The SUMER Data in the SOHO Archive

We have released an archive of all observational data of the VUV spectrometer Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on SOHO that has been acquired until now. The operational phase started with 'first light' observations on 27 January 1996 and will end in 2014. Future data will be added to the archive when they become available. The archive consists of a set of raw data (Level 0) and a set of data that are processed and calibrated to the best knowledge we have today (Level 1). This communication describes step by step the data acquisition and processing that has been applied in an automated manner to build the archive. It summarizes the expertise and insights into the scientific use of SUMER spectra that has accumulated over the years. It also indicates possibilities for further enhancement of the data quality. With this article we intend to convey our own understanding of the instrument performance to the scientific community and to introduce the new, standard-FITS-format database.Comment: 38 pages, 9 figures, accepted for publication by Solar Physic

The extreme UV imager telescope on-board the Solar Orbiter mission: overview of phase C and D

The Solar Orbiter mission is composed of ten scientific instruments dedicated to the observation of the Sun’s atmosphere and its heliosphere, taking advantage of an out-of ecliptic orbit and at perihelion reaching a proximity close to 0.28 A.U. On board Solar Orbiter, the Extreme Ultraviolet Imager (EUI) will provide full-Sun image sequences of the solar corona in the extreme ultraviolet (17.1 nm and 30.4 nm), and high-resolution image sequences of the solar disk in the extreme ultraviolet (17.1 nm) and in the vacuum ultraviolet (121.6 nm). The EUI concept uses heritage from previous similar extreme ultraviolet instrument. Additional constraints from the specific orbit (thermal and radiation environment, limited telemetry download) however required dedicated technologies to achieve the scientific objectives of the mission. The development phase C of the instrument and its sub-systems has been successfully completed, including thermo-mechanical and electrical design validations with the Structural Thermal Model (STM) and the Engineering Model (EM). The instrument STM and EM units have been integrated on the respective spacecraft models and will undergo the system level tests. In parallel, the Phase D has been started with the sub-system qualifications and the flight parts manufacturing. The next steps of the EUI development will be the instrument Qualification Model (QM) integration and qualification tests. The Flight Model (FM) instrument activities will then follow with the acceptance tests and calibration campaigns

The Lyman-α\alpha Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter

The hydrogen Lyman-$\alpha$ (H {\sc i} Ly$\alpha$) emission during solar flares has rarely been studied in spatially resolved images and its physical origin has not been fully understood. In this paper, we present novel Ly$\alpha$ images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-waveband and multi-perspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Ly$\alpha$ emission has a good temporal correlation with the thermal emissions at 1--8 \AA\ and 5--7 keV, indicating that the flaring Ly$\alpha$ is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Ly$\alpha$ at flare ribbons during the rise phase of the flare, as revealed by the hard X-ray imaging and spectral fitting. Besides originating from flare ribbons, the Ly$\alpha$ emission can come from flare loops, likely caused by plasma heating and also cooling that happen in different flare phases. It is also found that the Ly$\alpha$ emission shows fairly similar features with the He {\sc ii} 304 \AA\ emission in light curve and spatio-temporal variation along with small differences. These observational results improve our understanding of the Ly$\alpha$ emission in solar flares and also provide some insights for investigating the Ly$\alpha$ emission in stellar flares.Comment: 19 pages, 7 figures, and 2 tables. ApJ accepted. Comments are welcom

Investigating coronal loop morphology and dynamics from two vantage points

Coronal loops serve as the fundamental building blocks of the solar corona. Therefore, comprehending their properties is essential in unraveling the dynamics of the Sun's upper atmosphere. In this study, we conduct a comparative analysis of the morphology and dynamics of a coronal loop observed from two different spacecraft: the High Resolution Imager (HRI$_{EUV}$) of the Extreme Ultraviolet Imager aboard the Solar Orbiter and the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory. These spacecraft were separated by 43$^{\circ}$ during this observation. The main findings of this study are: (1) The observed loop exhibits similar widths in both the HRI$_{EUV}$ and AIA data, suggesting that the cross-sectional shape of the loop is circular; (2) The loop maintains a uniform width along its entire length, supporting the notion that coronal loops do not exhibit expansion; (3) Notably, the loop undergoes unconventional dynamics, including thread separation and abrupt downward movement. Intriguingly, these dynamic features also appear similar in data from both spacecraft. Although based on observation of a single loop, these results raise questions about the validity of the coronal veil hypothesis and underscore the intricate and diverse nature of complexity within coronal loops.Comment: Accepted for publication in A&A Letter

Slow solar wind sources

Context. The origin of the slow solar wind is still an open issue. One possibility that has been suggested is that upflows at the edge of an active region can contribute to the slow solar wind. Aims. We aim to explain how the plasma upflows are generated, which mechanisms are responsible for them, and what the upflow region topology looks like. Methods. We investigated an upflow region using imaging data with the unprecedented temporal (3 s) and spatial (2 pixels = 236 km) resolution that were obtained on 30 March 2022 with the 174 Å channel of the Extreme-Ultraviolet Imager (EUI)/High Resolution Imager (HRI) on board Solar Orbiter. During this time, the EUI and Earth-orbiting satellites (Solar Dynamics Observatory, Hinode, and the Interface Region Imaging Spectrograph, IRIS) were located in quadrature (∼92°), which provides a stereoscopic view with high resolution. We used the Hinode/EIS (Fe XII) spectroscopic data to find coronal upflow regions in the active region. The IRIS slit-jaw imager provides a high-resolution view of the transition region and chromosphere. Results. For the first time, we have data that provide a quadrature view of a coronal upflow region with high spatial resolution. We found extended loops rooted in a coronal upflow region. Plasma upflows at the footpoints of extended loops determined spectroscopically through the Doppler shift are similar to the apparent upward motions seen through imaging in quadrature. The dynamics of small-scale structures in the upflow region can be used to identify two mechanisms of the plasma upflow: Mechanism I is reconnection of the hot coronal loops with open magnetic field lines in the solar corona, and mechanism II is reconnection of the small chromospheric loops with open magnetic field lines in the chromosphere or transition region. We identified the locations in which mechanisms I and II work

Evolution of dynamic fibrils from the cooler chromosphere to the hotter corona

Dynamic fibrils (DFs) are commonly observed chromospheric features in solar active regions. Recent observations from the Extreme Ultraviolet Imager (EUI) aboard the Solar Orbiter have revealed unambiguous signatures of DFs at the coronal base, in extreme ultraviolet (EUV) emission. However, it remains unclear if the DFs detected in the EUV are linked to their chromospheric counterparts. Simultaneous detection of DFs from chromospheric to coronal temperatures could provide important information on their thermal structuring and evolution through the solar atmosphere. In this paper, we address this question by using coordinated EUV observations from the Atmospheric Imaging Assembly (AIA), Interface Region Imaging Spectrograph (IRIS), and EUI to establish a one-to-one correspondence between chromospheric and transition region DFs (observed by IRIS) with their coronal counterparts (observed by EUI and AIA). Our analysis confirms a close correspondence between DFs observed at different atmospheric layers, and reveals that DFs can reach temperatures of about 1.5 million Kelvin, typical of the coronal base in active regions. Furthermore, intensity evolution of these DFs, as measured by tracking them over time, reveals a shock-driven scenario in which plasma piles up near the tips of these DFs and, subsequently, these tips appear as bright blobs in coronal images. These findings provide information on the thermal structuring of DFs and their evolution and impact through the solar atmosphere.Comment: Accepted for publication in A&A Letters. Animation files are available https://drive.google.com/drive/folders/17-fqQz_P2T18llJ1jB6MJISMRvT5063F?usp=sharin

Evolution of dynamic fibrils from the cooler chromosphere to the hotter corona

Dynamic fibrils (DFs) are commonly observed chromospheric features in solar active regions. Recent observations from the Extreme Ultraviolet Imager (EUI) aboard the Solar Orbiter have revealed unambiguous signatures of DFs at the coronal base in extreme ultraviolet (EUV) emission. However, it remains unclear if the DFs detected in the EUV are linked to their chromospheric counterparts. Simultaneous detection of DFs from chromospheric to coronal temperatures could provide important information on their thermal structuring and evolution through the solar atmosphere. In this paper, we address this question by using coordinated EUV observations from the Atmospheric Imaging Assembly (AIA), Interface Region Imaging Spectrograph (IRIS), and EUI to establish a one-to-one correspondence between chromospheric and transition region DFs (observed by IRIS) with their coronal counterparts (observed by EUI and AIA). Our analysis confirms a close correspondence between DFs observed at different atmospheric layers and reveals that DFs can reach temperatures of about 1.5 million Kelvin, typical of the coronal base in active regions. Furthermore, the intensity evolution of these DFs, as measured by tracking them over time, reveals a shock-driven scenario in which plasma piles up near the tips of these DFs and, subsequently, these tips appear as bright blobs in coronal images. These findings provide information on the thermal structuring of DFs and their evolution and impact through the solar atmosphere

Signatures of dynamic fibrils at the coronal base: Observations from Solar Orbiter/EUI

The solar chromosphere hosts a wide variety of transients, including dynamic fibrils (DFs) that are characterised as elongated, jet-like features seen in active regions, often through H$\alpha$ diagnostics. So far, these features have been difficult to identify in coronal images primarily due to their small size and the lower spatial resolution of the current EUV imagers. Here we present the first unambiguous signatures of DFs in coronal EUV data using high-resolution images from the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. Using the data acquired with the 174~{\AA} High Resolution Imager (HRI$_{EUV}$) of EUI, we find many bright dot-like features (of size 0.3-0.5 Mm) that move up and down (often repeatedly) in the core of an active region. In a space-time map, these features produce parabolic tracks akin to the chromospheric observations of DFs. Properties such as their speeds (14 km~s$^{-1}$), lifetime (332~s), deceleration (82 m~s$^{-2}$) and lengths (1293~km) are also reminiscent of the chromospheric DFs. The EUI data strongly suggest that these EUV bright dots are basically the hot tips (of the cooler chromospheric DFs) that could not be identified unambiguously before because of a lack of spatial resolution.Comment: Accepted for publication in A&A Letters. Event movie can be downloaded from https://drive.google.com/file/d/1o_4jHA5JbyQtrpUBtB3ItE_s3HjF6ncc/view?usp=sharin

LEMUR: Large European Module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission

Understanding the solar outer atmosphere requires concerted, simultaneous solar observations from the visible to the vacuum ultraviolet (VUV) and soft X-rays, at high spatial resolution (between 0.1" and 0.3"), at high temporal resolution (on the order of 10 s, i.e., the time scale of chromospheric dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the chromosphere to the flaring corona), and the capability of measuring magnetic fields through spectropolarimetry at visible and near-infrared wavelengths. Simultaneous spectroscopic measurements sampling the entire temperature range are particularly important. These requirements are fulfilled by the Japanese Solar-C mission (Plan B), composed of a spacecraft in a geosynchronous orbit with a payload providing a significant improvement of imaging and spectropolarimetric capabilities in the UV, visible, and near-infrared with respect to what is available today and foreseen in the near future. The Large European Module for solar Ultraviolet Research (LEMUR), described in this paper, is a large VUV telescope feeding a scientific payload of high-resolution imaging spectrographs and cameras. LEMUR consists of two major components: a VUV solar telescope with a 30 cm diameter mirror and a focal length of 3.6 m, and a focal-plane package composed of VUV spectrometers covering six carefully chosen wavelength ranges between 17 and 127 nm. The LEMUR slit covers 280" on the Sun with 0.14" per pixel sampling. In addition, LEMUR is capable of measuring mass flows velocities (line shifts) down to 2 km/s or better. LEMUR has been proposed to ESA as the European contribution to the Solar C mission.Comment: 35 pages, 14 figures. To appear on Experimental Astronom

First Perihelion of EUI on the Solar Orbiter mission

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