AbstractThe Van Allen Probes‐A spacecraft observed an ∼9‐mHz ultralow‐frequency wave on 6 October 2012, at L ∼ 5.7, in the dawn sector, and very near the magnetic equator. The wave had a strong electric field that was initially stronger in the azimuthal component and later in the radial component, exhibited properties of a fundamental standing Alfvén wave, and was associated with giant pulsations observed on the ground near the magnetic field footprint of the spacecraft. The wave was accompanied by oscillations of the flux of energetic protons (jH+). The amplitude of
oscillations was large at equatorial pitch angles away from 90°, and the energy dependence of the phase and amplitude of the oscillations exhibited features consistent with drift resonance of ∼140‐keV protons with a westward‐propagating wave having an azimuthal wave number of ∼−40. The wave was detected when the spacecraft entered a region of an earthward gradient of the proton phase space density, in support of a theoretical prediction that such a gradient can drive fundamental poloidal waves.
