Growth and decay of field line resonances

The apparent growth rates of ULF field line resonances seen by the SuperDARN HF radars show a striking correlation with the azimuthal wave number m of the resonance. The observed high‐m (m>17) resonances have amplitudes that increase with time, indicating positive growth rates, while the low‐m resonances have decreasing amplitudes, indicating negative growth rates. The resonance growth rates and latitudinal phase shifts, a decrease for low‐m modes and an increase for high‐m modes, are found to be determined by the direction of the Poynting flux in the system. In the case of the low‐m resonance the Poynting flux direction is such that the energy flows from the fast wave driver into the shear Alfvén field line resonance and down into the ionosphere. In the case of the high‐m resonance an internal driver couples to the system and reverses the direction of the Poynting flux such that energy flows from the internal driver into the shear Alfvén field line resonance and out into the fast wave. There is evidence to suggest that the internal driver is in the form of a wave‐particle interaction.