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Session 80 - Solar Corona and Transition Region.
Oral session, Thursday, June 13
Wisconsin Center,

[80.09] Emission Measures and Electron Densities for the Solar Transition Region

G. A. Doschek (NRL)

I analyze high spectral resolution ultraviolet spectra in the wavelength region from 1200 to 2000 Årecorded by the Naval Research Laboratory slit spectrograph (S082-B) on the Skylab space station. The spectra were obtained from a quiet Sun region, a polar coronal hole, and two active regions. The major goal of this work is to determine electron densities and emission measures for transition region lines formed between about 3 x 10^4 K and 2.5 x 10^5 K, using the best available atomic data. The majority of these data were not available during the Skylab era. Electron densities are determined from density sensitive line ratios involving lines of C III, N III, O IV, O V, and Si III. The densities and inferred electron pressures show considerable variation with temperature. The overall shape of the differential emission measure distribution with temperature derived in this analysis is similar to that found from other data sets. However, as found recently by others, there are significant discrepancies in emission measures obtained for ions that should be formed at the same temperature. These discrepancies are not caused by variable element abundances. Apart from inaccuracies in atomic physics, some of the discrepancies in emission measures and electron pressures may be real and reflect the unresolved fine structure of the transition region. No indications of non-Maxwellian distributions were found from the lines analyzed. The temperature of Al III is consistent with the predicted temperature based on ionization equilibrium. These results are directly relevant to analysis of spectra from the SUMER spectrometer on SOHO. This work was supported by a NASA SRamp;T Grant (W-18218) from the Solar Physics Branch of the Space Physics Division.

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