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The Electron Pressure Differential between the Sunspot Plume and the Surrounding Region

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 $\approx$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 $\log (N_{\mbox{\scriptsize e}}/{\rm cm}^{-3}) \approx 9.9$ compared to $\log (N_{\mbox{\scriptsize e}}/{\rm cm}^{-3}) \approx 10.20$-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.

Figure: A TRACE 1600Å image of the sunspot and surrounding region (taken at 21:14 UT on 18 March 1999) with the position of the SUMER slit and TRACE white light contours over-plotted (left panel) outlining the umbra and penumbra. The middle panel shows SiIV 1402.77Å (at 20:13 UT) plus the 10arcsec regions centred around 40arcsec along the slit known as QS₁, while QS₂ is the region centred around 100arcsec. SS₁ and SS₂ are centred around 70arcsec and 80arcsec respectively along the slit. The right panel shows the three electron density diagnostic line ratios from within the OV 760Å multiplet, i.e. R₁ = 761.13/758.68, R₂ = 761.13/759.43 and R₃ = 761.13/761.99, plotted assuming the temperature of formation is $\log (T_{\rm e}/K) = 5.4$. The derived densities in all four regions are indicated.