  The dissipation of magnetized turbulence is an important paradigm for
describing heating and energy transfer in astrophysical environments such as
the solar corona and wind; however, the specific collisionless processes behind
dissipation and heating remain relatively unconstrained by measurements. Remote
sensing observations have suggested the presence of strong temperature
anisotropy in the solar corona consistent with cyclotron resonant heating. In
the solar wind, in situ magnetic field measurements reveal the presence of
cyclotron waves, while measured ion velocity distribution functions have hinted
at the active presence of cyclotron resonance. Here, we present Parker Solar
Probe observations that connect the presence of ion-cyclotron waves directly to
signatures of resonant damping in observed proton-velocity distributions. We
show that the observed cyclotron wave population coincides with both flattening
in the phase space distribution predicted by resonant quasilinear diffusion and
steepening in the turbulent spectra at the ion-cyclotron resonant scale. In
measured velocity distribution functions where cyclotron resonant flattening is
weaker, the distributions are nearly uniformly subject to ion-cyclotron wave
damping rather than emission, indicating that the distributions can damp the
observed wave population. These results are consistent with active cyclotron
heating in the solar wind.

Solar and Stellar Astrophysics;Plasma Physics;Space Physics