The Width, Density, and Outflow of Solar Coronal Streamers

Abstract

Characterizing the large-scale structure and plasma properties of the inner corona is crucial to understanding the source and subsequent expansion of the solar wind and related space weather effects. Here, we apply a new coronal rotational tomography method, along with a method to narrow streamers and refine the density estimate, to COR2A/Solar Terrestrial Relations Observatory observations from a period near solar minimum and maximum, gaining density maps for heights between 4 and 8 R ⊙ . The coronal structure is highly radial at these heights, and the streamers are very narrow: in some regions, only a few degrees in width. The mean densities of streamers is almost identical between solar minimum and maximum. However, streamers at solar maximum contain around 50% more total mass due to their larger area. By assuming a constant mass flux, and constraints on proton flux measured by Parker Solar Probe (PSP), we estimate an outflow speed within solar minimum streamers of 50–120 kms −1 at 4 R ⊙ , increasing to 90–250 kms −1 at 8 R ⊙ . Accelerations of around 6 ms −2 are found for streamers at a height of 4 R ⊙ , decreasing with height. The solar maximum slow wind shows a higher acceleration to extended distances compared with solar minimum. To satisfy the solar wind speeds measured by PSP, there must be a mean residual acceleration of around 1–2 ms −2 between 8 and 40 R ⊙ . Several aspects of this study strongly suggest that the coronal streamer belt density is highly variable on small scales, and that the tomography can only reveal a local spatial and temporal average.