ESA Voyage 2050 White Paper Solar Particle Acceleration, Radiation & Kinetics (SPARK) Point of Contact: Sarah Matthews, University College London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK Email:
[email protected] 5 August 2019 Executive Summary Particle acceleration is a ubiquitous process throughout the universe, observed in environments as diverse as stellar coronae, active galactic nuclei, the coronae of accretion disks around black holes, the magnetospheres of neutron stars and planetary atmospheres (including our own) interacting with the wind of their star. It operates both in very dynamic and explosive situations and in more steady phenomena where steep gradients, turbulence and instabilities exist. The product of this process, or processes, are energetic particles that fill the universe, and shape the way in which our own and other solar systems work, and what the conditions for the emergence and continuation of life might be. The Sun is a privileged place to study this fundamental astrophysical problem. It is the only astrophysical setting that allows us to probe particle acceleration by a combination of remote sensing (imaging spectroscopy) and in situ measurements, with time resolution that resolves many of the processes of acceleration and transport. This White Paper introduces a mission concept, SPARK, aimed at making significant advances in understanding particle acceleration and transport. Energetic particles are seen in nearly every manifestation of magnetic energy conversion, from large flares down to minor explosive events in active regions, and sometimes even the quiescent solar atmosphere. Several decades of hard X-ray (HXR) observations have provided substantial insights into electron acceleration, but the low energy end of the accelerated electron distribution remains poorly constrained, with profound consequences for the flare energy budget.