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S. R. Cranmer, J. L. Kohl (SAO)
In 1996, the Ultraviolet Coronagraph Spectrometer (UVCS) instrument aboard SOHO observed surprisingly broad line profiles of the O VI 1032, 1037 doublet in polar coronal holes. These measurements indicated perpendicular ion temperatures of at least 100--200 million K above two solar radii in the nascent high-speed solar wind. Since then, these observations have been supplemented by profiles of other ions, Doppler dimming measurements made possible by Spartan 201, and a great deal of theoretical work. This talk outlines the current state of understanding about coronal heating and solar wind acceleration that has been facilitated by UVCS.
The most promising mechanism for heating and accelerating minor ions remains the dissipation of high-frequency (10 to 10,000 Hz) ion cyclotron waves, but heating the protons is a more open question. The physics of the ion cyclotron interaction in the corona has only begun to be explored, and we will discuss recent insights into the generation and damping of these waves. A self-consistent theory of wave damping and turbulent cascade ``replenishment'' would allow the question of proton heating to be answered more definitively. Also, a kinetic approach to ion cyclotron heating yields non-bi-Maxwellian ``resonant shell'' velocity distributions that could produce emission line profiles narrower than expected from their most probable speeds. Thus, the UVCS measurements of 100--200 million K ion temperatures may only be lower limits.
This work is supported by the National Aeronautics and Space Administration under grant NAG5-7822 to the Smithsonian Astrophysical Observatory, by Agenzia Spaziale Italiana, and by the ESA PRODEX program (Swiss contribution).
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The author(s) of this abstract have provided an email address for comments about the abstract: scranmer@cfa.harvard.edu