The In Situ Signature of Cyclotron Resonant Heating

TL;DR

This paper presents Parker Solar Probe observations linking ion-cyclotron waves to resonant damping signatures in proton velocity distributions, providing direct evidence of cyclotron heating in the solar wind.

Contribution

It offers the first direct observational evidence connecting ion-cyclotron waves with resonant damping signatures in the solar wind.

Findings

Ion-cyclotron waves are observed alongside signatures of resonant damping.

Proton velocity distributions show flattening consistent with quasilinear diffusion.

Distributions near the damping threshold suggest active cyclotron heating.

Abstract

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.