  Context. The first studies with Parker Solar Probe (PSP) data have made
significant progress toward the understanding of the fundamental properties of
ion cyclotron waves in the inner heliosphere. The survey mode particle
measurements of PSP, however, did not make it possible to measure the coupling
between electromagnetic fields and particles on the time scale of the wave
periods.
  Aims. We present a novel approach to study wave-particle energy exchange with
PSP.
  Methods. We use the Flux Angle operation mode of the Solar Probe Cup in
conjunction with the electric field measurements and present a case study when
the Flux Angle mode measured the direct interaction of the proton velocity
distribution with an ion cyclotron wave.
  Results. Our results suggest that the energy transfer from fields to
particles on the timescale of a cyclotron period is equal to approximately 3-6%
of the electromagnetic energy flux. This rate is consistent with the hypothesis
that the ion cyclotron wave was locally generated in the solar wind.
