Identification of [Fe III] in the solar ultraviolet spectrum
Solar ultraviolet spectra have been obtained with high spectral and spatial resolution using the Naval Research Laboratory's high-resolution telescope and spectrograph1 (HRTS) flown on rockets since 1975 and most recently on the Spacelab 2 Shuttle flight. Because the solar spectrum between ∼1,170 and 1,719 Å has been well observed for some years, few lines of substantial intensity remain unidentified. (See for example the recent compilation by Sandlin et al.2). The longest exposures during the first rocket flight of the HRTS, which obtained spectra with a spatial resolution of ∼ 1 arc s along the slit, of length ∼ 1 solar radius, and a spectral resolution of ∼0.05 Å, showed a number of weak emission lines at the solar limb, in particular between 1,570 and 1,600 Å (see Plates 16 and 17 in ref. 3), that could not readily be identified. Improved observations of these lines were obtained during the flight of HRTS on the Spacelab 2 Shuttle flight in July–August 1985, by making longer exposures (60,100 and 250 s). The scope of the data obtained has been described4. We have now identified the emission lines concerned as forbidden (electric quadrupole and magnetic dipole) transitions in Fe III, the first detection of these particular transitions in any source. They must now be considered potential candidates for previously unidentified lines in other low-density (Ne ≲ 1010 cm−3) astrophysical sources.
Emission lines of H2 in the extreme-ultraviolet solar spectrum
We report the identification of emission lines of H/sub 2/ in the solar EUV spectrum. The observations were made during the first rocket flight of a new high-resolution telescope combined with a new stigmatic EUV spectrograph. The present paper concerns mainly the sunspot spectrum which shows approx.200 lines not previously reported in solar spectra. About 65 of the new lines have been identified as due to transitions in the Lyman bands of H/sub 2/. The identification of 29 of these has already been reported by the present authors.The strongest set of transitions is excited by fluorescence from H L..cap alpha... Photons emitted in the red wing of H L..cap alpha.. down toward the photosphere are absorbed by molecular hydrogen. The strong excitation takes place from the third vibrational state (..nu..''=2) of the ground state /sup 1/..sigma../sup +//sub g/ to the second vibrational states (..nu..'=1) of the excited state /sup 1/..sigma../sup +//sub u/. The observed lines then result from spontaneous emission over the progression ..nu..'=1..--> nu..''=2--9. Thus the strong lines represent only one line in the P and R branches of each band, giving rise to a characteristic fluorescence spectrum.Analysis of the line intensities now shows that other fluorescence routes aremore » also effective, and give rise to moderately strong progressions in the 0--..nu..'' bands (0<..nu..''<7. Other weaker fluorescence also occurs.At present the limiting factor in the interpretation of the line intensities is the lack of knowledge of the line profile of H L..cap alpha.. where it reaches the H/sub 2/ molecules. However, three contributing factors to the enhancement of H/sub 2/ over the sunspot are identified. The absolute and relative intensities are discussed as far as is possible at present. The conditions under which polarization of the lines might be observed are discussed briefly. Finally, the implications of the H/sub 2/ observations for other areas of astrophysics are mentioned.« less
CO fluorescence in the extreme-ultraviolet solar spectrum
Emission lines in the fourth positive system of CO have been identified in the extreme-ultraviolet solar spectrum 1540-1660 A. These lines are excited by the C IV transition-zone lines at 1548 and 1551 A. They are strong in the spectrum of a sunspot and in parts of the adjacent active region. Some of them appear as weak, broad emission features in the quiet sun.