Proton-Acoustic Wave Effects on the Relaxation of Proton Transverse Heating in Magnetized Plasmas
Transverse electromagnetic and electrostatic plasma wave modes propagating along a background magnetic field $\vec{B}_0$ are independent according to linear kinetic theory. However, resonant interactions and energy exchange between waves and particles break this linear decoupling. This work tracks the coupled evolution of Alfvén-cyclotron (ACWs) and Ion-acoustic waves (IAWs) by solving moment-based quasilinear equations for a collisionless plasma of bi-Maxwellian protons and Maxwellian electrons. Unlike earlier quasilinear studies that adopt the cold-electron limit, our formulation retains the full kinetic response of both species, treating the electrons as a thermal reservoir to isolate proton heating. A parameter survey over $0.01\leqβ_{\parallel p}\leq10$ and $1\le T_e/T_p\le10$ shows that an ambient spectrum of ACWs can drive significant perpendicular proton heating and raise the temperature anisotropy from initially isotropic conditions at low $β_{\parallel p}\lesssim0.1$, thereby triggering cyclotron instabilities. The quasilinear evolution self-regulates the ACW, driving the system toward a quasi-stationary state with $γ/Ω_p<10^{-1}$ and reduced anisotropy. As $T_e/T_p$ increases, IAWs become less damped and absorb a larger share of the fluctuation energy through Landau resonance, reducing the efficiency of ACW-driven proton heating and thus regulating the instability. For sufficiently large $β_{\parallel p}$ or $T_e/T_p\gtrsim5$, ACWs become inefficient drivers of perpendicular heating, leaving IAWs as the dominant dissipation channel. These results explain how modest electrostatic activity in low-$β$ environments such as the inner heliosphere and planetary magnetosheaths can regulate, but not indefinitely sustain, cyclotron instabilities.