observations of the streamer belt with Metis and EUI instruments on Solar Orbiter

Abstract This paper presents Heliospheric conneXions for Coronal mass ejections (HeXCor), a new coronal mass ejection (CME) flux rope (FR) reconstruction model that combines the geometry of the graduated cylindrical shell (GCS) model, defined from remote sensing imagery, with a modified cylindrical magnetic field solution inferred from in situ data. The usage of synergistic remote sensing and in situ data allows HeXCor to reconstruct an advanced magnetic FR model of CMEs with both global 3D geometry and internal magnetic field defined. The model is validated with four case studies of Earth-affecting CMEs from 2010 to 2014 observed in the corona by Solar and Heliospheric Observatory and Solar TErrestrial RElations Observatory’s spacecraft with large separation angles. It is shown that the in situ fitting capability is comparable to traditional cylindrical fittings while uniquely providing global structural context. The model also uniquely defines both the poloidal and toroidal impact parameters to fully describe the characteristics of CME impacts. Two of the events (2012 July 12 and 2011 March 25 CMEs) are found to follow the basic assumptions of radial and self-similar expansion from the Sun to Earth, while the other two (2010 May 23 and 2013 April 11 CMEs) likely experience deflection in the propagation direction and change in axial orientation. These results demonstrate how HeXCor provides a realistic and informative CME model that is comprised of accurate 3D global geometry with internal magnetic fields. 

HeXCor: A Hybrid Magnetic Flux-rope Model of Coronal Mass Ejections Combining In Situ Measurements with Remote Sensing Observations

A point-spread function describes the optics of an imaging system and can be used to correct collected images for instrumental effects. The state of the art for deconvolving images with the point-spread function is the Richardson–Lucy algorithm; however, despite its high fidelity, it is slow and cannot account for light scattered out of the field of view of the detector. We reinstate the Basic Iterative Deconvolution (BID) algorithm, a deconvolution algorithm that considers photons scattered out of the field of view of the detector, and extend it for image subregion deconvolutions. Its runtime is 1.8 to 7.1 faster than the Richardson–Lucy algorithm for 4096×4096\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$4096 \times 4096$\end{document} pixel images and up to an additional factor of 150 for subregions of 250×250\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$250 \times 250$\end{document} pixels. We test the extended BID algorithm for solar images taken by the Atmospheric Imaging Assembly (AIA), and find that the reconstructed intensities between BID and the Richardson–Lucy algorithm agree within 1%.

The Basic Iterative Deconvolution: A Fast Instrumental Point-Spread Function Deconvolution Method That Corrects for Light That Is Scattered Out of the Field of View of a Detector

Most observations of the solar corona beyond 2 Rs consist of broadband visible light imagery from coronagraphs. The associated diagnostics mainly consist of kinematics and derivations of the electron number density. While the measurement of the properties of emission lines can provide crucial additional diagnostics of the coronal plasma (temperatures, velocities, abundances, etc.), these observations are comparatively rare. In visible wavelengths, observations at these heights are limited to total eclipses. In the VUV range, very few additional observations have been achieved since the pioneering results of UVCS. One of the objectives of the Full Sun Imager (FSI) channel of the EUI telescope on board the Solar Orbiter mission has been to provide very wide field-of-view EUV diagnostics of the morphology and dynamics of the solar atmosphere in temperature regimes that are typical of the lower transition region and of the corona. FSI carries out observations in two narrowbands of the EUV spectrum centered on 17.4 nm and 30.4 nm that are dominated, respectively, by lines of Fe IX/X (formed in the corona around 1 MK) and by the resonance line of He II (formed around 80 kK in the lower transition region). Unlike previous EUV imagers, FSI includes a moveable occulting disk that can be inserted in the optical path to reduce the amount of instrumental stray light to a minimum. FSI detects signals at 17.4 nm up to the edge of its FOV (7~Rs), which is about twice further than was previously possible. Comparisons with observations by the LASCO and Metis coronagraphs confirm the presence of morphological similarities and differences between the broadband visible light and EUV emissions, as documented on the basis of prior eclipse and space-based observations. The very-wide-field observations of FSI are paving the way for future dedicated instruments.

/pdf/beyond-the-disk-euv-coronagraphic-observations-of-the-1goetgei.pdf

Beyond the disk: EUV coronagraphic observations of the Extreme Ultraviolet Imager on board Solar Orbiter

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Beyond the disk: EUV coronagraphic observations of the Extreme Ultraviolet Imager on board Solar Orbiter