Ion Cyclotron Diffusion of Velocity Distributions in the Extended Solar Corona

The Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO) has revealed strong kinetic anisotropies and extremely large perpendicular temperatures of heavy ions in the extended solar corona. These observations have revived interest in the idea that the high-speed solar wind is heated and accelerated by the dissipation of ion cyclotron resonant Alfvén waves. This process naturally produces departures from Maxwellian and bi-Maxwellian velocity distributions. Here it is argued that these departures must be taken into account in order to understand the resonant velocity-space diffusion, the wave damping, and the formation of ultraviolet emission lines. Time-dependent ion velocity distributions are computed for a fixed spectrum of waves in a homogeneous plasma, and the moments of the distributions are compared with simple bi-Maxwellian models. The existence of a boundary, in parallel velocity space, between resonance and nonresonance produces an effective saturation of the velocity-space diffusion that bi-Maxwellian models could not predict. The damping of an input wave spectrum is computed for a coronal population of 1000 ion species with the above saturation effect included. For realistic levels of fluctuation power, it is concluded that waves propagating solely from the coronal base would not be able to heat and accelerate the ions that have been observed to exhibit strong energization, and that local wave generation is required. Ultraviolet emission line profiles are computed for the derived non-Maxwellian distributions, and possible unique identifiers of the ion cyclotron resonance mechanism are noted.