Excitation of poloidal standing Alfvén waves through drift resonance wave-particle interaction

Drift-resonance wave-particle interaction is a fundamental collisionless plasma process studied extensively in theory. Using cross-spectral analysis of electric field, magnetic field, and ion flux data from the Van Allen Probe (Radiation Belt Storm Probes) spacecraft, we present direct evidence identifying the generation of a fundamental mode standing poloidal wave through drift-resonance interactions in the inner magnetosphere. Intense azimuthal electric field (E_φ) oscillations as large as 10mV/m are observed, associated with radial magnetic field (B_r) oscillations in the dawn-noon sector near but south of the magnetic equator at L∼5. The observed wave period, E_φ/B_r ratio and the 90° phase lag between B_r and E_φ are all consistent with fundamental mode standing Poloidal waves. Phase shifts between particle fluxes and wave electric fields clearly demonstrate a drift resonance with ∼90 keV ring current ions. The estimated earthward gradient of ion phase space density provides a free energy source for wave generation through the drift-resonance instability. A similar drift-resonance process should occur ubiquitously in collisionless plasma systems. One specific example is the "fishbone" instability in fusion plasma devices. In addition, our observations have important implications for the long-standing mysterious origin of Giant Pulsations. Key Points Unambiguous identification of drift-resonance in magnetosphere Broad implications for ring current and ground observations Drift-resonance similar to fishbone instability in Tokamak.