The emission-line spectrum of a sunspot in the far-ultraviolet

The emission-line spectrum between 1200 and 1817 A from the sunspot in McMath region 12510 near Sun center is discussed. The spectrum was obtained by the NRL normal-incidence spectrograph on Skylab. The spectral resolution is 0.06 A and the projected slit area on the Sun was 2'' x 60''. The principal results are: (1) The line widths of emission lines originating in the chromosphere and lower transition region over the sunspot are much narrower than those previously reported for a polar coronal hole observed above the limb and a quiet Sun chromospheric network observed near Sun center. This indicates that the mass motions in the sunspot are less than in these other regions. Typical mass motions in the spot are approx.13 km s-1 for an electron temperature of approx.105 K. (2) The sunspot spectrum, aside from the narrow line widths of emission lines, is similar to the spectra from the chromospheric network boundary. The intensities of lines in the sunspot are much enhanced relative to the network interior. From the full-width at half-maximum (FWHM) of the 1207 A Si III line, an optical depth at line center 3.6 is deduced. An electron density of approx.2.7 x 1010 cm-3 and a characteristic lenght of approx.330 km are estimated for the Si III region over the spot.Comparison with Parker's theory of sunspots shows that, if the enhancement of emission lines is due to enhanced transport of hydromagnetic waves generated in the sunspot convective zone, the mode of the hydromagnetic waves is predominately Alfvenic