Effect of the non-uniform solar chromospheric Lyα radiation on determining the coronal H I outflow velocity

¹ INAF–Catania Astrophysical Observatory, Via Santa Sofia 78, 95123 Catania, Italy
e-mail: sergio.dolei@inaf.it
² INAF–Turin Astrophysical Observatory, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
³ INAF–Capodimonte Astronomical Observatory, Salita Moiariello 16, 80131 Napoli, Italy
⁴ CNR–Institute of Photonics and Nanotechnologies, Via Trasea 7, 35131 Padova, Italy
⁵ INAF–Arcetri Astrophysical Observatory, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁶ University of Padova–Dept. of Physics and Astronomy, Via Marzolo 8, 35131 Padova, Italy
⁷ University of Florence–Dept. of Physics and Astronomy, Largo Enrico Fermi 2, 50125 Firenze, Italy

Received: 11 January 2019
Accepted: 22 May 2019

Abstract

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 H I Lyα 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 Lyα 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 Lyα 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 H I 121.6 nm and He II 30.4 nm line intensities was used to construct Carrington rotation maps of the non-uniform solar chromospheric Lyα radiation and thus to compute the Lyα line irradiance throughout the outer corona. Approximations concerning the temperature of the scattering H I 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 effect of the chromospheric Lyα brightness distribution on the resulting H I 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 50−60 km s⁻¹ closer to the Sun. This difference 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.