Session 1 - Chromosphere, Corona, Flares.
Display session, Friday, June 27
Ballroom B, Chair: Charles Kankelborg

[1.23] Model Computations of the Line Profiles of O VI 1032 and 1037 Åin the Fast Solar Wind and Comparison with UVCS Observations

X. Li, S. R. Habbal, R. Esser (Harvard-Smithsonian Center for Astrophysics)

The profiles of the O VI 1032 and 1037 Å\ spectral lines are calculated for the fast solar wind. The computed theoretical line profiles are based on high speed solar wind models where the O^+5 ions are treated as test particles in a three-fluid (electrons, protons and alphas) background solar wind flow. The background solar wind model matches the observational constraints of particle flux, flow speeds and temperatures at 1 AU, and the electron density profiles in the inner corona. Line of sight effects are included in the calculations of the line profile. Preferential heating is applied to the oxygen ions to produce an ion temperature in the inner corona around 2 \times 10^8 K, as inferred from the UVCS observations on SOHO, and a flow speed exceeding that of the protons beyond several solar radii. Both resonantly scattered and collisionally excited components of the oxygen 1032 and 1037 Å\ lines are computed. We find that for flow speeds below 100 km/s, the two components have approximately the same width. However, once the speed exceeds 150 km/s, the collisionally excited component becomes much wider than the resonantly scattered component. This effect can be explained by the strong Doppler dimming of the resonantly scattered component as the flow speed increases. Consequently, when the integration along the line of sight is taken into account, ions traveling along trajectories away from the plane of the sky will contribute to the the broadening of the collisionally excited component and to the dimming of the resonantly scattered component. The model computations thus suggest that the broad oxygen line profiles observed by the UVCS instrument on SOHO reflect the existence of very high oxygen flow speeds very close to the coronal base, in addition to the high kinetic temperatures.