The SWAP EUV Imaging Telescope Part I: Instrument Overview and Pre-Flight Testing

The Sun Watcher with Active Pixels and Image Processing (SWAP) is an EUV solar telescope on board ESA's Project for Onboard Autonomy 2 (PROBA2) mission launched on 2 November 2009. SWAP has a spectral bandpass centered on 17.4 nm and provides images of the low solar corona over a 54x54 arcmin field-of-view with 3.2 arcsec pixels and an imaging cadence of about two minutes. SWAP is designed to monitor all space-weather-relevant events and features in the low solar corona. Given the limited resources of the PROBA2 microsatellite, the SWAP telescope is designed with various innovative technologies, including an off-axis optical design and a CMOS-APS detector. This article provides reference documentation for users of the SWAP image data.Comment: 26 pages, 9 figures, 1 movi

EUV fine structure and variability associated with coronal rain revealed by Solar Orbiter/EUI HRIEUV and SPICE

Coronal rain is the most dramatic cooling phenomenon of the solar corona and an essential diagnostic tool for the coronal heating properties. A puzzling feature of the solar corona, besides the heating, is its EUV filamentary structure and variability. We aim to identify observable features of the TNE-TI scenario underlying coronal rain at small and large spatial scales, to understand the role it plays in the solar corona. We use EUV datasets at unprecedented spatial resolution of ~240 km from EUI/HRIEUV and SPICE of Solar Orbiter from the spring 2022 perihelion. EUV absorption features produced by coronal rain are detected at scales as small as 260 km. As the rain falls, heating and compression is produced immediately downstream, leading to a small EUV brightening accompanying the fall and producing a "fireball" phenomenon. Just prior to impact, a flash-like EUV brightening downstream of the rain, lasting a few minutes is observed for the fastest events. For the first time, we detect the atmospheric response to the rain's impact on the chromosphere and consists of upward propagating rebound shocks and flows partly reheating the loop. The observed widths of the rain clumps are 500 +- 200 km. They exhibit a broad velocity distribution of 10 - 150 km s^-1, peaking below 50 km s^-1. Coronal strands of similar widths are observed along the same loops co-spatial with cool filamentary structure, which we interpret as the CCTR. Matching with the expected cooling, prior to the rain appearance sequential loop brightenings are detected in gradually cooler lines from corona to chromospheric temperatures. Despite the large rain showers, most cannot be detected in AIA 171 in quadrature, indicating that LOS effects play a major role in coronal rain visibility. Still, AIA 304 and SPICE observations reveal that only a small fraction of the rain can be captured by HRIEUV.Comment: Astronomy & Astrophysics; 32 Pages, 24 Main Figures, Appendi

Prominence-cavity regions observed using SWAP 174A filtergrams and simultaneous eclipse flash spectra

Images from the SWAP (Proba 2 mission) taken at 174A in the Fe IX/X lines are compared to simultaneous slitless flash spectra taken during the last solar total eclipse of July, 11th 2010. Many faint low excitation emission lines together with the HeI and HeII Paschen Alpha chromospheric lines are recorded on eclipse spectra where regions of limb prominences are obtained with space-borne imagers. We consider a deep flash spectrum obtained by summing 80 individual spectra to show the intensity modulations of the continuum. Intensity depressions are observed around the prominences in both eclipse and SWAP images. The prominence cavities are interpreted as a relative depression of plasma density, produced inside the corona surrounding the prominences. Photometric measurements are shown at different scales and different, spectrally narrow, intervals for both the prominences and the coronal background.Comment: 22 pages, 14 figures, accepted to publish in Sol. Phy

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

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

Formation of a White-Light Jet within a Quadrupolar Magnetic Configuration

We analyze multi-wavelength and multi-viewpoint observations of a large-scale event viewed on 7 April 2011 originating from an active region complex. The activity leads to a white-light jet being formed in the outer corona. The topology and evolution of the coronal structures were imaged in high resolution using the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). In addition, large field-of-view images of the corona were obtained using the Sun Watcher using Active Pixel System detector and Image Processing (SWAP) telescope onboard the PRoject for Onboard Autonomy (PROBA2) microsatellite, providing evidence for the connectivity of the coronal structures with outer coronal features that were imaged with the Large Angle Spectrometric Coronagraph (LASCO) C2 on Solar and Heliospheric Observatory (SOHO). The data-sets reveal an Eiffel-tower type jet configuration extending into a narrow jet in the outer corona. The event starts from the growth of a dark area in the central part of the structure. The darkening was also observed in projection on the disk by the Solar TErrestrial RElations Observatory-Ahead (STEREO-A) spacecraft from a different point of view. We assume that the dark volume in the corona descends from a coronal cavity of a flux rope that moved up higher in the corona but still failed to erupt. The quadrupolar magnetic configuration corresponds to a saddle-like shape of the dark volume and provides a possibility for the plasma to escape along the open field lines into the outer corona, forming the white-light jet.Comment: 15 pages, 10 figure

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

The EUI flight instrument of Solar Orbiter: from optical alignment to end-to-end calibration

The Extreme Ultraviolet Imager (EUI) instrument for the Solar Orbiter mission will image the solar corona in the extreme ultraviolet (17.1 nm and 30.4 nm) and in the vacuum ultraviolet (121.6 nm) spectral ranges. The development of the EUI instrument has been successfully completed with the optical alignment of its three channels’ telescope, the thermal and mechanical environmental verification, the electrical and software validations, and an end-toend on-ground calibration of the two-units’ flight instrument at the operating wavelengths. The instrument has been delivered and installed on the Solar Orbiter spacecraft, which is now undergoing all preparatory activities before launch