Authors: Evan L Yerger (UNH), Benjamin Chandran (UNH), Vincent David (UNH)

Parallel ion cyclotron waves (PICWs) are often observed by spacecraft in fast solar wind streams. While the origin of the observed PICWs remains uncertain, it is likely they are the result of a local instability, rather than direct driving by the oblique cascade. Recent observations, suggest these waves tend to heat, rather than cool, the solar wind, as one would expect from an instability. We argue that this apparent contradiction is in fact self-consistent. First, we show how PICWs with parallel wave numbers below a critical value are driven unstable by the turbulent cascade via quasi-linear focusing. We then argue that unstable PICWs will propagate and, as the solar wind is an inhomogeneous medium, that the wave frequencies and wave numbers evolve according to WKB theory. The effect of this evolution is that waves which cool the solar wind at one radius will propagate and heat it at a larger one. We call this effect `cyclotron breaking’, in analogy with ocean waves breaking on the shore. In addition to discussions of cyclotron breaking phenomenology and the conditions under which PICWs on average heat the solar wind, we also present our efforts to quantitatively model the effect for PSP encounter 10. Our model produces both partial and integrated PICW heating rates that generally agree with observations.