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: http://star.arm.ac.uk/annrep/annrep2003/node49.html
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It was shown in the early 1980's that spectral lines formed around 1-2.5 x 105K were enhanced in radiance by factors up to 15-25 over the normal quiet Sun. From the dynamical viewpoint, this would suggest a pressure imbalance between the sunspot and the surrounding region. In another collaborative project, Maria Madjarska and Gerry Doyle have thrown some light on this topic.
Here, the electron density was evaluated over a bright sunspot plume region using lines within the OV 760Å multiplet. The plume showed an intensity enhancement factor of 9 in the OV lines compared to regions outside the sunspot umbra. Internal agreement between the various ratios is excellent, which would suggest that the OV lines do not suffer from blending problems. The derived mean electron densities for the sunspot plume is compared to -10.45 in the surrounding area. The derived gas pressure in the plume compared to that outside lends weight to the suggestion that it is plasma flowing from outside the spot into the umbra at transition region temperatures that is the main cause of the down-flows. The plume non-thermal velocities are 5 to 10km s-1 smaller than those measured in regions external to the spot, suggesting significantly less turbulence within the umbra.