An analysis of the weak permitted line spectra of the Orion Nebula and of the planetary nebulae NGC 7027 and NGC 7662 has been undertaken in order to identify the excitation mechanism for each observed line. With a few exceptions, model calculations show that some combination of recombination, resonance fluorescence by starlight, and resonance fluorescence by other nebular emission lines can successfully account for the observed line strengths. Among the exceptions with no satisfactory explanation of their excitation mechanism are several planetary nebula lines arising from doubly excited levels.
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Steven A. Grandi (1976) studied this question.