GROWTH AND DECAY OF FIELD LINE RESONANCES

The apparent growth rates of ULF field line resonances seen by the Sup erDARN HF radars show a striking correlation with the azimuthal wave n umber m of the resonance. The observed high-m (m>17) resonances have a mplitudes that increase with time, indicating positive growth rates, w hile the low-m resonances have decreasing amplitudes, indicating negat ive growth rates. The resonance growth rates and latitudinal phase shi fts, 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 sy stem 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 A lfven held line resonance and down into the ionosphere. In the case of the high-m resonance an internal driver couples to the system and rev erses the direction of the Poynting flux such that energy flows from t he internal driver into the shear Alfven held line resonance and out i nto the fast wave. There is evidence to suggest that the internal driv er is in the form of a wave-particle interaction.