Magnetosonic Waves Above the Lower Hybrid Frequency in Cyclotron Resonance With the Van Allen Radiation Belt Electrons

Cyclotron resonance with magnetosonic waves was usually considered unimportant for the Van Allen radiation belt electron dynamics because it occurs at energies that are too high. Here, we show that for magnetosonic waves with frequencies above the lower hybrid frequency and normal angles below 87 degrees in the high-density plasmasphere, the electron cyclotron resonances of +/- 1, +/- 2, and +/- 3 orders can extend down to energies of 10-100s keV. These high-frequency, non-quasi-perpendicular magnetosonic waves are found to occur predominantly in the duskside outer plasmasphere, with an amplitude on the order of 10 pT following strong substorms. They probably originate from midlatitudes of the outer plasmasphere, different from the commonly known magnetosonic waves generated near the equator.