Beam-Driven Instabilities and the Formation of Anisotropic Proton Populations in the Young Solar Wind

Motivated by Parker Solar Probe (PSP) observations of anisotropic proton populations and core–beam structures, we perform an advance analysis of right-hand (RH) proton instabilities using both linear theory and quasilinear modeling. Our linear study reveals a cumulative RH proton (CRHP) instability driven by the interplay between beam drift and temperature anisotropy, significantly enhancing growth rates and lowering instability thresholds. Suprathermal beams, modeled by bi-Kappa distributions, and anisotropic electrons further amplify this effect by increasing wave–particle coupling. A quasilinear approach to young solar wind conditions from PSP measurements demonstrates that the ion-ion resonant (IIR) instability can scatter beam protons and induce strong perpendicular anisotropies consistent with observed proton velocity distributions. The modeled quasilinear (dynamic) paths adjust to PSP events, reproducing both the anisotropy levels and the observed RH wave signatures. These findings establish a compelling link between kinetic instabilities and PSP-observed structures, and motivate future numerical modeling of these cumulative effects in simulations.