Observations of multiple ion populations in solar wind velocity distributions with Solar Orbiter-PAS

Particles in the solar wind show a variety of deviations from thermodynamic equilibrium. These non-thermal features include secondary particle beams drifting relative to the main core population. Several mechanisms have been proposed to explain the formation of such beams, but the topic remains debated.
Recently, thanks to the excellent energy resolution of the Proton Alpha Sensor (PAS) on board Solar Orbiter, the technique described in De Marco et al. (A&A, 2023) has made it possible, in many cases, to clearly identify, not only proton beam, but also the more elusive alpha-particle beam. In this preliminary work, we present observations in which the proton velocity distribution function, instead of following a simple core+beam scenario, displays a more complex structure, exhibiting modulations consistent with the superposition of several sub-populations. Such multi-beam configurations are typically short-lived, representing transient stages in the evolution of proton beams. These observations indicate that proton populations may be continuously reshaped by local kinetic processes, providing an observational basis for future studies on the formation and evolution of multiple proton populations. Furthermore, these complex distributions can offer valuable insight into wave–particle interactions in the solar wind, helping to connect kinetic-scale structures with plasma turbulence and instabilities.