Alfvén-cyclotron fluctuations: Linear Vlasov theory

[1] Linear Vlasov dispersion theory for a homogeneous, collisionless electron-proton plasma with Maxwellian velocity distributions is used to examine the damping of Alfven-cyclotron fluctuations. Fluctuations of sufficiently long wavelength are essentially undamped, but as k∥, the wave vector component parallel to the background magnetic field Bo, reaches a characteristic dissipation value kd, the protons become cyclotron resonant and damping begins abruptly. For proton cyclotron damping, kdc/ωp ∼ 1 for 10−3 ≲ βp ≲ 10−1, where βp ≡ 8πnpkBTp/Bo2 and ωp/c is the proton inertial length. At k∥ < kd, me/mp < βe, and βp ≲ 0.10 the electron Landau resonance becomes the primary contributor to fluctuation dissipation, yielding a damping rate that scales as ωr (k⊥c/ωp)2, where ωr is the real frequency and k⊥ is the wave vector component perpendicular to Bo. As βp increases from 0.10 to 10, the proton Landau resonance makes an increasing contribution to damping of these waves at k∥ < kd and 0° < θ < 30°, where θ = arccos( · o). The maximum damping rate due to the proton Landau resonance scales approximately as βp(kc/ωp)2 over 0.50 ≤ βp ≤ 10. Both magnetic transit time damping and electric Landau damping may contribute to Landau resonant dissipation; in the electron Landau resonance regime the former is important only at propagation almost parallel to Bo, whereas proton transit time damping can be relatively important at both quasi-parallel and quasi-perpendicular propagation of Alfven-cyclotron fluctuations.