Abstract :
Cyclotron resonances were first studied to explain observations showing that solar wind heavy ions flow faster and are hotter than the protons. About ten years ago the resonances were applied to the corona. Those models predicted high proton temperatures, and were discounted. The SOHO/UVCS data now indicate that coronal protons are in fact hot, and that heavy ions are more than mass-proportionally hotter; protons and ions are hotter than the electrons. The ions, and probably the protons, are heated primarily perpendicularly to the magnetic field. These are all indications that the corona is heated by ion-cyclotron resonances, and that the wind is largely driven by the proton pressure. We will discuss the basic physics of the cyclotron interaction, and show in simple terms how it can qualitatively explain the observations. Illustrative models will be used to show that the idea works quantitatively, though there may be problems with simultaneously reproducing the UVCS proton and oxygen data. We will emphasize that there are basic questions concerning the wave source: does the Sun directly launch high-frequency (kHz) waves, or does the Sun launch lower frequency waves which subsequently undergo a turbulent cascade to the resonant frequencies? We will further emphasize that a full description must be in terms of the particle distribution functions, which will evolve in such a way as to become unstable to the generation of sunward-propagating waves, which may lead to novel effects.
