The Solar Orbiter Solar Wind Analyser (SWA) suite

¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
e-mail: c.owen@ucl.ac.uk
² INAF-Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
³ Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
⁴ Institut de Recherche en Astrophysique et Planétologie, 9, Avenue du Colonel ROCHE, BP 4346, 31028 Toulouse Cedex 4, France
⁵ Laboratoire de Physique des Plasmas, Ecole Polytechnique, Palaiseau, France
⁶ Centre National d’Etudes Spatiales, DCT/PO/EU – B.P.I. 2220, 18, Avenue Edouard Belin, 31401 Toulouse Cedex 9, France
⁷ Techno System Developments S.R.L., Via Provinciale Pianura 2, Int. 23, San Martino Zona Industriale 80078, Pozzuoli, Italy
⁸ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, Maryland 20771, USA
⁹ Planetek Italia S.R.L., Via Massaua, 12, 70132 Bari, BA, Italy
¹⁰ Space Science Center, University of New Hampshire, Morse Hall, Durham, NH 03824, USA
¹¹ University of Michigan, 2455 Hayward St, Ann Arbor, MI 48109, USA
¹² LEONARDO, Viale del lavoro, 101, 74123 Taranto, Italy
¹³ SITAEL S.p.A., Via San Sabino 21, 70042 Mola di Bari, BA, Italy
¹⁴ ESA-ESTEC/SCI-PS, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹⁵ Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, 18000 Prague 8, Czech Republic
¹⁶ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹⁷ CGC Instruments, Chemnitz, Germany
¹⁸ Space and Atmospheric Physics, The Blackett Laboratory, Imperial College London, London SW7 2AZ, UK
¹⁹ LESIA, Observatoire de Paris, Universite PSL, CNRS, Sorbonne Universite, Universite de Paris, 5 Place Jules Janssen, 92195 Meudon, France
²⁰ Space Research Group, University of Alcalá, 28801 Alcalá de Henares, Spain
²¹ Agenzia Spaziale Italiana, Via del Politecnico, snc, 00133 Roma, Italy
²² ESA-ESAC, Camino bajo del Castillo s/n, 28692 Villafranca del Castillo, Madrid, Spain
²³ Institute of Experimental and Applied Physics, Kiel University, Leibnizstrasse 11, 24118 Kiel, Germany
²⁴ Physikalisches Institut, Universitat Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
²⁵ Incom, Inc., 242 Sturbridge Road, Charlton, MA 01507, USA

Received: 5 December 2019
Accepted: 8 July 2020

Abstract

The Solar Orbiter mission seeks to make connections between the physical processes occurring at the Sun or in the solar corona and the nature of the solar wind created by those processes which is subsequently observed at the spacecraft. The mission also targets physical processes occurring in the solar wind itself during its journey from its source to the spacecraft. To meet the specific mission science goals, Solar Orbiter will be equipped with both remote-sensing and in-situ instruments which will make unprecedented measurements of the solar atmosphere and the inner heliosphere. A crucial set of measurements will be provided by the Solar Wind Analyser (SWA) suite of instruments. This suite consists of an Electron Analyser System (SWA-EAS), a Proton and Alpha particle Sensor (SWA-PAS), and a Heavy Ion Sensor (SWA-HIS) which are jointly served by a central control and data processing unit (SWA-DPU). Together these sensors will measure and categorise the vast majority of thermal and suprathermal ions and electrons in the solar wind and determine the abundances and charge states of the heavy ion populations. The three sensors in the SWA suite are each based on the top hat electrostatic analyser concept, which has been deployed on numerous space plasma missions. The SWA-EAS uses two such heads, each of which have 360° azimuth acceptance angles and ±45° aperture deflection plates. Together these two sensors, which are mounted on the end of the boom, will cover a full sky field-of-view (FoV) (except for blockages by the spacecraft and its appendages) and measure the full 3D velocity distribution function (VDF) of solar wind electrons in the energy range of a few eV to ∼5 keV. The SWA-PAS instrument also uses an electrostatic analyser with a more confined FoV (−24° to +42° × ±22.5° around the expected solar wind arrival direction), which nevertheless is capable of measuring the full 3D VDF of the protons and alpha particles arriving at the instrument in the energy range from 200 eV/q to 20 keV/e. Finally, SWA-HIS measures the composition and 3D VDFs of heavy ions in the bulk solar wind as well as those of the major constituents in the suprathermal energy range and those of pick-up ions. The sensor resolves the full 3D VDFs of the prominent heavy ions at a resolution of 5 min in normal mode and 30 s in burst mode. Additionally, SWA-HIS measures 3D VDFs of alpha particles at a 4 s resolution in burst mode. Measurements are over a FoV of −33° to +66° × ±20° around the expected solar wind arrival direction and at energies up to 80 keV/e. The mass resolution (m/Δm) is > 5. This paper describes how the three SWA scientific sensors, as delivered to the spacecraft, meet or exceed the performance requirements originally set out to achieve the mission’s science goals. We describe the motivation and specific requirements for each of the three sensors within the SWA suite, their expected science results, their main characteristics, and their operation through the central SWA-DPU. We describe the combined data products that we expect to return from the suite and provide to the Solar Orbiter Archive for use in scientific analyses by members of the wider solar and heliospheric communities. These unique data products will help reveal the nature of the solar wind as a function of both heliocentric distance and solar latitude. Indeed, SWA-HIS measurements of solar wind composition will be the first such measurements made in the inner heliosphere. The SWA data are crucial to efforts to link the in situ measurements of the solar wind made at the spacecraft with remote observations of candidate source regions. This is a novel aspect of the mission which will lead to significant advances in our understanding of the mechanisms accelerating and heating the solar wind, driving eruptions and other transient phenomena on the Sun, and controlling the injection, acceleration, and transport of the energetic particles in the heliosphere.