How Alfvén Waves, Expansion, and Kinetic Instabilities Shape Proton Distributions in the Solar Wind

Recent observations from the Parker Solar Probe have confirmed that Alfvénic fluctuations are ubiquitous in the solar wind, particularly within streams originating from coronal holes. These fluctuations persist from regions close to the Sun out to distances beyond 1 AU. While often described using incompressible models, the nonlinear coupling between Alfvén and compressible modes introduces transient effects that facilitate wave-particle interactions. These interactions contribute to particle acceleration—such as the formation of proton beams—and to plasma heating. In this work, we present results from solar wind data analysis using both ongoing missions (Parker Solar Probe and Solar Orbiter) and past missions (Helios and Ulysses), alongside hybrid particle-in-cell (PIC) simulations of the self-consistent formation of proton beams in the expanding solar wind. By comparing observations and simulations, we discuss the critical role of compressible effects and wave-particle interactions in determining the observed evolution of proton beams in the solar wind.