The Astrophysical Journal, 881:116 (20pp), 2019 August 20 https://doi.org/10.3847/1538-4357/ab289a © 2019. The American Astronomical Society. All rights reserved. Current Sheets, Magnetic Islands, and Associated Particle Acceleration in the Solar Wind as Observed by Ulysses near the Ecliptic Plane Olga Malandraki1 , Olga Khabarova2,17 , Roberto Bruno3 , Gary P. Zank4 , Gang Li4 , Bernard Jackson5 , Mario M. Bisi6 , Antonella Greco7 , Oreste Pezzi8,9,10 , William Matthaeus11 , Alexandros Chasapis Giannakopoulos11 , Sergio Servidio7, Helmi Malova12,13 , Roman Kislov2,13 , Frederic Effenberger14,15 , Jakobus le Roux4 , Yu Chen4 , Qiang Hu4 , and N. Eugene Engelbrecht16 1 IAASARS, National Observatory of Athens, Penteli, Greece 2 Heliophysical Laboratory, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of the Russian Academy of Sciences (IZMIRAN), Moscow, Russia; [email protected] 3 Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica (IAPS-INAF), Roma, Italy 4 Center for Space Plasma and Aeronomic Research (CSPAR) and Department of Space Science, University of Alabama in Huntsville, Huntsville, AL 35805, USA 5 University of California, San Diego, CASS/UCSD, La Jolla, CA, USA 6 RAL Space, United Kingdom Research and Innovation—Science & Technology Facilities Council—Rutherford Appleton Laboratory, Harwell Campus, Oxfordshire, OX11 0QX, UK 7 Physics Department, University of Calabria, Italy 8 Gran Sasso Science Institute, Viale F. Crispi 7, I-67100 L’Aquila, Italy 9 INFN/Laboratori Nazionali del Gran Sasso, Via G. Acitelli 22, I-67100 Assergi (AQ), Italy 10 Dipartimento di Fisica, Universita’ della Calabria, I-87036 Cosenza, Italy 11 Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA 12 Scobeltsyn Nuclear Physics Institute of Lomonosov Moscow State University, Moscow, Russia 13 Space Research Institute (IKI) RAS, Moscow, Russia 14 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany 15 Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035, USA 16 Center for Space Research, North-West University of Potchefstroom, South Africa Received 2018 August 28; revised 2019 May 23; accepted 2019 June 7; published 2019 August 20 Abstract Recent studies of particle acceleration in the heliosphere have revealed a new mechanism that can locally energize particles up to several MeV nucleon–1. Stream–stream interactions, as well as the heliospheric current sheet (CS)— stream interactions, lead to formation of large magnetic cavities, bordered by strong CSs, which in turn produce secondary CSs and dynamical small-scale magnetic islands (SMIs) of ∼0.01 au or less owing to magnetic reconnection. It has been shown that particle acceleration or reacceleration occurs via stochastic magnetic reconnection in dynamical SMIs confined inside magnetic cavities observed at 1 au. The study links the occurrence of CSs and SMIs with characteristics of intermittent turbulence and observations of energetic particles of keV– MeV nucleon–1 energies at ∼5.3 au. We analyze selected samples of different plasmas observed by Ulysses during a widely discussed event, which was characterized by a series of high-speed streams of various origins that interacted beyond Earth’s orbit in 2005 January. The interactions formed complex conglomerates of merged interplanetary coronal mass ejections, stream/corotating interaction regions, and magnetic cavities. We study properties of turbulence and associated structures of various scales. We confirm the importance of intermittent turbulence and magnetic reconnection in modulating solar energetic particle flux and even local particle acceleration. Coherent structures, including CSs and SMIs, play a significant role in the development of secondary stochastic particle acceleration, which changes the observed energetic particle flux time–intensity profiles and increases the final energy level to which energetic particles can be accelerated in the solar wind. Key words: acceleration of particles – magnetic reconnection – solar wind – Sun: heliosphere – Sun: magnetic fields – turbulence Supporting material: animation 1. Introduction heliosphere are known to be accelerated during solar flares and at coronal mass ejection (CME) driven shocks. In the latter Energetic particles in the heliosphere and magnetosphere case, the acceleration process is most effective in the corona. pose significant radiation hazards for astronauts in current and Generated energetic particles are referred to as solar energetic future space missions in our solar system (e.g., to Mars) and for particles (SEPs; Reames 1999; Malandraki et al. 2012; Desai & communication satellites (e.g., Iucci et al. 2006; National Giacalone 2016; Malandraki & Crosby 2018a, 2018b). Another Research Council 2006; Malandraki 2015; Miroshnichenko – –1 / ) source of keV MeV nucleon energetic particles is stream 2015; Malandraki & Crosby 2018a, 2018b . Studies of their corotating interaction regions (SIR/sCIRs). In this case, origin and properties are very important for timely protection of acceleration takes place at SIR/CIR-driven shocks usually onboard equipment and human resources. Protons and formed at 2–3au(Malandraki et al. 2007; Tsubouchi 2014; – electrons of keV MeV energies observed in the inner Richardson 2018). In summary, the dominant paradigm suggests that energetic particles in the keV–MeV energy range 17 Author for correspondence. propagate to 1 au either from the Sun or from distant sources 1 The Astrophysical Journal, 881:116 (20pp), 2019 August 20 Malandraki et al. associated with CIRs, and thus the effect of local processes can accordance with expectations of SEP observations at 1 au, but be neglected. keV–MeV energy ion flux shows variations, increases, and The standard mechanism typically invoked to explain the time delays corresponding to propagation of some solar wind energization of particles locally in the solar wind is that of structures (see Khabarova et al. 2015a and references therein). diffusive shock acceleration (DSA), as shown by Lee & Ryan The role of DSA in local particle acceleration has been found (1986) and Zank et al. (2000), although other mechanisms have not to be of primary importance in many events. Khabarova & been proposed (e.g., Fisk & Gloeckler 2012, 2014; Laurenza Zank (2017) illustrated how to distinguish between effects of et al. 2016; Pallocchia et al. 2017). The basic physics were local DSA and particle energization associated with another elucidated especially well in the seminal papers of Axford local source, which can be more effective, just through the et al. (1982) and Bell (1978a, 1978b) in the context of the visual inspection of time–intensity profiles of the energetic astrophysical problem of cosmic-ray acceleration at supernova particle flux in different energy channels (e.g., see their remnants. However, detailed interplanetary observations are Figure 12). Khabarova & Zank (2017) carried out an extensive not easily interpreted in terms of the simple original steady- analysis of particle energization in the vicinity of reconnecting state models of particle acceleration at shock waves. Three exhausts observed in the supersonic solar wind by the ACE fundamental aspects make the interplanetary problem more spacecraft. Both the case study analysis and the superposed complicated than the typical astrophysical problem: the time epoch analysis of 126 reconnection exhaust events, which dependence of the acceleration and the solar wind background, mainly occurred within interplanetary coronal mass ejections the geometry of the shock, and the long mean free path for (ICMEs), showed that energetic ions are accelerated up to at particle transport away from the shock. Consequently, the least 5 MeV and electrons up to 0.315 MeV within a time shock itself introduces a multiplicity of timescales, ranging interval of ±30 hr around reconnecting current sheets (CSs). from shock propagation timescales to particle acceleration Khabarova & Zank (2017) concluded that the observed long- timescales at parallel and perpendicular shocks. Many of these timescale atypical energetic particle event (AEPE) encompass- — timescales feed into other timescales these can determine ing the reconnection exhausts is very likely related to secondary maximum particle energy scalings or escape timescales, for reacceleration of energetic particles in SMIs or medium-scale example, from which accelerated particle distribution functions magnetic islands surrounding the reconnecting CSs. Their are predicted that can be broken power laws or power laws with results are in a very good agreement with predictions based on exponential rollovers in velocity space. a theory of stochastic particle energization in the supersonic Models describing the acceleration of ions at CME-driven solar wind via numerous dynamically interacting small-scale shocks have become increasingly sophisticated and capable flux ropes (Zank et al. 2014, 2015a, 2015b; le Roux et al. of explaining many observations associated with gradual 2015, 2016, 2018a, 2018b). Initial particle acceleration may events (Zank et al. 2000; Rice et al. 2003; Li et al. 2003; occur in different ways, including acceleration at interplanetary Verkhoglyadova et al. 2015). Only relatively low-energy shocks combined with reconnection at CSs. energetic particles tend to remain localized near the shock Reconnecting small-scale CSs are observed very often where DSA occurs. These spectra,