The EPD/Suprathermal Ion Spectrograph (SIS) For Solar Orbiter

Understanding the physical processes operating in Solar Energetic Particle (SEP) events is a major goal of the Solar Orbiter mission because of the importance of acceleration processes in solar system and astrophysical sites, and because of the potential impact of these events on space hardware. The association of the largest SEP events with shocks propagating out from explosions on the Sun seemed to imply that the energetic particle seed population was the relatively abundant solar wind. However, recent studies have now determined the heavy ion composition (He-Fe) of the solar wind and the accelerated energetic particles with great accuracy, and this has revealed complex differences that do not appear to be the result of the physics of the acceleration process. Rather, a large body of evidence now suggests that the seed particles energized by shocks come from the suprathermal ion pool, that is, the ions with energy range from a few times the solar wind speed up to a few hundred keV/nucleon. The "head start" these particles have compared to the bulk solar wind gives an apparently conclusive advantage in the acceleration process, so that the suprathermals can dominate the energetic particle population even though they are roughly 10,000 times less abundant than the bulk plasma. Decisive progress in advancing our knowledge of this process requires traveling to the inner solar system in order to (1) measure the contents, properties, and variations of the suprathermal ion pool, and (2) close in on the shock acceleration sites in the inner solar system so that the particle signatures reflect the acceleration process itself before the spectra and timing information have become blurred by the effects of particle scattering near the shock and on the way to 1 AU. We describe progress possible with a high mass resolution sensor whose precise composition measurements make it possible to distinguish different particle sources, and whose sensitivity is sufficient to characterize the suprathermal ion pool in periods preceding large SEP events. The Suprathermal Ion Spectrograph (SIS) instrument for Solar Orbiter is a portion of the Energetic Particle Detector (EPD) suite which covers the full range of energetic particle measurements required to achieve the science goals of the mission.