First light observations of the solar wind in the outer corona with the Metis coronagraph

¹ University of Firenze, Florence, Italy
e-mail: marco.romoli@unifi.it
² INAF – Associate Scientist, Italy
³ INAF – Astronomical Observatory of Capodimonte, Naples, Italy
⁴ University of Catania, Catania, Italy
⁵ INAF – Astrophysical Observatory of Catania, Catania, Italy
⁶ IFN-CNR, Padova, Italy
⁷ Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
⁸ Predictive Science Inc., San Diego, CA, USA
⁹ INAF – Astrophysical Observatory of Torino, Torino, Italy
¹⁰ Astronomical Institute of the Czech Academy of Sciences, Ondřejov, Czech Republic
¹¹ University of Torino, Torino, Italy
¹² University of Padova, Padova, Italy
¹³ INAF – Institute for Space Astrophysics and Cosmic Physics, Milan, Italy
¹⁴ Naval Research Laboratory, Washington, DC, USA
¹⁵ Catholic University of America, Washington, DC, USA
¹⁶ University of Urbino, Urbino, Italy
¹⁷ INAF – Astrophysical Observatory of Trieste, Trieste, Italy
¹⁸ Politecnico di Torino, Torino, Italy
¹⁹ NASA HQ, Washington, DC, USA
²⁰ IEIIT-CNR, Padova, Italy
²¹ ASI – Agenzia Spaziale Italiana, Roma, Italy
²² Institut d’Astrophysique Spatiale, Paris, France
²³ University of Wrocław – Astronomical Institute, Wrocław, Poland
²⁴ INAF – Institute for Space Astrophysics and Planetology, Rome, Italy
²⁵ INAF – Astronomical Observatory of Palermo, Palermo, Italy
²⁶ Laboratoire Atmosphères, Milieux et Observations Spatiales, Guyancourt, France
²⁷ University of Pavia, Pavia, Italy
²⁸ University of Calabria, Cosenza, Italy
²⁹ University of Palermo, Palermo, Italy
³⁰ University of Athens, Athens, Greece
³¹ University of California, Los Angeles, CA, USA

Received: 1 April 2021
Accepted: 25 May 2021

Abstract

In this work, we present an investigation of the wind in the solar corona that has been initiated by observations of the resonantly scattered ultraviolet emission of the coronal plasma obtained with UVCS-SOHO, designed to measure the wind outflow speed by applying Doppler dimming diagnostics. Metis on Solar Orbiter complements the UVCS spectroscopic observations that were performed during solar activity cycle 23 by simultaneously imaging the polarized visible light and the H I Lyman-α corona in order to obtain high spatial and temporal resolution maps of the outward velocity of the continuously expanding solar atmosphere. The Metis observations, taken on May 15, 2020, provide the first H I Lyman-α images of the extended corona and the first instantaneous map of the speed of the coronal plasma outflows during the minimum of solar activity and allow us to identify the layer where the slow wind flow is observed. The polarized visible light (580–640 nm) and the ultraviolet H I Lyα (121.6 nm) coronal emissions, obtained with the two Metis channels, were combined in order to measure the dimming of the UV emission relative to a static corona. This effect is caused by the outward motion of the coronal plasma along the direction of incidence of the chromospheric photons on the coronal neutral hydrogen. The plasma outflow velocity was then derived as a function of the measured Doppler dimming. The static corona UV emission was simulated on the basis of the plasma electron density inferred from the polarized visible light. This study leads to the identification, in the velocity maps of the solar corona, of the high-density layer about ±10° wide, centered on the extension of a quiet equatorial streamer present at the east limb – the coronal origin of the heliospheric current sheet – where the slowest wind flows at about 160 ± 18 km s⁻¹ from 4 R_⊙ to 6 R_⊙. Beyond the boundaries of the high-density layer, the wind velocity rapidly increases, marking the transition between slow and fast wind in the corona.