Emission-line spectra of two active regions on the solar limb - 1175-1940 A

Abstract

Emission-line spectra of two active regions on the solar limb are discussed. The spectra cover the wavelength range from 1175 to 1940 A, and were recorded by the NRL spectrograph on Skylab. The spectral resolution is 0.06 A and the spatial resolution is 2'' x 60''. This paper is a sequel to our earlier papers on the limb spectra of a quiet Sun and a coronal-hole region. The intensities and profiles of selected chromospheric and transition-zone lines are given at positions ranging from --12'' inside the limb to +12'' outside the limb for the active regions. Similar data are presented for the coronal forbidden line of Fe XII at 1242 A. The emission from this line arises partly from active-region loops and partly from the background corona. Density-sensitive line ratios of lines of Si III, C III, O IV, and O V are used to infer electron densities in the active regions. The densities in these regions range from approx.2 x 10/sup 10/cm/sup -3/ to approx.3 x 10/sup 11/cm/sup -3/. The volumes of the emitting regions are derived from the densities and volume emission measures of allowed lines. The volumes can be quite small, approx.4 x 10/sup 23/cm/sup 3/. Themore » continuum emission between 1300 A and 1900 A is also discussed and compared with the north polar coronal-hole continuum emission. (The coronal-hole and quiet-Sun continua are the same.) For example, at 1500 A and 0'' (at the limb) the continuum for one of the active regions is about 5 times larger than the coronal-hole continuum. This is due to the free-bound Si I edge at approx.1526 A (/sup 3/P/sub 2/ limit). Outside the limb at 2'' the free-bound emission is still strong. Evidently the free-bound continuum is formed high in the chromosphere.« less