Vincenzo Andretta

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

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 A 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.

Using Strong Solar Coronal Emission Lines as Coronal Flux Proxies

We investigate the possibility that strong EUV lines observed with the Goddard Solar EUV Rocket Telescope and Spectrograph (SERTS) provide good proxies for estimating the total coronal flux over shorter wavelength ranges. We use coordinated SERTS and Yohkoh observations to obtain both polynomial and power-law fits relating the broad-band soft X-ray fluxes to the intensities of Fexvi 335 Ú and 361 Ú, Fexv 284 Ú and 417 Ú, and Mgix 368 Ú measured with SERTS. We found that the power-law fits best cover the full range of solar conditions from quiet Sun through active region, though not surprisingly the ‘cooler’ Mgix 368 Ú line proves to be a poor proxy. The quadratic polynomial fits yield fair agreement over a large range for all but the Mgix line. However, the linear fits fail conspicuously when extrapolated into the quiet-Sun regime. The implications of this work for the Heii 304 Ú line formation problem are also briefly considered.