Solar energetic particles in the inner heliosphere

Abstract

The upcoming missions Solar Orbiter (SolO) and Solar Probe Plus (SPP) will explore the inner region of the heliosphere. To understand the environment in which these missions will operate, we studied the main characteristics of the Interplanetary Magnetic Field (IMF). The IMF can be described by an Archimedean spiral whose curvature is more pronounced for small values of the solar wind speed. In the inner heliosphere (r < 0.3 AU), the IMF can be described by a radial magnetic field. At 0.05 AU (SPP perihelia), the angle between the magnetic field vector and the radial direction is of only ≃ 3◦ for a solar wind speed of 400 km/s. Under the assumption of particle scatter-free transport along the IMF, we studied the degree of anisotropy expected at different radial distances from the Sun and how this depends on the location where the particle release occurred. We obtained that the particle pitch angle, , decreases with radial distance and the decrease is faster for inner source locations. Then the distribution is more beamed for inner source positions and for positions of the observer further out from the Sun. In addition, the study of the particle equation of motion shows that for a fixed distance along the field line particles coming from different source positions reach the observer at different times. Particles released closer to the Sun will arrive later to the same observer. Finally, we analyzed the focusing cone expected in different regions of the heliosphere due to different source positions. We found broader PADs in the inner heliosphere and for more external sources, because these particles suffer less focusing