The Solar Orbiter EUI instrument: The Extreme Ultraviolet Imager

Abstract

Context. The Extreme Ultraviolet Imager (EUI) is part of the remote sensing instrument package of the ESA/NASA Solar Orbiter mission that will explore the inner heliosphere and observe the Sun from vantage points close to the Sun and out of the ecliptic. Solar Orbiter will advance the “connection science” between solar activity and the heliosphere. Aims. With EUI we aim to improve our understanding of the structure and dynamics of the solar atmosphere, globally as well as at high resolution, and from high solar latitude perspectives. Methods. The EUI consists of three telescopes, the Full Sun Imager and two High Resolution Imagers, which are optimised to image in Lyman-α and EUV (17.4 nm, 30.4 nm) to provide a coverage from chromosphere up to corona. The EUI is designed to cope with the strong constraints imposed by the Solar Orbiter mission characteristics. Limited telemetry availability is compensated by state-of-the-art image compression, onboard image processing, and event selection. The imposed power limitations and potentially harsh radiation environment necessitate the use of novel CMOS sensors. As the unobstructed field of view of the telescopes needs to protrude through the spacecraft’s heat shield, the apertures have been kept as small as possible, without compromising optical performance. This led to a systematic effort to optimise the throughput of every optical element and the reduction of noise levels in the sensor. Results. In this paper we review the design of the two elements of the EUI instrument: the Optical Bench System and the Common Electronic Box. Particular attention is also given to the onboard software, the intended operations, the ground software, and the foreseen data products. Conclusions. The EUI will bring unique science opportunities thanks to its specific design, its viewpoint, and to the planned synergies with the other Solar Orbiter instruments. In particular, we highlight science opportunities brought by the out-of-ecliptic vantage point of the solar poles, the high-resolution imaging of the high chromosphere and corona, and the connection to the outer corona as observed by coronagraphs.

Lyman‐alpha Variability During Solar Flares Over Solar Cycle 24 Using GOES‐15/EUVS‐E

“…Future flare studies could look at the link between Lyα and LyC (Machado et al, 2018), or Lyα and Hα (Canfield et al, 1981), for example. The recent launch of Solar Orbiter also included an Extreme Ultraviolet Imager (EUI; Rochus et al, 2020;Schühle et al, 2011) that contains a Lyα channel as part of its High Resolution Imager (HRI) suite. EUI will image the Sun in Lyα at <1 s cadence, at 1 ′′ resolution at 0.3AU.…”

Solar Irradiance Variability Due to Solar Flares Observed in Lyman-Alpha Emission

“…Although the sensitivity of GOES-15/EUVS-E might be limited to larger flares, the new Lyα irradiance instruments that are part of the Extreme Ultraviolet and X-ray Irradiance Sensor (EXIS; Eparvier et al, 2009;Chamberlin et al, 2009) suite on the newly launched GOES-R series of spacecraft (GOES-16 and GOES-17 were launched in February 2017 and June 2018, respectively, with GOES-18 and GOES-19 to follow) will have a greater dynamic range, as well as will provide pseudo line profiles by sampling the Lyα profile in five wavelength pixels rather than as broadband measurements. Similarly, the imaging capability of the EUV Imager (EUI; Schühle et al, 2011;Rochus et al, 2020) on board Solar Orbiter will be able to spatially resolve flares in Lyα for the first time. The findings presented illustrate that even minor flares can produce small, but perceptible changes in the solar Lyα irradiance, and should therefore serve as a baseline for the advent of new Lyα flare observations and advanced numerical simulations that will become available during Solar Cycle 25.…”

Future Prospects for Solar EUV and Soft X-Ray Spectroscopy Missions

“…Multilayer coatings are ideal for solar imaging as they naturally yield a narrow bandpass that can be tuned to specific emission lines in the EUV spectrum. The first multilayer imaging telescope flown on a spacecraft was EIT (Delaboudinière et al, 1995) on SOHO, launched in 1995, followed by TRACE in 1998, EUVI on the STEREO spacecraft in 2006, SWAP on the Proba-2 spacecraft in 2009 (Seaton et al, 2013), AIA on the SDO spacecraft in 2010 (Lemen et al, 2012), SUVI on the GOES-16 and 17 spacecraft in 2016 and 2018 (Vasudevan et al, 2019), and EUI on Solar Orbiter in 2020 (Rochus et al, 2020).…”

“…For example the 4k × 4k detectors of the AIA instrument on SDO take about 3 s. CMOS sensors (also referred to as Active Pixel Sensors, APS) have a much faster readout performance and are likely to be used more extensively in the coming decade. Like CCDs, they can be used in a back-illuminated configuration, as demonstrated by EUI on Solar Orbiter (Rochus et al, 2020). A further advantage is that they are much more robust to harsh radiation environments, hence they have been used for the imaging instruments on board Solar Orbiter and PSP, which are operating far beyond the protective bubble of the Earth's magnetosphere.…”