A&A 642, A6 (2020) Astronomy https://doi.org/10.1051/0004-6361/201937032 & c F. Auchère et al. 2020 Astrophysics The Solar Orbiter mission Special issue Coordination within the remote sensing payload on the Solar Orbiter mission F. Auchère1, V. Andretta13, E. Antonucci2, N. Bach12, M. Battaglia5, A. Bemporad2, D. Berghmans9, E. Buchlin1, S. Caminade1, M. Carlsson25, J. Carlyle6, J. J. Cerullo31, P. C. Chamberlin11, R.C. Colaninno4, J. M. Davila11, A. De Groof12, L. Etesi5, S. Fahmy6, S. Fineschi2, A. Fludra3, H. R. Gilbert11, A. Giunta3, T. Grundy3, M. Haberreiter22, L. K. Harra22,32, D. M. Hassler10, J. Hirzberger8, R. A. Howard4, G. Hurford14, L. Kleint5, M. Kolleck8, S. Krucker5, A. Lagg8, F. Landini26, D. M. Long21, J. Lefort12, S. Lodiot23, B. Mampaey9, S. Maloney30, F. Marliani6, V. Martinez-Pillet27, D. R. McMullin17, D. Müller6, G. Nicolini2, D. Orozco Suarez18, A. Pacros6, M. Pancrazzi20, S. Parenti1,9, H. Peter8, A. Philippon1, S. Plunkett33, N. Rich4, P. Rochus7, A. Rouillard24, M. Romoli20, L. Sanchez12, U. Schühle8, S. Sidher3, S. K. Solanki8,29, D. Spadaro16, O. C. St Cyr11, T. Straus13, I. Tanco23, L. Teriaca8, W. T. Thompson19, J. C. del Toro Iniesta18, C. Verbeeck9, A. Vourlidas15, C. Watson12, T. Wiegelmann8, D. Williams12, J. Woch8, A. N. Zhukov9,28, and I. Zouganelis12 (Affiliations can be found after the references) Received 31 October 2019 / Accepted 22 January 2020 ABSTRACT Context. To meet the scientific objectives of the mission, the Solar Orbiter spacecraft carries a suite of in-situ (IS) and remote sensing (RS) instruments designed for joint operations with inter-instrument communication capabilities. Indeed, previous missions have shown that the Sun (imaged by the RS instruments) and the heliosphere (mainly sampled by the IS instruments) should be considered as an integrated system rather than separate entities. Many of the advances expected from Solar Orbiter rely on this synergistic approach between IS and RS measurements. Aims. Many aspects of hardware development, integration, testing, and operations are common to two or more RS instruments. In this paper, we describe the coordination effort initiated from the early mission phases by the Remote Sensing Working Group. We review the scientific goals and challenges, and give an overview of the technical solutions devised to successfully operate these instruments together. Methods. A major constraint for the RS instruments is the limited telemetry (TM) bandwidth of the Solar Orbiter deep-space mission compared to missions in Earth orbit. Hence, many of the strategies developed to maximise the scientific return from these instruments revolve around the optimisation of TM usage, relying for example on onboard autonomy for data processing, compression, and selection for downlink. The planning process itself has been optimised to alleviate the dynamic nature of the targets, and an inter-instrument communication scheme has been implemented which can be used to autonomously alter the observing modes. We also outline the plans for in-flight cross-calibration, which will be essential to the joint data reduction and analysis. Results. The RS instrument package on Solar Orbiter will carry out comprehensive measurements from the solar interior to the inner heliosphere. Thanks to the close coordination between the instrument teams and the European Space Agency, several challenges specific to the RS suite were identified and addressed in a timely manner. Key words. space vehicles: instruments – Sun: general – instrumentation: polarimeters – instrumentation: spectrographs – telescopes 1. The remote sensing challenge try (TM) rate varies greatly along a given orbit and from orbit to orbit (Müller et al. 2020), and as a result it is highly con- Due to its comprehensive payload and unique mission profile, strained compared to recent missions in Earth orbit, such as Solar Orbiter is very different from previous solar and helio- for example Hinode (Kosugi et al. 2007) and the Solar Dynam- spheric missions. The top-level scientific objectives of the mis- ics Observatory (SDO, Pesnell et al. 2012). While this is a con- sion (Müller et al. 2013; Müller et al. 2020) require close inter- straint for the full Solar Orbiter payload, it is felt more strongly action between the different instruments. This has motivated the by the RS instruments since they are inherently capable of formation of the Remote Sensing Working Group (RSWG) and producing orders of magnitude more data than can be trans- the in-situ Working Group (Walsh et al. 2020) in charge of coor- ferred to the ground. As a mitigation, the RS science opera- dinating the science operations of the remote sensing (RS) and tions have been restricted – from the early mission studies – to in-situ (IS) instruments, respectively. Initially, the two work- three ten-day Remote Sensing Windows (RSW) per six-month ing groups were mostly devoted to hardware and spacecraft orbit, centred on the a priori most interesting vantage points: (S/C) interface developments, topics that justified the distinction the perihelion and the two extreme solar latitudes. As a result, between RS and IS instruments. As the hardware development the RS suite will not continuously observe the solar activity as progressed, the two groups worked on joint operations within the has been the case for previous missions. However, when possi- activities of the Science Operations Working Group (SOWG). ble, these nominal periods of operation will be complemented The Solar Orbiter mission profile poses several challenges by continuous low-cadence, synoptic type observations outside for the RS instruments. As a deep space mission, the teleme- the RSWs (see Sect.4) providing contextual information to the A6, page 1 of 12 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. for S few days before weeks if before they observations, are withthe resource-neutral actual modifications (e.g. observations. only The TM, detailed possiblening power) commanding for a is each instrument frozen must two six-month be finalised planning several months period. before during Thus, the next the orbit, typically high-level sixa months science selection before of the plan- scientific start objectives of to each be addressed by the payload Fox et al. IS payload and other observatories (e.g. A6, page 2 of these di flares. Section as prominence eruptions,a coronal major mass hurdle ejections forthe (CMEs), the RSWs and and observation limited of TM, intermittent this events, was such identified very early on as the solar limbbe pointed (which anywhere allows(Sect. on EUI thethe solar range disc, allowed forSun-centered by example targets the towards by Metisand o coronagraph Helioseismic safety Imager constraint of (PHI) the will ExtremeCoronal be Ultraviolet Environment Imager able (SPICE) and (EUI) to thependicular and high-resolution observe channels to the non- Polarimetric thethe orbital nominal plane. rollS The angle SPectral defined Imagingof by of view the the (FOVs) S ment are papers illustrated givenmarised in in as Fig. Table references. The RS instruments fields Zouganelis et al. the concept ofmust be Solar planned in Orbiter aacross coordinated Observing the way. This is Plans fullAs achieved through most science (SOOPs, Solar payload, see Orbiter the2.1. objectives instrument Planning require strategy observations consistent data2. sets Remote sensing windows and planning synoptic type. (SoloHI) have been designedX-rays to mostly provide (STIX) observationstrograph. of and Conversely, the the Spectrometer from Solar the large orbiterdata selection parameter Heliospheric or space processing,tion. Imager but o On flexibility of thememory operation other comes equivalent hand, totion SPICE or and will larger extensivedownlink perform onboard than volume, these little processing, their instrumentsing by onboard TM are at including capable orbit of internal highimages alloca- data (0.28 cadence selec- (downclosest approach, to EUI and PHI sub-second). willoperational provide profiles To very-high-resolution for reduce the various their instruments. For example, at ferent scientific objectives will bethis prioritised will accordingly. result in aCombined variety with of the geometric changing configurations. solarbetween The latitude perihelion dif- (e.g. and Figs. high-latitudesions, windows the (Figs. distance totions the at Sun close will vary SunSect. by distances. about Unlike a previous factor solarSPICE of physics to two mis- perform limited o / C is nominally pointed at Sun center (Figs. Because each orbit is di The main characteristics of the six RS instruments are sum- The top-level scientific objectives are translated into di 2.3 / 2.3 C safety reasons. Combined with the limited duration of 2016 , limb-pointing is not compatible with Metis observa- ffi and 12 culties. Antonucci et al. ). 2.2 00 2020 1 equivalent from Earth) and are capable of observ- discusses the strategies devised to overcome . Details can be found in the respective instru- ). These are coordinated campaigns of 2020 ). The S ff 1 ff ff for four cases. The -pointing the S erent, the planning concept includes -limb observations). As detailed in 1 a,c, and d), ff / HRI ered by it being a spec- / Telescope for Imaging 174 / C Z-axis being per- Parker / 1 , EUI C (Fig. a and d), 1 / a and b). C boresight can 1 / HRI Solar Probe, b) within Ly A&A 642, A6 (2020) α ff , and erent Table 1. Main characteristics of the Solar Orbiter remote-sensing instruments. Name (reference) Measurements Channel / Wavelength/ Angular Spectral Typical Allocated detector FOV energy range resolution resolution cadence TM (Gbits) EUI (Rochus et al. 2020) High-resolution and wide field images of the solar disc and corona FSI 3:8◦ × 3:8◦ square 17.4 and 30.4 nm 900 – 10 min 53 0 0 00 in the UV / EUV. HRI174 16:8 × 16:8 square 17.4 nm 1 – 1–10 s 0 0 00 HRILyα 16:5 × 16:5 square 121.6 nm 1 – 0.1–10 s Metis (Antonucci et al.