Oxygen temperature anisotropy and solar wind heating above coronal holes out to 5 R$_\odot

The purpose of the paper is to measure the degree of temperature anisotropy of the oxygen ions in the outer corona. The ratio of the Doppler dimmed O VI 1037-1032 line intensity as a function of the velocity of the fast solar wind, computed for typical values of coronal density, is consistent with the observed ratio, only when a significant temperature anisotropy is established in polar coronal holes. The oxygen ion velocity distribution is constrained to be bi-Maxwellian from 2$R_\odot$ to 3.7$R_\odot$, where the lowest degree of anisotropy compatible with the observational data increases up to ~7 at 2.9$R_\odot$, proving that the oxygen ions are accelerated across the magnetic field, in accordance with a preferential energy deposition perpendicular to the field lines, consistent with the process of ion-cyclotron dissipation of Alfvén waves. The most plausible evolution of the velocity distribution of the O+5 ions departs from the bi-Maxwellian configuration at 2$R_\odot$, according to an anisotropy ratio that reaches its maximum value $T_\perp/T_\parallel$~14 at 2.9$R_\odot$, and further out approaches isotropy, at 3.7$R_\odot$. In response to the acceleration across the field, energy redistribution along the magnetic field lines accelerates the oxygen component of the solar wind to velocities of 760 km s-1 at 5$R_\odot$. The variation of the anisotropy ratio with the heliocentric distance might be satisfactorily explained by theoretical models of the fast solar wind heating based on the oxygen cyclotron instability or the fast shock mechanism. 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.