  Wave-particle resonance, a ubiquitous process in the plasma universe, occurs
when resonant particles observe a constant wave phase to enable sustained
energy transfer. Here, we present spacecraft observations of simultaneous
Landau and anomalous resonances between oblique whistler waves and the same
group of protons, which are evidenced, respectively, by phase-space rings in
parallel-velocity spectra and phase-bunched distributions in gyro-phase
spectra. Our results indicate the coupling between Landau and anomalous
resonances via the overlapping of the resonance islands.
