Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002a%26a...394..707k&link_type=abstract
Astronomy and Astrophysics, v.394, p.707-715 (2002)
Astronomy and Astrophysics
Astrophysics
10
Sun: Atmosphere, Sun: Magnetic Fields, Atomic Data, Atomic Processes, Line: Formation, Polarization
Scientific paper
In recent years, Landi Degl'Innocenti (\cite{Landi-98}, \cite{Landi-99}) has proposed a model of the polarization spectrum of the Na I D lines observed near the solar limb, based on lower level polarization effects. By so doing, he obtains a remarkable agreement between his model and the observations, so that he comes to a conclusion about a paradox, because the existence of lower level polarization is incompatible with a magnetic field strength higher than 10 mGauss (except vertical), and with possible depolarizing collision effects. In the present paper, we investigate the depolarizing collision effects (collisions with neutral hydrogen) by using the collisional rates computed with ab-initio and quantum chemistry methods by Kerkeni (\cite{Kerkeni-01}; see also Kerkeni et al. \cite{Kerkeni-etal-00}, and Kerkeni \cite{Kerkeni-02}). We solve the statistical equilibrium equations for the atomic density matrix, taking into account these depolarizing collisions. We investigate the effect of a weak magnetic field (Hanle effect). Our results indicate that the lower levels should be completely depolarized by the collisions at the depth where the Na I D lines are formed. Furthermore, large values of the lower level alignment such as those introduced by Landi Degl'Innocenti in his model to get a good theoretical fit of the observations, seem to us unlikely, as our computations confirm. Thus, as the agreement between the model by Landi Degl'Innocenti and the observations is however very convincing, the paradox is confirmed, reinforced and increased by our results.
Bommier Veronique
Kerkeni B.
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