The exact numerical solutions, the real frequency a) and the growth rate b), c), d) for the PFHI as provided by Eq. (7) for κ_p = 3 (dotted line), κ_p = 4 (dashed line), κ_p = 8 (long-dashed line), and κ_p → ∞ (solid line), and typical solar wind plasma parameters T_e,∥/T_p,∥ = 2, β_e,∥ = 8, (β_p,∥ = 4), T_e,⊥ = T_e,∥ ≃ 10⁶ K and different values of the proton anisotropy T_p,⊥/T_p,∥ = 0.25 a), b), 0.6 c), 0.8 d). Electrons are assumed to be isotropic and Maxwellian (κ_e → ∞). (W_r = ω_r/Ω_p, W_i = ω_i/Ω_p, K = kc/ω_p,p).