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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.
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