The Effect of Temperature Anisotropy on Observations of Doppler Dimming and Pumping in the Inner Corona

Abstract

Recent observations of the spectral line profiles and intensity ratio of the O VI λλ1032 and 1037.6 doublet by the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO), made in coronal holes below 3.5 RS, provide evidence for Doppler dimming of the O VI λ1037.6 line and pumping by the chromospheric C II λ1037.0182 line. Evidence for a significant kinetic temperature anisotropy of O5+ ions was also derived from these observations. We show in this Letter how the component of the kinetic temperature in the direction perpendicular to the magnetic field, for both isotropic and anisotropic temperature distributions, affects both the amount of Doppler dimming and pumping. Taking this component into account, we further show that the observation of the O VI doublet intensity ratio less than unity can be accounted for only if pumping by C II λ1036.3367 in addition to C II λ1037.0182 is in effect. The inclusion of the C II λ1036.3367 pumping implies that the speed of the O5+ ions can reach 400 km s-1 around 3 RS, which is significantly higher than the reported UVCS values for atomic hydrogen in polar coronal holes. These results imply that oxygen ions flow much faster than protons at that heliocentric distance.