Astron. Astrophys. 340, L15-L17 (1998) 3. Theoretical line ratiosThe O V model ion used in this work consists of the twelve energetically lowest LS terms, making a total of 20 fine-structure levels, and was first presented in Ryans et al. (1998). Relative level populations, line intensities and resulting line ratios were calculated for electron temperatures and densities in the range of and , appropriate for the solar transition region and corona. We used the energies from Moore (1980), electron impact excitation rates of Kato, Lang & Berrington (1990), Einstein A-coefficients from Hibbert (1980), as well as the proton excitation rates given in Ryans et al. (1998), and the equilibrium code of Dufton (1977). In Fig. 2 we plot the log of the theoretical O V forbidden to intercombination line ratio , as a function of electron density for three electron temperatures; 5.2, 5.4 and 5.6. It can be seen from the figure that R is very sensitive to . As the radiative decay rate of the 2s2p ^{3}P_{1}, ^{3}P_{2} levels (or upper levels of the forbidden and intercombination lines) are very small, collisional de-excitation becomes an important depopulation mechanism over an appropriate range (, where A is the spontaneous radiative de-excitation rate, C is the electron collisional rate per unit and m represents the metastable state). We can also see that R is temperature insensitive. This is because the upper energy levels of the lines are very close together, and R essentially becomes a function of the effective collision strengths of the levels, which are related to the collisional excitation rate coefficients, and are only slowly varying functions of (see Keenan 1992; Mason 1997; for a detailed discussion).
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