Linking Sun and Heliosphere with Remote Sensing and in situ Measurements - Solar Orbiter and Solar Probe Plus

Abstract:

The solar wind creates a plasma bubble in our very local interstellar medium (VLISM), the heliosphere. Its structure is determined by dynamic processes and by the boundary conditions at the Sun and in the VLISM. Because of the supersonic expansion of the solar wind the structure of the inner (several AU) heliosphere is (nearly) exclusively determined by the Sun. As simple as this may all appear, the problem of linking heliospheric structure to solar features is remarkably complex and has so far eluded satisfactory solutions. ESA and NASA have implemented the Solar Orbiter and Solar Probe Plus missions to tackle and solve the mystery of how the Sun creates and controls the heliosphere.

Previous missions, especially the twin Helios mission, lacked two crucial elements, remote-sensing of solar features and their dynamics, and composition measurements of the solar plasma, wind, and energetic particles. Solar Orbiter has both elements in its highly sophisticated payload and will allow us to link solar features to the solar wind sampled in situ by using composition and energetic particles as tracers. The composition of the solar wind is altered from its photospheric origin by two processes very probably acting at different altitudes in the solar atmosphere. Elemental composition of the solar wind appears to be fractionated by its First Ionization Potential (FIP) or time (FIT), indicating that some mechanism separates neutral atoms from ions. This requires temperatures low enough to allow a substantial neutral fraction of the solar plasma and therefore the FIP-effect is believed to act primarily in the chromosphere. Charge states on the other hand are determined by the expansion and acceleration of the solar wind and the electron temperature high in the corona. In the case of energetic particles, charge state composition affects measured elemental abundances by m/q-dependent acceleration/fractionation processes. Solar Orbiter will allow remote-sensing measurements of the elemental composition – and, to a limited degree – source temperatures of solar features such as coronal holes, active regions, or even flares. Comparison with in situ measurements of solar wind and energetic particle composition and their timing will provide key information about the linkage between Sun and heliosphere.