Authors: Yogesh (CUA/NASA GSFC), L. Ofman (CUA/NASA GSFC), S. A. Boardsen (UMBC/NASA GSFC), Kristopher G. Klein (LPL/University of Arizona), Mihailo M. Martinović (LPL/University of Arizona), V. M. Sadykov (GSU),, J. L. Verniero (NASA GSFC), Niranjana Shankarappa (LPL/University of Arizona), L.K. Jian (NASA GSFC), Parisa Mostafavi (Johns Hopkins University/APL), Kristoff Paulson (Smithsonian Astrophysical Observatory), J. Huang (SSL/UC Berkeley)

Recent in situ observations from the Parker Solar Probe (PSP) near perihelia have revealed the presence of ion beams, temperature anisotropies, and enhanced kinetic wave activity—features that are likely connected to the processes of solar wind heating and acceleration. During Encounters 13 and 17, the PSP/FIELDS instrument detected intensified ion-scale wave activity which occur concurrently with deviations from local thermodynamic equilibrium (as evident from non-Maxwellian features) in the ion velocity distribution functions (VDFs) measured by the SPAN-I instrument. During Encounter 13, ion-scale waves were linked to both proton and alpha particle beams. During this interval, ion-scale waves propagating in both sunward and anti-sunward directions were observed. In a separate event during Encounter 17, proton VDFs exhibited pronounced secondary beam populations, and bi-Maxwellian fits showed that the proton beam temperature is anisotropic, and the beam density often increased during periods of wave activity—at times even exceeding the core proton population. Intriguingly, a reduction in wave power was observed when the beam density surpassed that of the core.

Linear wave analysis indicates that the proton and alpha beams could either drive wave growth or enhance wave damping. Hybrid kinetic simulations extend the evolution of the instability to the nonlinear regime and highlight the energy exchange mechanisms between particles and waves. Collectively, these findings provide critical new insights into wave–particle interactions and the role of ion-scale dynamics in shaping solar wind evolution and heating.