Van Allen Probes Observations of a Fundamental Poloidal Wave in the Plasmasphere

All past spacecraft observations of fundamental poloidal Alfvén (P1) waves attributed to ion drift resonance have been made outside the plasmasphere. In this study, we present the first spacecraft observations of a plasmaspheric P1 wave driven by that resonance. The wave was observed on 6 March 2014 by the Van Allen Probes A and B spacecraft as they moved from = 3 to = 5 in the prenoon sector. The wave had a frequency of 4 mHz, and its harmonic order is determined from the phase difference between the magnetic and electric field components and a theoretical model of the frequency. The wave was accompanied by proton flux oscillations in the energy range 70–100 keV, which exhibited energy‐dependent amplitude and phase. This flux behavior is explained by proton drift resonance with the P1 wave, which is inferred to have propagated westward with an azimuthal wave number, , of ∼. The wave was detected on the ground near the magnetic footprints of the spacecraft as magnetic pulsations in the east‐west component, resembling giant pulsations. The proton phase space density evaluated with the first two adiabatic invariants fixed at the values specified at the center of the wave event exhibits a negative radial gradient in the region of the P1 wave, supporting the theory of drift resonance excitation of P1 waves. Pitch angle scattering of ring current ions by electromagnetic ion cyclotron waves may contribute to the formation of the phase space density gradient.