Outflow velocity of the O+5ions in polar coronal holes out to 5R$_\odot$

Abstract

The purpose of the paper is to extend the measurement of the kinetic temperature and outflow velocity of the oxygen ions in the outer corona above polar holes out to 5 $R_\odot$. An analytical model of the solar corona at the minimum of activity has been employed in order to synthesize the spectral line profiles to be fitted with the data: the kinetic temperature of the O +5 ions on the plane of the sky is derived from the width of the O VI 1031.9 A line by applying the $\chi^2$ minimization. The oxygen temperature peaks at about 2.9 $R_\odot$, reaching a value of 1.5 $\times$ 10 8 K, and further out it is somehow flattening. The outflow velocity of the oxygen component of the fast solar wind, derived from the intensity ratio of the Doppler dimmed O VI doublet, increases outward to reach $550{-}760$ km s -1 at 5 $R_\odot$. The upper and lower limits of the outflow speed are due to the fact that its measurement depends on the velocity distribution of the oxygen ions which cannot be directly measured along the radial direction, but only along the line of sight. Hence the uncertainty is related to the temperature anisotropy assumed in the analysis. For this reason in this paper the measurement of the temperature anisotropy, found beyond 2 $R_\odot$ according to the previous literature on UVCS results, has been as well extended at higher altitudes, and it is found that above 3.7 $R_\odot$ anisotropy can still exist but not necessarily. The observations of the extended corona analyzed in this paper are performed with the Ultraviolet Coronagraph Spectrometer on board the Solar Heliospheric Observatory, during the solar minimum activity period 1996-1997.