Abstract
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 8R ⊙. 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 4R ⊙, increasing to 90–250 kms−1 at 8R ⊙. Accelerations of around 6 ms−2 are found for streamers at a height of 4R ⊙, 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 40R ⊙. 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.
Highlights
Coronal rotational tomography (CRT) of coronagraph imaging data currently offers the only direct method to reconstruct the global coronal density structure
Mean density values are calculated for a set of tomography maps for both the 2018/11/11 solar minimum and 2010/09/19 ascending phase at 0.5R height increments between 4 and 8R
A new coronal tomography method, based on spherical harmonics and regularized least-squares inversion gives a robust reconstruction of the coronal density structure
Summary
Coronal rotational tomography (CRT) of coronagraph imaging data currently offers the only direct method to reconstruct the global coronal density structure. Estimates of the 3D distribution of density is crucial for interpretation of observations by other instruments (Frazin et al 2003), and as boundary constraints for large-scale models (Frazin et al 2005) These methods, and their findings, are described in the review of Aschwanden (2011). An extension and refinement to the CRT method is presented, necessary to replicate the apparent narrowness of streamers in the coronagraph images.
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