Off-Equatorial Source Of Magnetosonic Waves Extending Above The Lower Hybrid Resonance Frequency In The Inner Magnetosphere

Inner magnetospheric magnetosonic waves are usually considered to grow from the near-equatorial ion Bernstein instability below the lower hybrid resonance frequency and contribute significantly to the radiation belt electron dynamics. We here present unusual observations of magnetosonic waves extending above the local lower hybrid resonance frequency in the midlatitude similar to 15 degrees plasmasphere after the substorm proton injection. Linear instability analyses and ray-tracing simulations show that, because of the latitudinal variation of normal angles, these waves have little growth near the equator but are amplified accumulatively to the observable level over broad latitudes |lambda| < 30 degrees during the bounce-drift propagation. Our data and modeling illustrate a previously unexplored scenario that off-equatorial proton ring distributions could be a significant source of magnetosonic waves in the inner magnetosphere. Such off-equatorially generated magnetosonic waves might differ from the near-equatorially generated ones in latitudinal coverage, wavevector distribution, and then effect on the radiation belt electrons.