The degree of LP as a function of φ. The red solid line, blue dashed line, black dotted line, and orange dotted–dashed line indicate φ _t = 2/γ, ω = ω _c ; φ _t = 1/2γ, ω = ω _c ; φ _t = 1/2γ, ω = 0.1ω _c ; φ _t = 1/2γ, ω = 10ω _c , respectively. We adopt below physical parameters: ρ = 10⁷ cm, γ = 100, Δθ _s = 10⁻³.