4.4 Plasma emission

Although simple models of the coronal plasma emission may be derived by populating PFSS magnetic field lines with hydrostatic atmospheres (Jardine et al., 2002b), self-consistent models of plasma density and temperature require full-MHD models (Section 3.4). Indeed, a key emphasis of global MHD models has been direct comparison of the steady state solutions with either white-light (Rušin et al., 2010Jump To The Next Citation Point) or multi-spectral EUV and X-Ray observations (Lionello et al., 2009Jump To The Next Citation Point; Downs et al., 2010Jump To The Next Citation Point). Early global MHD models using a simplified polytropic energy equation were able to produce a good representation of the Suns large-scale magnetic field (Mikić et al., 1999), however they were unable to reproduce realistic emission profiles in EUV and X-rays. This was attributed to the fact that they did not produce a sufficiently high density and temperature contrast between open and closed field regions. To improve the models, a more realistic energy equation has been incorporated, including the effects of thermal conduction, radiative losses, and coronal heating, along with modeling the upper chromosphere and transition region (Lionello et al., 2001, 2009Jump To The Next Citation Point; Downs et al., 2010Jump To The Next Citation Point).

Figure 18View Image shows a comparison of the global MHD model of Lionello et al. (2009Jump To The Next Citation Point) with observations for CR 1913 (August – September 1996). The left hand column shows three EUV pass bands (171, 195, 284 Å) along with a Yohkoh/SXT X-ray image. The other three columns show synthetic emission profiles constructed from density and temperature distributions in the global MHD simulation. Each column uses exactly the same MHD model: the only difference is the form of coronal heating applied (Hch in Equation (47View Equation)). While each solution produces roughly the same magnetic structure, the most important factor in reproducing the emissions is the coronal heating. The second column, which uses an exponential form falling off with height, gives the worst comparison. When spatially varying heating is applied (either from Schrijver et al., 2004 or the composite form of Lionello et al., 2009), the simulation captures many of the features of the observed emission. Similar results were also found in the paper of Downs et al. (2010). In an alternative comparison, Rušin et al. (2010Jump To The Next Citation Point) compared the output from the global MHD model shown in Figure 18View Image with white light eclipse observations from 1 August 2008. To compare the model and observations, the authors simulated the white light emission of the corona by integrating the electron density along the line-of-sight. The results of the comparison can be seen in Figure 5 of Rušin et al. (2010) where the model successfully reproduced the overall shape of the corona along with the shape and location of three helmet streamers.


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