Solar Wind Speed Maps from the Metis coronagraph observations

This presentation outlines the Doppler dimming diagnostic method, the computational tools developed, and the solar wind speed maps derived from Metis data spanning a wide range of heliocentric distances, from approximately 1.5 to 10 solar radii, during the solar activity minimum.

The Doppler dimming diagnostic, which combine simultaneous Metis observations from its two channels, polarized broadband visible light and narrowband ultraviolet H I Lyα intensity, allows for the measurement of the expanding coronal plasma speed. The presented wind speed maps are obtained from Metis data sets collected during the cruise phase of the Solar Orbiter mission, shortly after its first perihelion at approximately 0.5 AU in June 2020, as well as before and after its second perihelion in January and February 2021. These observations provide critical insights into the solar corona above approximately 3.0 solar radii, while future Solar Orbiter perihelion passages will extend this analysis to distances as close as 1.7 solar radii.

The reliability of the derived wind speed values is evaluated, considering instrumental uncertainties, the inherent limitations of the diagnostic method, and key assumptions about solar corona model parameters, including electron and neutral atom kinetic temperatures and the 3D geometric configuration of the corona.

As expected at the minimum of the solar cycle, the obtained maps show a clear bipolar topology with a rapid transition to higher speeds in the interface zone between equatorial streamers and high-latitude regions. The equatorial regions, where intense and relatively stable streamer belt persists, do not exhibit speeds higher than 220 km/s up to the maximum observed distance, around 6 solar radii. In contrast, within the coronal holes, already just above the minimum observed heights, around 3.5 solar radii, the wind speed reaches the maximum values detectable using Doppler dimming diagnostics applied to the neutral hydrogen line, approximately 350 km/s.

The method and algorithms are further tested and applied to a sample of daily ultraviolet intensity images reconstructed from spectral data obtained by the UVCS instrument, in conjunction with visible light coronal observations performed by LASCO, both instruments onboard the SOHO mission during solar cycle 23. This extended analysis provides valuable insights into the solar wind acceleration region, covering distances between approximately 1.5 and 4.0 solar radii, encompassing nearly a full solar activity cycle, providing essential context to complement the present Solar Orbiter observations.