Electromagnetic proton cyclotron anisotropy instability: Wave-particle scattering rate

The electromagnetic proton cyclotron instability is driven by a proton temp erature anisotropy T-perpendicular to p/T-parallel to p > 1 where the direc tional subscripts denote directions relative to the background magnetic fie ld. Enhanced field fluctuations from instability growth lead to wave-partic le scattering which reduces the anisotropy; previous research has demonstra ted that the anisotropy is constrained by a power-law function of beta(para llel to p). Here two-dimensional hybrid simulations in a homogeneous, magne tized, collisionless plasma are used to derive a scaling relation for the m aximum scattering rate of proton anisotropy reduction. The result is a func tion only of the anisotropy and beta(parallel to p) and so is appropriate f or use in fluid models of anisotropic plasmas such as the magnetosheath.