Poloidal Mode Wave‐Particle Interactions Inferred From Van Allen Probes and CARISMA Ground‐Based Observations

Ultralow‐frequency wave and test particle models are used to investigate the pitch angle and energy dependence of ion differential fluxes measured by the Van Allen Probes spacecraft on 6 October 2012. Analysis of the satellite data reveals modulations in differential flux resulting from drift resonance between H+ ions and fundamental mode poloidal Alfvén waves detected near the magnetic equator at L ∼ 5.7. Results obtained from simulations reproduce important features of the observations, including a substantial enhancement of the differential flux between ∼20 and 40° pitch angle for ion energies between ∼90 and 220 keV and an absence of flux modulations at 90°. The numerical results confirm predictions of drift‐bounce resonance theory and show good quantitative agreement with observations of modulations in differential flux produced by ultralow‐frequency waves. Key Points ULF wave and test particle models are used to investigate the pitch angle and energy dependence of H+ fluxes observed by Van Allen Probes Trapping of a population of drift‐resonant ions in a range of L is observed The dependence of the drift‐resonance energy on pitch angle leads to butterfly shaped ion distributions