Hydrogen and oxygen temperatures in coronal holes Original Research Article

The analysis of the O VI λ 1032 and 1037 and H I Lyα λ 1216 line profiles, observed with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard the Solar Heliospheric Observatory (SOHO) in a polar coronal hole within 1.5 R⊙ and 3.5 R⊙ during the solar activity minimum, indicates the existence of a large anisotropy in the velocity distribution of the oxygen ions across the magnetic field lines. This is evidence for temperatures across the field lines that at 3.1 R⊙ exceed by ≥1.6-1.7 × 108 K the oxygen temperature inferred along the radial direction, which is ∼107 K. The upper limit for the neutral hydrogen/proton temperature is 2.4-3.0 × 106 K. The bulk motions across the magnetic field lines, due to wave motions and nonradial coronal expansion, cannot exceed ∼170 km s−1. The results of the present analysis imply that oxygen ions are heated much more effectively than protons in the first solar radius of the solar atmosphere and that protons can be heated more efficiently than electrons. Furthermore, in addition to the process which heats the oxygen ions to 107 K, a very strong acceleration operating only perpendicularly to the magnetic field has to be invoked to justify the temperature excess, of the order of 108 K, in this direction.