Circularly polarized waves at ion-scales are frequently observed in the inner heliosphere by Parker Solar Probe (PSP), with their occurrence increasing with closer radial distances. Ion-scale waves play a major role in mediating the non-Maxwellian features of solar wind ions and the solar wind heating. They have also been proposed to mediate the flow of energy from the turbulent cascade to small parallel scales via the “Helicity Barrier” mechanism. At the closest observed distances, ion-scale waves are seen by PSP in at least 30% of the intervals, are preferentially left-hand (LH) polarized, and are preferentially observed when the acute angle between the magnetic field and solar wind velocity is small.

Due to the amplitude of turbulent fluctuations, these waves might not be observed — even when they are present — when this observing angle is large. We identify the circularly polarized waves as a function of time using the observed spacecraft frame circular polarization for PSP Encounters 1-22 and statistically analyze their occurrence as a function of plasma and solar wind parameters. The LH waves are intermittently observed over long periods (entire days) in later PSP encounters where the proton temperature anisotropy remains close to the ion cyclotron instability threshold.

We find that the presence (absence) of these LH waves is correlated with the acute angle between magnetic field and solar wind velocity being lower (higher) than a threshold. We therefore propose that these LH waves are part of parallel propagating Ion Cyclotron Wave Storms that last longer than the timescales of the observed patches of high circular polarization. Such wave storms suggest continual quasi-steady generation and absorption of such ICWs over extended periods.