Effect of the non-uniform solar chromospheric Ly α radiation on determining the coronal H i outflow velocity

We derived maps of the solar wind outflow velocity of coronal neutral hydrogen atoms at solar minimum in the altitude range
1.5–4.0 R. We applied the Doppler dimming technique to coronagraphic observations in the UV HI Ly line at 121.6 nm. The technique
exploits the intensity reduction in the coronal line with increasing velocities of the outflowing plasma to determine the solar
wind velocity by iterative modelling. The Ly line intensity is sensitive to the wind outflow velocity and also depends on the physical
properties of coronal particles and underlying chromospheric emission. Measurements of irradiance by the chromospheric Ly
radiation in the corona are required for a rigorous application of the Doppler dimming technique, but they are not provided by past
and current instrumentations. A correlation function between the HI 121.6 nm and He II 30.4 nm line intensities was used to construct
Carrington rotation maps of the non-uniform solar chromospheric Ly radiation and thus to compute the Ly line irradiance
throughout the outer corona. Approximations concerning the temperature of the scattering HI atoms and exciting solar disc radiation
were also adopted to significantly reduce the computational time and obtain a faster procedure for a quick-look data analysis of future
coronagraphic observations. The eect of the chromospheric Ly brightness distribution on the resulting HI outflow velocities was
quantified. In particular, we found that the usual uniform-disc approximation systematically leads to an overestimated velocity in
the polar and mid-latitude coronal regions up to a maximum of about 5060 km s1 closer to the Sun. This dierence decreases at
higher altitudes, where an increasingly larger chromospheric portion, including both brighter and darker disc features, contributes to
illuminate the solar corona, and the non-uniform radiation condition progressively approaches the uniform-disc approximation.