Ion Cyclotron Damping in the Solar Corona and Solar Wind

The solar corona is the hot, ionized outer atmosphere of the Sun. Coronal plasma expands into interplanetary space as a supersonic bulk outflow known as the solar wind. This tenuous and unbounded medium is a unique laboratory for the study of kinetic theory in a nearly collisionless plasma, as well as magnetohydrodynamic waves, shocks, and jets. Particle velocity distributions in the solar wind have been probed directly by spacecraft (outside the orbit of Mercury), and indirectly by ultraviolet spectroscopy (close to the Sun). Fluctuations in the plasma properties and in electromagnetic fields have been measured on time scales ranging from seconds to years. Despite more than a half-century of study, though, the basic physical processes responsible for heating the million-degree corona and accelerating the solar wind past the Sun’s escape velocity are still not known with certainty. Understanding the basic physics of the solar wind is necessary to predict the Sun’s impact on the Earth’s climate and local space environment. This presentation will review the kinetic origins of several physical processes that are currently believed to be important in depositing energy and momentum in coronal particle velocity distributions. Because ions in the solar wind are heated and accelerated more than would be expected in either thermodynamic equilibrium or via a mass-proportional process, an ion cyclotron resonance has been suggested as a likely mechanism. Other evidence for gyroresonant wave dissipation in the corona will be presented, and possible generation mechanisms for the (as yet unobserved) highfrequency cyclotron waves will be reviewed. The mean state of the coronal and heliospheric plasma is intimately coupled with kinetic fluctuations about that mean, and theories of turbulence, wave dissipation, and instabilities must continue to be developed along with steady state descriptions of the solar wind.