4.2 Coronal streamers

The first coronal tomographic reconstructions of Altschuler (1979) from Skylab data visualized a streamer belt map from a polar viewing angle. Tomographic inversion of Mark III K-coronameter data of the minimum corona revealed pronounced longitudinal density variations in the streamer belt (Zidowitz, 1999). Using a higher cadence of 4 polarized brightness (pB) images per day from LASCO C2, instead of 1 pB image per day, the quality of tomographic reconstruction could be dramatically improved (Frazin et al., 2007), especially at a height of 2.55 R⊙ that serves as a reference level of magnetic potential-field source surface (PFSS) models (Figure 15View Image, top left). 3D reconstructions of coronal streamers from LASCO images at 2.5 R ⊙ were compared with PFSS magnetic field lines and were found to coincide closely with the heliospheric current sheet, often associated with strong magnetic field active regions (Liewer et al., 2001Jump To The Next Citation Point). Based on this it was concluded that many of the bright streamers are the result of scattering from regions of enhanced density associated with active region outflows, and not a result of line-of-sight viewing through folds in a warped current sheet with uniform density (Liewer et al., 2001). However, more detailed tomography with LASCO data revealed double plasma sheets and triple current sheets that are not reproduced by standard PFSS extrapolations (Saez et al., 2005Jump To The Next Citation Point) (Figure 19View Image, top left), visible also in pseudo-tomography maps that subtract a radial gradient filter but do not preserve the electron density (Morgan et al., 2009Jump To The Next Citation Point; Morgan and Habbal, 2010Jump To The Next Citation Point) (Figure 19View Image, right). True tomography of the 3D density distribution with two simultaneous spacecraft observations (STEREO/A and B, COR-1) was first performed by Kramar et al. (2009Jump To The Next Citation Point) (Figure 19View Image, bottom left), which demonstrated that the equatorial streamer belt is largely consistent with the variation of the current sheet derived from magnetic potential field models.
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Figure 19: Examples of tomographic streamer belt reconstructions. Top left: Streamer belt density reconstruction with LASCO C-2 at r = 2.5R ⊙ (Saez et al., 2005); Bottom left: Electron density at height r = 2.0R ⊙ reconstructed from STEREO COR-1, overlaid with magnetic field contours from NSO/GONG (Kramar et al., 2009); Right: Qualitative density maps at r = 4.0 R⊙ reconstructed from LASCO C-2 data (Morgan et al., 2009) (top), (Morgan and Habbal, 2010) (bottom).

While streamers were mostly considered as static objects in tomographic reconstruction, dynamic phenomena were also tracked with stereoscopic triangulation, such as streamer blobs that gradually expand outward (Sheeley Jr et al., 2009), and become swept up and compressed by the fast solar wind from low-latitude coronal holes (Sheeley Jr and Rouillard, 2010), or slow streamer-blowout CMEs that are considered as a natural consequence of the corona’s adjustment to the long term evolutionary driving of the photospheric magnetic field (Lynch et al., 2010). The thermodynamics and kinematics of coronal streamers was also simulated with resistive MHD models and PFSS magnetic models, which allowed the authors to test steady-state flows versus polytropic solutions (Airapetian et al., 2011).

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