Abstract
Circularly polarized, nearly parallel propagating waves are prevalent in the
solar wind at ion-kinetic scales. At these scales, the spectrum of turbulent
fluctuations in the solar wind steepens, often called the transition-range,
before flattening at sub-ion scales. Circularly polarized waves have been
proposed as a mechanism to couple electromagnetic fluctuations to ion
gyromotion, enabling ion-scale dissipation that results in observed ion-scale
steepening. Here, we study Parker Solar Probe observations of an extended
stream of fast solar wind ranging from 15-55 solar radii. We demonstrate that,
throughout the stream, transition-range steepening at ion-scales is associated
with the presence of significant left handed ion-kinetic scale waves, which are
thought to be ion-cyclotron waves. We implement quasilinear theory to compute
the rate at which ions are heated via cyclotron resonance with the observed
circularly polarized waves given the empirically measured proton velocity
distribution functions. We apply the Von Karman decay law to estimate the
turbulent decay of the large-scale fluctuations, which is equal to the
turbulent energy cascade rate. We find that the ion-cyclotron heating rates are
correlated with, and amount to a significant fraction of, the turbulent energy
cascade rate, implying that cyclotron heating is an important dissipation
mechanism in the solar wind.