Oxygen outflow velocities in a polar coronal hole

The expansion velocity of the solar corona in a polar coronal hole during the minimum of solar activity is inferred from an analysis of the intensities and profiles of the O VI λ 1032, 1037 lines observed with the Ultraviolet Coronagraph Spectrometer (UVCS), operating onboard the Solar Heliospheric Observatory (SOHO). The outflow velocity of the oxygen ions carried by the solar wind reaches values ≥ 300 km s −1 above 2.1 R ⊙. This is evidence for ion acceleration primarily near 1.6-2.1 R ⊙. This analysis also shows that beyond 1.8 R ⊙ the velocity distribution of the oxygen ions in the frame of reference of the solar wind is highly anisotropic, that is, this distribution is much broader along the line of sight than along the radial. The velocity anisotropy found in the region of acceleration of the fast solar wind is evidence for a heating process operating preferentially in the direction perpendicular to the magnetic field.