Fast solar wind velocity in a polar coronal hole during solar minimum

We present a study of the outflow velocity of the fast wind in the northern polar coronal hole observed on 21 May 1996, during the minimum of solar activity, in the frame of a joint observing program of the SOHO (Solar Heliospheric Observatory) mission. The outflow velocity is inferred from an analysis of the Doppler dimming of the intensities of the O vi lambda 1032, 1037 and H i L alpha lambda 1216 lines observed between 1.5 R-. and 3.5 R-. with the Ultraviolet Coronagraph Spectrometer (UVCS), operating onboard SOHO. The analysis shows that for a coronal plasma characterized by low density, as derived for a polar hole at solar minimum by Guhathakurta et al. (1999), and low temperature, as directly measured at the base of this coronal hole by David et al. (1998), the oxygen outflow speed derived spectroscopically is consistent with that implied by the proton flux conservation. The hydrogen outflow is also consistent with flux conservation if the deviation from isotropy of the velocity distribution of the hydrogen atoms is negligible. Hence, for this cool and tenuous corona, the oxygen ions and neutral hydrogen atoms flow outward roughly at the same speed, which increases from 40 km s(-1) at 1.5 R-. to 360 km s(-1) at 3.1 R-., with an average acceleration of the order of similar to4.5x10(3) cm s(-2). The highly anisotropic velocity distributions of the O vi ions found in the analysis confirm that the process which is heating the oxygen ions acts preferentially across the magnetic field.