Observations and Statistics of Secondary Alpha Populations in the Inner Heliosphere

Abstract:

Three-dimensional measurements of the alpha particle velocity distribution functions (VDFs) by the SPAN-Ion instrument on Parker Solar Probe (PSP) allow us for the first time to resolve and measure separate alpha core and beam populations in a statistical sense. Secondary proton populations in the solar wind have been known about and studied since the earliest days of in situ solar wind measurements. Such asymmetric proton beams or “shoulders” are ubiquitous in the proton VDFs observed by PSP, and are often coincident with ion-cyclotron wave storms. While instances of secondary alpha particle populations have appeared in the literature before, these have mainly been isolated examples, or in the context of distinct double humped distributions representing interpenetrating streams. Our measurements suggest that alpha beams, just like proton beams, are very common. In this work we characterise the secondary alpha beam population in fast, near-Sun solar wind in terms of density and drift speed relative to the core population, finding that alpha beams are much more dense and travel more slowly relative to the core than is the case for protons, as well as analyse the relative coincidence of alpha beams with proton beams and their dependence on collisional age.