Plasma Mechanisms Behind Hammerhead Proton Populations Observed by Parker Solar Probe

The Parker Solar Probe (PSP) has provided unprecedented detailed in-situ measurements of proton velocity distributions (VDs) in the young solar wind, unveiling striking hammerhead features. The first interpretations and analyses, including PIC simulations of these unexpected shapes, suggested the involvement of more complex processes, especially kinetic instabilities. Recently, in A&A, 692, L6 (2024), we have identified a self-generated instability triggered by proton beams, whose back-reaction on the proton VDs can form the hammerhead proton population. An effective and numerically less-expensive quasi-linear approach enabled us to explore how this plasma micro-instability reshapes proton distribution, reducing beam drift and inducing a strong perpendicular temperature anisotropy, the main feature of the hammerhead structure. Our results align with PSP's in situ data and provide a fresh perspective on these distributions' dynamic and transient nature. These findings offer new insights into the role of kinetic instabilities in shaping space plasma dynamics.