Solar Orbiter - Solar Wind Analyser (SWA) Suite: Initial Results of an Alpha-particle Relative Abundance Study within 0.5-1 AU

Alpha particles play an important role in solar wind dynamics. Their relative abundance (A_He) and relative drift to protons are usually associated with the solar wind source region. However, observations near the Earth indicate that these relative properties could also be altered by processes occurring in the interplanetary space, such as interactions of solar wind streams with different speeds. When the fast solar wind pushes up against the slow solar wind ahead of it, the co-rotating interaction region (CIR) is formed. Mirroring of the multicomponent solar wind in the converging magnetic field that develops within CIRs could lead to changes of A_He and reduction of the alpha-proton relative drift. When the high-speed stream outruns the slow solar wind, the co-rotating region of rarefied solar wind (CRR) is formed where A_He variations were also observed. Since CRRs are associated with the regions of small longitudinal extent on the Sun, these variations probably arise along the path from the Sun to Earth.

Passing the first perihelion, Solar Orbiter - the ESA/NASA common mission to study the Sun and inner heliosphere - entered the cruise phase in June 2020. Currently, two sensors of the Solar Wind Analyser Suite - Proton-Alpha Sensor (PAS) and Heavy Ion Sensor (HIS) provide valuable data on alpha-particle properties along the outbound part of the first Solar Orbiter orbit. In this contribution, we briefly describe the PAS processing methods related to alpha particles and existing caveats and we present the initial results on the alpha particle relative abundance observations. Also, we would like to combine the Solar Orbiter data on a half a way to the Sun with observations at 1 AU (Wind, ACE) to study the mechanisms leading to variations of alpha-proton properties.