Mapping the solar wind HI outflow velocity in the inner heliosphere by coronagraphic ultraviolet and visible-light observations^★

¹ INAF–Catania Astrophysical Observatory, Via Santa Sofia 78, 95123 Catania, Italy
e-mail: sdo@oact.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 Padova–Dept. of Information Engineering, Via Gradenigo 6/B, 35131 Padova, Italy
⁸ University of Firenze–Dept. of Physics and Astronomy, Largo Enrico Fermi 2, 50125 Firenze, Italy

Received: 17 October 2017
Accepted: 27 December 2017

Abstract

We investigated the capability of mapping the solar wind outflow velocity of neutral hydrogen atoms by using synergistic visible-light and ultraviolet observations. We used polarised brightness images acquired by the LASCO/SOHO and Mk3/MLSO coronagraphs, and synoptic Lyα line observations of the UVCS/SOHO spectrometer to obtain daily maps of solar wind H I outflow velocity between 1.5 and 4.0 R_⊙ on the SOHO plane of the sky during a complete solar rotation (from 1997 June 1 to 1997 June 28). The 28-days data sequence allows us to construct coronal off-limb Carrington maps of the resulting velocities at different heliocentric distances to investigate the space and time evolution of the outflowing solar plasma. In addition, we performed a parameter space exploration in order to study the dependence of the derived outflow velocities on the physical quantities characterising the Lyα emitting process in the corona. Our results are important in anticipation of the future science with the Metis instrument, selected to be part of the Solar Orbiter scientific payload. It was conceived to carry out near-sun coronagraphy, performing for the first time simultaneous imaging in polarised visible-light and ultraviolet H I Lyα line, so providing an unprecedented view of the solar wind acceleration region in the inner corona.