On the Role of the Solar Corona and Transition Region in the Excitation of the Spectrum of Neutral Helium

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Abstract
We investigate the formation of the spectrum of neutral helium in the solar atmosphere by solving the corresponding non-LTE problem in an extended grid of model atmospheres. From the results we infer several general properties and scaling laws that can be used to discriminate the different proposed formation mechanisms. Some of the scaling laws have also been tested in a comparison with previous calculations made with different assumptions. In our models, the excitation of the spectrum by direct photoionization by EUV radiation shortward of 504 Å followed by recombinations (PR mechanism), seems to be capable of influencing significantly the resonance continuum and the subordinate lines, even in the presence of other (collisional) excitation mechanisms. While this influence is almost unavoidable in most atmospheric features, it is hardly justifiable as the only possible contribution. Moreover, the resonance lines seem inclined to respond much more effectively to the conditions of the lower transition region, even in the presence of a significant coronal EUV illumination. With the help of the detailed non-LTE calculations and of the derived scaling laws, we explore the interplay of the possible formation mechanisms and their effect on the individual spectral features.