Helium line formation in a Solar Active Region

MAUAS, P. J. D.; ANDRETTA, V.; FALCHI, A.; FALCIANI, R.; TERIACA, L.; CAUZZI, G.

Hamburgo, Alemania

Workshop; 13th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun; 2004

European Space Agency

An observing campaign coordinated between groundbased and SOHO instruments has been planned to obtainsimultaneous spectroheliograms of the same area in severalspectral lines. The chromospheric lines Ca ii K, Hα andNa i D, as well as the He i 10830, 5876, 584 and He ii 304˚Alines, have been observed. These observations allow usto build semi-empirical models of the chromosphere andlow transition region of an active region, taking into accountthe photoionizing photons impinging on it. We obtaineda model that matches very well all the observedline profiles, using the standard value of the He abundance([He]=0.1). We studied the influence of the coronalradiation on the computed helium lines and found that,even in an active region, the incident coronal radiationhas a limited effect on the UV He lines, while it results offundamental importance for the D3 and 10830 lines. Finally,we built two more models assuming values of the Heabundance [He]=0.07 and 1.5, only in the region where thetemperatures are larger than 10000 K and found that theobservations can still be well reproduced in both cases.This implies that, to choose between different values of[He], it is necessary to constrain the transition region withdifferent observables, independent of the He lines.