Abstract
We develop a new technique to determine the plasma parameters in a polar coronal hole. This method makes use of the line intensities of the H I Lyα λ1215.6 line and of the O VI λλ1031.9, 1037.6 doublet, measured with the Ultraviolet Coronagraph Spectrometer (UVCS) on board the ESA-NASA solar spacecraft Solar and Heliospheric Observatory (SOHO) during 1996 August. The observed intensities are self-consistently reproduced with a two-dimensional semiempirical coronal hole model, for heliocentric distances between 1.4 and 2.6 R☉ and latitudes between 90° (north pole) and 40°. Electron densities are derived by separating the O VI doublet collisional components from those due to resonant scattering. The calculated electron density radial profiles are consistent with typical polar coronal hole data and show only a moderate increase with latitude decreasing, in regions close to the equatorial streamer. The outflow speeds of protons and O VI ions are determined by means of the Doppler dimming technique. In the Doppler dimming analysis we use kinetic temperatures Tk derived from UVCS observations of the line profiles, whenever available, or we keep Tk as a free parameter if not provided by data. Mass flux conservation along the magnetic field lines is studied adopting a simple analytical model for the geometry of the magnetic flux tubes. Our model shows that protons and O VI ions accelerate outward, but their outflow speed turns out to decrease slowly as latitude decreases. The O VI speed, initially comparable to the speed of protons, exceeds the proton speed beyond ~1.7 R☉. Anisotropic O VI kinetic temperatures, T∥ and T⊥, turn out to be necessary to ensure the consistency of the model parameters with mass flux conservation, while the H kinetic temperature distribution is kept isotropic. Results from our model are compared with those from other two-dimensional models recently developed.
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