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Astron. Astrophys. 346, L69-L72 (1999)

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1. Introduction

Emission line ratios of ions in the sodium isoelectronic sequence have been recognised as powerful electron temperature ([FORMULA]) diagnostics for the solar atmosphere since the work of Flower & Nussbaumer (1975). This is because the difference in excitation energies of the lines needs to be large compared to the thermal energy of the exciting electrons. Unfortunately, this condition often means that the wavelength separation of two such diagnostic lines is large, which can lead to problems with instrumental calibration. Some ions however, produce these [FORMULA]-sensitive emission lines relatively close together in wavelength. Si IV is one such species, and has ultraviolet emission lines grouped around [FORMULA]800, 1100 and 1400 Å. These emission lines, as well as being [FORMULA]-sensitive, are insensitive to the electron density ([FORMULA]), since the upper levels of the line transitions are not metastable. The theoretical temperature of maximum ionization fraction ([FORMULA]) for Si IV is close to [FORMULA] K, with Arnaud & Rothenflug (1985) and Mazzotta et al. (1998) predictions of [FORMULA]=[FORMULA] and [FORMULA] K respectively. An observational benefit for Si IV is that this predicted formation temperature is close to the temperature of minimum emission measure in the solar atmosphere (Doschek et al. 1997). These considerations make Si IV emission lines potentially very useful [FORMULA]-diagnostics.

Until recently, studies of Si IV line pairs have been confined to lines at wavelengths greater than 912 Å. This has been largely due to difficulties with the aluminium-coated mirrors used in instruments such as those on Skylab and the OSO IV satellite. The reflectance of these mirrors decreased rapidly below 912 Å which resulted in poor instrumental calibration, precluding accurate intensity measurements. Keenan et al. (1986) reported a large discrepancy between theory and low resolution ([FORMULA] Å) OSO IV observations of the I(1128 Å)/I(1394 Å) ratio, and suggested a blend in the 1128 Å line due to ions of low ionization potential. Keenan & Doyle (1988) however pointed out that it was more likely that there was an error in the 1394 Å line flux, as it was near the edge of the instrumental spectral coverage, and hence its intensity may not have been well determined. They examined spectra of slightly higher resolution ([FORMULA]1.6 Å) obtained using the Harvard S055 EUV spectrometer on board Skylab , and concluded that the 1128 Å line was relatively unblended (at least at the solar limb), whereas the 1122 Å line was probably blended with a low ionization potential Fe III transition, as previously suggested by Feldman & Doschek (1977).

The situation changed with the launch of the Solar and Heliospheric Observatory (SOHO) on December 2, 1995. The Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on board SOHO is designed to obtain observations of the solar chromosphere, transition region and corona, and is ideal for measuring detailed spectroscopic line ratios between [FORMULA]800 and 1600 Å in first order. SUMER has an angular resolution of [FORMULA]1.0 arcsecond and a resolving power of [FORMULA]/[FORMULA] = 17700-38300 at 800-1610 Å in first order (see Wilhelm et al. 1995for a detailed description). Doschek et al. (1997) recently examined SUMER spectra containing some Si IV emission lines. They compared the I(1128 Å)/I(1403 Å) line ratio with the theoretical results of Keenan, Dufton & Kingston (1986) for the quiet Sun cell centre and network. They considered the Si IV 1394 Å line too strong to be placed on the most sensitive part of the detector, and ignored the Si IV 1122 Å line, since it had previously been identified as badly blended by other instruments. The Si IV lines around 800 Å were not considered.

In this paper, we present SUMER observations of a quiet solar region covering six Si IV emission lines around [FORMULA]800, 1100 and 1400 Å, and compare eight [FORMULA]-sensitive diagnostic line ratios and three line ratios insensitive to changes in [FORMULA] and [FORMULA] with more recent level population calculations, and discuss line blending. We also investigate the hypothesis of Schmahl & Orral (1979), who suggest that Lyman continuum absorption may significantly decrease the intensities of solar emission lines at wavelengths below 912 Å.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999