Astron. Astrophys. 340, L15-L17 (1998)

4. Results and discussion

We found the wavelengths of the two lines by best-fit Gaussians to the data of Fig. 1. These were found to be 1218.35 Å for the forbidden line, and 1213.95 Å for the intercombination line, in good agreement with 1218.35 Å and 1213.90 Å from Sandlin, Brueckner & Tousey (1977). Our line separation of 4.50.1 Å is in excellent agreement with the theoretical prediction of 4.540.01 Å from Edlén (1983), and the observed value of 4.62 0.12 Å from McKenna et al. (1997). The full-width-at-half-maxima were found to be 0.290.03 Å and 0.250.03 Å respectively, which are the same to with the errors, as one would expect for two lines from the same ion. A linear sloping continuum was subtracted off the spectra to calculate line fluxes, and the resulting line ratio , corresponding to an electron density .

Fig. 3. Four EIT images of the observed region, as well as the forbidden O V image. The bottom left-hand corner of each image is at the point 318 arcseconds east of sun centre, 154 arcseconds north of sun centre, and each image is 45 by 300 arcseconds in extent.

By summing over the forbidden O V line, and subtracting off the Lyman- wing, an image of the Solar surface in forbidden O V was produced. Fig. 3 shows a smoothed version of this image plotted along side images of the same region taken by the Extreme Ultraviolet Imaging telescope (EIT) on SOHO, at approximately the same time. The images are in order of decreasing formation temperature, and the main features of the EIT images are present in the forbidden O V image. The O V bright points are about 20 arcseconds across. It can also be seen that the contrast between the intensity of the network (the bright points) and the intensity of the cell (in between the bright points) is high, similar to the He II image (which is a chromospheric line), but markedly different from the other images which include higher temperature coronal lines. Both these features are consistent with emission from the chromosphere/transition region, and the bright point dimension agrees well with that observed by Gallagher et al. (1998), who presented intensity cuts across Coronal Diagnostic Spectrograph O V line features.

The detection of these O V lines in the quiet Sun has prompted us to search for the line in the HST spectra of stellar sources. As the intensity of the line increases significantly at low densities, a low activity star like Procyon is a good candidate for this study. In Fig. 4 we present the HST spectrum of Procyon in the Lyman wavelength region. The intercombination line is evident whereas the forbidden line is not detected. The upper limit to the line flux would imply a and hence . This value is consistent with the chromospheric and coronal models of Procyon constructed by Brown & Jordan (1981), which imply at a temperature of 2 10⁵ K.

Fig. 4. The HST spectrum of Procyon in the O V wavelength region.

In summary, the high spectral and spatial resolution provided by SUMER has allowed us to unambiguously detect for the first time the O V 1213.9 Å forbidden line for the quiet Sun. The ratio of this line to the O V 1218.35 Å intercombination line gives an electron density of about 3cm^-3.

© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998
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