A qualitative interpretation of the second solar spectrum of Ce ll

¹ High Altitude Observatory, National Center for Atmospheric Research, PO Box 3000, Boulder CO 80307-3000, USA e-mail: rms@ucar.edu
² Dipartimento di Astronomia e Scienza dello Spazio, Università degli Studi di Firenze, Largo Enrico Fermi 2, 50125 Firenze, Italy
³ Instituto de Astrofísica de Canarias, Av. Vía Láctea s/n, 38205, La Laguna, Tenerife, Spain
⁴ Consejo Superior de Investigaciones Científicas, Spain

Received: 2 August 2005
Accepted: 24 October 2005

Abstract

This is a theoretical investigation on the formation of the linearly polarized line spectrum of ionized cerium in the sun. We calculate the scattering line polarization pattern emergent from a plane-parallel layer of Ce ii atoms illuminated from below by the photospheric radiation field, taking into account the differential pumping induced in the various magnetic sublevels by the anisotropic radiation field. We find that the line polarization pattern calculated with this simple model is in good qualitative agreement with reported observations. Interestingly, the agreement improves when some amount of atomic level depolarization is considered. We find that the best fit to the observations corresponds to the situation where the ground and metastable levels are depolarized to about one fifth of the corresponding value obtained in the absence of any depolarizing mechanism. One possibility to have this situation is that the depolarizing rate value of elastic collisions is exactly , which is rather unlikely. Therefore, we interpret that fact as due to the presence of a turbulent magnetic field in the limit of saturated Hanle effect for the lower-levels. For this turbulent magnetic field we obtain a lower limit of 0.8 Gauss and an upper limit of 200–300 Gauss.