TWO-DIMENSIONAL STRUCTURE OF A POLAR CORONAL HOLE AT SOLAR MINIMUM: NEW SEMIEMPIRICAL METHODOLOGY FOR DERIVING PLASMA PARAMETERS

摘要

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 solar radius 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 T_k derived from UVCS observations of the line profiles, whenever available, or we keep T_k 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 solar radius. 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.