The width, density and outflow of solar coronal streamers

Characterising the large-scale structure and plasma properties of the inner corona is crucial to understand 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/STEREO observations from a period near solar minimum and maximum, gaining density maps for heights between 4 and 8\Rs. 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\ at 4\Rs, increasing to 90-250\kms\ at 8\Rs. Accelerations of around 6\mss\ are found for streamers at a height of 4\Rs, decreasing with height. The solar maximum slow wind shows a higher acceleration to extended distances compared to solar minimum. To satisfy the solar wind speeds measured by PSP, there must be a mean residual acceleration of around 1-2\mss\ between 8 and 40\Rs. 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.

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