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. Between 24 and 25 December 2021 a sungrazing comet (SOHO-4341) approached the Sun, being observed by “classical” visible light (VL) coronagraphs on board the SOHO and STEREO missions, and also by the innovative Metis coronagraph on board the ESA-NASA Solar Orbiter mission in the VL and ultraviolet (UV H I Lyman-α) band.
Aims. We show how VL data acquired by the Metis coronagraph can be combined with those provided by other space-based coronagraphs to reconstruct the comet orbit, but also to provide information on the dust composition from the polarized VL emission. Moreover, we show how the UV emission can be employed to measure local plasma parameters of the ambient solar wind.
Methods. By using the comet positions tracked with VL Metis images (with spatial resolution that is four times better than UV), the UV images (with a time cadence that is five times faster than VL) have been coaligned to maximize the signal-to-noise ratio in the UV band. The local electron density ne was measured from the observed exponential decay of the UV Lyman-α intensity along the tail, while the solar wind speed vwind was measured from the UV Lyman-α tail inclination with respect to the cometary orbital path deprojected in 3D. Moreover, the proton kinetic temperature Tk was also obtained by the aperture angle of the UV Lyman-α tail.
Results. When the comet was at an average heliocentric distance of 14.3 R⊙, the comet had a radial speed of 155 km s−1 and a tangential speed of 59 km s−1. The comet had a UV Lyman-α tail extending in the anti-solar direction over more than 1.5 R⊙. From the analysis of the tail shape in UV we obtained the local solar wind speed (vwind = 190 km s−1), electron density (ne = 1.5 × 104 cm−3), and proton temperature (Tk = 1.2 × 106 K). Moreover, theoretical analysis of the measured UV Lyman-α intensity allowed us to estimate the radius of the cometary nucleus (Rcom = 65 m) and the water outgassing rate (QH2O = 4.8 × 1028 molec s−1).
Conclusions. These results show that sungrazing comets are unique “local probes” for the ambient coronal plasma, providing measurements that are not as affected by the line-of-sight integration effects as those provided by remote sensing instruments, in regions of the Heliosphere that are not explored in situ by the ongoing space missions.

Analysis of the first coronagraphic multi-band observations of a sungrazing comet

Tomography of the Solar Corona with the Metis Coronagraph II: Three-Dimensional Reconstructions of the Electron Density and Comparison with Reconstructions Based on LASCO-C2

Photometric Comparison of Metis and LASCO-C2 Polarized Brightness Images

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

Context. We present the results for the in-flight radiometric calibration performed for the Visible Light (VL) channel of the Metis coronagraph on board Solar Orbiter. Aims. The radiometric calibration is a fundamental step in building the o ffi cial pipeline of the instrument, devoted to producing the calibrated data in physical units (L2 data). Methods. To obtain the radiometric calibration factor ( ϵ VL ), we used stellar targets transiting the Metis field of view. We derived ϵ VL by determining the signal of each calibration star by means of the aperture photometry and calculating its expected flux in the Metis band pass. The analyzed data set covers the time range from the beginning of the Cruise Phase of the mission (June 2020) until March 2021. Results. Considering the uncertainties, the estimated factor ϵ VL is in a good agreement with that obtained during the on-ground calibration cam-paign. This implies that up to March 2021 there was no measurable reduction in the VL channel throughput. Finally, we compared the total and polarized brightness visible light images of the solar corona acquired with Metis and STEREO-A / COR2 during the November 2020 superior conjunction of these instruments. A general good agreement was obtained between the images of these instruments for both the total and polarized brightness.

In-flight radiometric calibration of the Metis Visible Light channel using stars and comparison with STEREO-A/COR2 data

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

In-flight radiometric calibration of the Metis Visible Light channel using stars and comparison with STEREO-A/COR2 data