1 POLARIS+: POLAR Investigation of the Sun using Solar Sailing Malcolm Macdonald 1 , Thierry Appourchaux 2 , Vincenzo Andretta 3 , Frédéric Auchère 2 , Frédéric Baudin 2 , Patrick Boumier 2 , Allan Sacha Brun 4 , Thierry Corbard 5 , Andrew N. Fazakerley 6 , Silvano Fineschi 7 , Wolfgang Finsterle 8 , Louise Harra 6 , Richard A. Harrison 9 , Donald Hassler 2, 10 , John Leibacher 2, 11 , Chris J. Owen 6 , Milan Maksimović 12 , Valentín Martínez Pillet 13 , Neil Murphy 14 , Giampiero Naletto 15 , Pierre Rochus 16 , Marco Romoli 17 , Werner Schmutz 8 , Takashi Sekii 18 , Daniele Spadaro 19 1 University of Strathclyde, Glasgow, Scotland 2 Institut d’Astrophysique Spatiale, Orsay, France 3 INAF / Osservatorio Astronomico di Capodimonte, Napoli, Italy 4 Commissariat à l'Energie Atomique, Gif-sur-Yvette, France 5 Observatoire de la Côte d’Azur, Nice, France 6 University College London, Mullard Space Science Laboratory, Dorking, United Kingdom 7 Osservatorio Astrofisico di Torino, Italy 8 PMOD/WRC, Davos Dorf, Switzerland 9 RAL Space, STFC Rutherford Appleton Laboratory, Harwell, United Kingdom 10 Southwest Research Institute, Boulder, Colorado, USA 11 National Solar Observatory, Tucson, Arizona, USA 12 Observatoire de Paris-Meudon, France 13 National Solar Observatory, Boulder, USA 14 Jet Propulsion Laboratory / California Institute of Technology, Pasadena, California, USA 15 University of Padova, Italy 16 Centre Spatial de Liège, Belgium 17 University of Florence, Italy 18 National Astronomical Observatory of Japan, Tokyo, Japan 19 INAF/Osservatorio Astrofisico di Catania, Italy This paper presents an overview of the POLARIS+ (POLAR Investigation of the Sun) proposal made to the European Space Agency’s 2016 call for New Ideas for the Science Programme. The POLARIS+ mission concept uses solar sail propulsion, plus an optional Venus gravity assist, to place a spacecraft in a 0.48 au circular orbit around the Sun, with an inclination of 75° with respect to the solar equator. In this orbit, at least 59% of the time will be spent at latitudes higher than the maximum latitude reached by Solar Orbiter, and each pole is visible to a view-zenith angle of less than 60° for over 37 days. This first extended view of the high-latitude regions of the Sun will enable crucial observations not possible from the ecliptic viewpoint, or from Solar Orbiter. While Solar Orbiter will give the first glimpse of the high-latitude magnetic field and flows to probe the solar dynamo, it does not provide sufficient viewing of the polar regions to achieve the primary objective of POLARIS+, to determine the relationship between the magnetism and dynamics of the Sun's polar regions and the solar cycle. This paper presents the science objectives of the POLARIS+ mission concept, alongside the scientific payload needed to achieve these objectives and a system level analysis of this concept. It is clearly identified that the use of solar sail propulsion is the major challenge of the POLARIS+ concept, and as such the motivation and contribution of this paper. Keywords: Solar, sail, Sun, remote sensing, poles, space weather 1. Introduction In late-2018, a new era in solar physics will begin with the launch of Solar Orbiter. This mission will get close to the Sun, 0.28 au perihelion, and out of the ecliptic plane, 25° at the end of nominal mission and up to 34° in the extended phase. This will be the first ever view of the solar poles, and will open a new world in understanding the dynamo and how the activity cycle works. Following the pioneering steps of Solar Orbiter, it is anticipated that for a detailed understanding of the activity cycle of our star, regular and long duration studies of the poles will be necessary. Solar Orbiter will not have sufficiently long observations of the polar regions, and indeed plans only occasional orbits to concentrate on helioseismology due to telemetry restrictions. In order to determine the relation between magnetism and dynamics of the Sun’s polar region, and the solar cycle, requires a different, longer-term approach. This fundamental science question, drives not only the heliosphere in our own solar system, but also aids understanding of other stars with their associated exoplanets. The POLARIS+ mission concept seeks to place a spacecraft in a 0.48 au, circular orbit around the Sun with an inclination of 75° with respect to the solar equator; the solar obliquity to ecliptic is assumed to be 7.25°. The mission concept to place a spacecraft into a high-inclination, or polar, orbit over the Sun using solar sail propulsion has been extensively studied since the nineteen seventies. [1–14] From the earliest studies, it has been clear that such a mission is