Solar wind speed maps from the Metis coronagraph observations

¹ INAF – Torino Astrophysical Observatory, Via Osservatorio 20, I-10025 Pino Torinese, (TO), Italy
² INAF – Catania Astrophysical Observatory, Via Santa Sofia 78, I-95123 Catania, Italy
³ INAF – Capodimonte Astronomical Observatory, Salita Moiariello 16, I-80131 Naples, Italy
⁴ Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
⁵ University of Firenze – Physics and Astronomy Department, Via Sansone 1, I-50019 Sesto Fiorentino, (FI), Italy
⁶ INAF – Arcetri Astrophysical Observatory, Largo Enrico Fermi 5, I-50125 Florence, Italy
⁷ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
⁸ INAF –Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Alfonso Corti 12, I-20133 Milan, Italy

^⋆ Corresponding author: silvio.giordano@inaf.it

Received: 11 February 2025
Accepted: 21 July 2025

Abstract

Context. The solar wind plays a crucial role in shaping the heliosphere and influencing space weather. Understanding its origin and acceleration requires measurements of coronal dynamics. The Metis coronagraph on board of Solar Orbiter provides high-resolution simultaneous imaging of the middle solar corona in the polarized visible light and ultraviolet H I Lyα, which allows us to derive solar wind speed maps.

Aims. We determine solar wind speed maps by applying the Doppler dimming technique to Metis observations in the distance range from about 3.0 to 7.6 R_⊙. The goal is to present the detailed algorithm, investigate the dependence of the speed on the parameters of the coronal model, and provide maps of the solar wind speed at the minimum of the solar activity. This is useful to improve our understanding of the physical processes that accelerate the wind.

Methods. Solar wind speeds are inferred by analyzing H I Lyα intensities in combination with electron density maps derived from visible-light polarized-brightness data. The Doppler dimming effect is used to estimate outflow velocities with different coronal model parameters, such as the electron temperature, the kinetic temperature, and the helium abundance, which are tested to assess their effect on the results.

Results. The wind speed maps confirm the bimodal distribution of the solar wind outflow velocities that characterize the near-minimum phases of solar activity. The slow wind (100−200 km s⁻¹) confined to the equatorial streamer belt and fast wind (250−400 km s⁻¹) originates from the polar coronal holes. The transition between these regions is sharp, with a steep velocity gradient at mid-latitudes. Variations in the coronal model parameters significantly affect the inferred speeds. This highlights the need for precise constraints on the coronal conditions.

Conclusions. Our method allows a systematic mapping of the solar wind speed and can be applied to data that are daily acquired by Metis throughout the current solar cycle. This provides new information on regions in which the wind is accelerated and on their evolution. These results provide valuable constraints for heliospheric models and theoretical studies of the formation of the solar wind. Future observations, in particular, during closer Solar Orbiter perihelia, will refine these measurements and improve our understanding of the solar corona and solar wind dynamics.