Ion-Scale Waves Regulated by Plasma Beta and Cross Helicity: Observational Evidence for the Helicity Barrier in Imbalanced Solar Wind Turbulence
Ion-scale waves are believed to play an important role in heating of the solar corona and wind, though their generation mechanism remains unclear. Based on Parker Solar Probe observations, this Letter investigates the occurrence of ion-scale waves in the near-Sun solar wind with heliocentric distances between 0.1 and 0.2 au. Results show that the occurrence rate of left-handed polarized waves significantly depends on the plasma beta ($\beta$) and cross helicity ($\sigma_c$). The occurrence rate rapidly increases with decreasing $\beta$ and increasing $\sigma_c$. Overall, the occurrence rate exceeds 45$\%$ when $\beta<0.2$ and $\sigma_c > 0.7$ are satisfied. These observations are consistent with the direct predictions of the helicity barrier theory that suggests the generation of ion cyclotron waves via imbalanced magnetized turbulence.