Proton Temperature Anisotropy Constraint Associated With Alpha Beam Instability in the Solar Wind

Solar wind observations in the space of the proton temperature anisotropy $T_{p\perp }/T_{p\Vert }$ versus parallel proton beta ${\beta }_{p\Vert }$ always show a distorted rhomboid-like pattern, where the boundaries of this plot are associated with several instabilities. However, the specific mechanism on the constraint of the proton temperature anisotropy is unclear in the low-beta plasma. In this work, we study the kinetic instabilities driven by proton temperature anisotropy and alpha beam with the Vlasov theory and investigate the nonlinear evolution of these instabilities with hybrid simulations. We also compare the theoretical and simulation results with Wind observations. The alpha beam with drift velocity $v_{\alpha }/v_A>1$ leads to a new kind of Alfvén/ion-cyclotron instability ($\alpha$A/IC instability) in the region of ${\beta }_{p\Vert }\le 0.2$. In the region of ${\beta }_{p\Vert }\le 0.2$ and , the nonlinear wave-particle interaction through the $\alpha$A/IC instability can decelerate alpha beams to $v_{\alpha }/v_A\le 1$ and result in the anti-correlation between $T_{p\perp }/T_{p\Vert }$ and ${\beta }_{p\Vert }$. In the region of ${\beta }_{p\Vert }\le 0.2$ and $T_{p\perp }\ge T_{p\Vert }$, the nonlinear wave-particle interaction through the $\alpha$A/IC instability further leads to slight increase of $T_{p\perp }/T_{p\Vert }$. The present results provide a potential mechanism for regulating the proton temperature anisotropy in the low-beta plasma with in the solar wind.