Simulating Differences in Helium Abundances between Fast and Slow Solar Winds

Abstract

To forecast the solar wind arriving at Earth in advance, and hence its impacts on technology, it is important to have a good understanding of how the solar wind is generated by the solar corona. Helium is a major constituent of the corona/solar wind that plays an important role in the energy budget of the corona and the acceleration of the solar wind. Helium abundance varies significantly with the solar cycle and in the different types of fast and slow solar winds. We present the newly-developed multi-species IRAP Solar Atmospheric Model (ISAM) which solves for the coupled transport of both neutral and charged particles between the chromosphere and the corona, including a self-consistent treatment of collisional and ionisation processes. We exploit ISAM to study the mechanisms that regulate Helium abundance in the source region of the fast and slow solar winds and contrast numerical results with and without Helium included in the model. We compare our model outputs with as many observations as possible, including Helios, Parker Solar Probe and Solar Orbiter data, in particular from the Proton and Alpha particle Sensor, SWA-PAS. We show that our simulations are in good agreement with previous studies, in particular that the solar cycle variation in Helium abundance can be explained by differences in abundance found for each solar wind type by ISAM, when just using diffusion in the model and not (yet) including the ponderomotive force.