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J. Space Weather Space Clim., 6, A31 (2016) DOI: 10.1051/swsc/2016024 Ó C. Plainaki et al., Published by EDP Sciences 2016 TOPICAL REVIEW OPEN ACCESS Planetary space weather: scientific aspects and future perspectives Christina Plainaki1,2,*, Jean Lilensten3, Aikaterini Radioti4, Maria Andriopoulou5, Anna Milillo1, Tom A. Nordheim6, Iannis Dandouras7, Athena Coustenis8, Davide Grassi1, Valeria Mangano1, Stefano Massetti1, Stefano Orsini1, and Alice Lucchetti9 1 INAF-IAPS, via del Fosso del Cavaliere 100, 00133 Rome, Italy 2 Nuclear and Particle Physics Department, Faculty of Physics, National and Kapodistrian University of Athens, 15784 Athens, Greece *Corresponding author: [email protected] 3 Institut de Planétologie et d’Astrophysique de Grenoble, CNRS/UGA, 38041 Grenoble, France 4 Laboratoire de Physique Atmosphérique et Planétaire, Institut d’Astrophysique et de Géophysique, Université de Liège, 4000 Liege, Belgium 5 Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria 6 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr, Pasadena, 91109 CA, USA 7 IRAP, University of Toulouse/CNRS, 31028 Toulouse, France 8 LESIA, Observ. Paris-Meudon, CNRS, Univ. P. et M. Curie, Univ. Paris-Diderot, 92195 Meudon, France 9 CISAS, University of Padova, via Venezia 15, 35131 Padova, Italy Received 27 November 2015 / Accepted 6 June 2016 ABSTRACT In this paper, we review the scientific aspects of planetary space weather at different regions of our Solar System, performing a comparative planetology analysis that includes a direct reference to the circum-terrestrial case. Through an interdisciplinary analysis of existing results based both on observational data and theoretical models, we review the nature of the interactions between the environment of a Solar System body other than the Earth and the impinging plasma/radiation, and we offer some considerations related to the planning of future space observations. We highlight the importance of such comparative studies for data interpretations in the context of future space missions (e.g. ESA JUICE; ESA/JAXA BEPI COLOMBO). Moreover, we discuss how the study of planetary space weather can provide feedback for better understanding the traditional circum- terrestrial space weather. Finally, a strategy for future global investigations related to this thematic is proposed. Key words. Space weather, Planetary atmospheres, Planetary magnetospheres, Exospheres, Interactions, Comparative planetology, Future missions, JUICE, BEPI COLOMBO 1. Introduction To account for this necessity, the US National Space Weather 1.1. Definition of planetary space weather and analogy Program (http://www.nswp.gov/, version 25/06/2014) proposed with the circum-terrestrial case the following definition: ‘‘space weather refers to conditions on the Sun and in the space environment that can influence the In a seminal paper on our understanding of space environments performance and reliability of space-borne and ground-based around Solar System bodies other than Earth, Lilensten et al. technological systems, and can endanger human life or health’’. (2014) discussed the characteristics of planetary space weather, Although this new definition referred clearly to both the condi- an emerging aspect of the space weather discipline. The earliest tions on the Sun and in the actual space environment of a body, definition of space weather, provided by the US National Space the use of the term ‘‘ground-based technological systems’’ Weather Plan (1995, 2000), referred to the ‘‘conditions on the excluded gaseous planets. Sun and in the solar wind, magnetosphere, ionosphere, and In a 2008 definition agreed among 24 countries (Lilensten thermosphere that can influence the performance and reliability & Belehaki 2009), it was stated that ‘‘Space Weather is the of space-borne and ground-based technological systems and can physical and phenomenological state of natural space environ- endanger human life or health’’. In 2010, this definition was ments; the associated discipline aims, through observation, slightly modified and the significant role of the variability of monitoring, analysis and modelling, at understanding and the Sun and of the environment conditions was evidenced: predicting the state of the Sun, the interplanetary and planetary ‘‘the term ‘space weather’ refers to the variable conditions on environments, and the solar and non-solar driven perturbations the Sun, throughout space, and in the Earth’s magnetic field that affect them; and also at forecasting and now-casting the and upper atmosphere that can influence the performance of possible impacts on biological and technological systems’’. space-borne and ground-based technological systems and We note that in this definition the phenomenon and the associ- endanger human life or health’’. In both of these definitions, ated discipline are clearly distinguished. Moreover, the 2008 however, the term ‘‘space weather’’ was restricted to the definition focuses on space environments stating that planetary terrestrial case and did not include planetary environments. space weather cannot be disconnected from the temporal This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. J. Space Weather Space Clim., 6, A31 (2016) variability of either the solar activity or the magnetospheric and ionospheres; the variability of the planetary plasma behaviour at planetary systems. Therefore, the disci- magnetospheric regions under different external plasma pline of planetary space weather refers to the study of the vari- conditions; the interactions of a planet’s radiation belts with ability of planetary (or satellite) environments (e.g. atmospheres, satellites and rings; space weathering and atmospheres, exospheres1 (often referred to as ‘‘tenuous planetary or lunar surface charging at bodies possessing atmospheres’’ Johnson et al. 2009), intrinsic magnetospheres) tenuous atmospheres. determined by the variability of the solar activity or/and The main agents determining the space weather conditions the interplanetary space dynamics (or/and the dynamics of around a body in the Solar System are: the magnetosphere in which the Solar System body may be embedded). d the distance of the body from the Sun, determining the Recently, the World Meteorological Organisation (WMO) properties of solar wind and solar energetic particles proposed the following definition2: ‘‘Space Weather is defined (SEPs), the solar photon flux and the properties of the as the physical and phenomenological state of the natural space Interplanetary Magnetic Field (IMF) at that location; environment including the Sun, the solar wind, the magneto- d the presence (or absence) of a dense atmosphere; sphere, the ionosphere and the thermosphere, and its interac- d the optical thickness (wavelength dependent) of the tion with the Earth. The associated discipline aims, through body’s atmosphere and the mean free paths for neutrals observation, monitoring, analysis and modelling, at under- and ions travelling therein; standing the driving processes, predicting the state of the d in case of the Giant planet moons, the existence of a Sun, the interplanetary and planetary environments including strong (or weak) magnetosphere in which the body is the Earth’s magnetic field, their disturbances, and forecasting embedded (e.g. Europa and Ganymede; Titan and and nowcasting the potential impacts of these disturbances Enceladus versus Miranda and Ariel; Naiade, Talassa on ground-based or space-based infrastructure and human life and Triton); or health’’. This definition re-introduces the Earth as a unique d the possession by the body of an intrinsic or induced object of study and is therefore not suited for planetary space magnetic field; weather. Nevertheless, a WMO task group has defined the d the existence of endogenic sources (e.g. plumes, space environment as ‘‘the physical and phenomenological volcanoes); state of the natural space environment, including the Sun and d galactic cosmic rays (GCRs); the interplanetary and planetary environments’’, and as d micrometeorites and dust. meteorology of space ‘‘the discipline which aims at observing, understanding and predicting the state of the Sun, of the Any variability related to the energy release from the Sun, interplanetary and planetary environments, their disturbances, in the form of photon flux (in the UV, EUV and X-ray bands), and the potential impacts of these disturbances on biological 3 solar wind (streams), coronal mass ejections (CMEs) and and technological systems’’. This definition certainly includes SEPs, has been known to be the principal agent determining the planetary space weather case, nevertheless, it is focused on circum-terrestrial space weather. At other planetary bodies in the Sun as the main driver for the related phenomena. the Solar System, solar-driven perturbations also play a role However, at large distances from the Sun, other agents in atmospheric and magnetospheric processes, however (e.g. cosmic rays, accelerated magnetospheric ions) may be the details of such interactions are planet-dependent and in the primary drivers of space weather. In the context of a more some cases (e.g. Jupiter), internal processes dominate over global perspective, therefore, it is important to mention external drivers. In particular,
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