The cyclotron resonance: Heating of protons and oxygen in coronal holes

The UVCS/SOHO data have offered remarkable evidence that the coronal holes and acceleration region of the fast solar wind are heated by ion-cyclotron waves. We here summarize the basic physics of the cyclotron resonance, and show why ions such as 0(+5) can be heated to more than mass-proportional temperatures compared to the protons. The mirror force provides the main acceleration out of the corona, yielding heavy ions which flow faster than the protons. We quantify these ideas by following an average test particle. Agreement with observation is achieved, but only if we take a steep power spectrum. Particular attention is given to the behavior of T(perpendicular to) for 0(+5), which seems to increase with distance from the Sun out to the limits of the observations; this observation is a major constraint.