Remote sensing of solar activity by energetic charged and neutral particles with Solar Orbiter

The solar wind represents a uniquely accessible domain of space, where fundamental physical processes common to solar, astrophysical and laboratory plasma can be studied in detail. Due to the unique orbit of Solar Orbiter one of the prime objectives are in situ measurements of different energetic particle populations in the inner heliosphere. These measurements include charged as well as neutral particles. Solar flares and coronal mass ejections occur when magnetic energy is suddenly released in the Sun's atmosphere. In these eruptions, solar energetic particles including electrons, protons, and heavy nuclei are released. In extreme cases the energies exceed the nuclear binding limits and a particle zoo is created in high-energy collisions, accompanied by the emission of γ-rays. In addition neutrons are released and escape due to charge-neutrality freely into space. Because neutrons decay after 15 min only a mission like Solar-Orbiter can observe neutrons below 100 MeV in sizable quantities. In this paper, we will review some aspects of our current understanding of energetic particles and emphasize the potential of Solar Orbiter to provide information on solar activity by means of measuring energetic charged and neutral particles.