Resonance amplification factor, W, vs. frequency of oscillation, ω (solid red line). The vertical dashed violet line marks the resonance peak at ω = ω_res, and the dashed green line indicates the frequency of the tidal potential component that is responsible for exciting the oscillations, ω_mn. When the magnetic field, B, of the flux tube increases, the resonance peak moves toward the right because ω_res increases. On the contrary, when the magnetic field strength decreases, the peak moves toward the left. Therefore, the perturbationΔB² produced by the oscillation displacement, ξ_r, can lock the system into resonance, with ω_mn being slightly greater than ω_res, as illustrated (see text).