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Дата изменения: Wed Jan 15 21:17:53 2003 Дата индексирования: Fri Feb 28 01:55:43 2014 Кодировка: Поисковые слова: dra |
Polarization is related to the presence of all kinds of anisotropy: the interaction of atoms with an anisotropic radiation field, with an ensemble of colliding particles having an anisotropic velocity distribution, with a (static) magnetic or electric field, can give rise to polarization in spectral lines. Furthermore, if the radiation passes through an optically thick medium like a stellar atmosphere, such polarization is generally modified by dichroism and anomalous dispersion effects. It follows that polarization contains information on the physical conditions of the medium: this information is mainly - but not solely - related to anisotropies. In particular, the analysis of polarization is the principal tool, and often the only possibility, for measuring the magnetic field vector on the Sun and stars.
Similarly to the "classical" approach to stellar atmospheres (where the radiation is described via the only intensity and the atoms via the only level populations), the study of the polarization observed in spectral lines is based on two equations: the set of the statistical equilibrium equations, describing how the state of atoms is affected by the interaction with the surrounding medium, and the transfer equation describing the modifications undergone by radiation in the propagation process. These equations generalize their "classical" counterparts to take the polarization properties of light into account. Such generalization involves considerable difficulties, both of geometrical nature (because anisotropy itself is a geometrical factor) and of physical nature in a broader sense. The radiation must be described with 4 quantities (e.g., the Stokes parameters) rather than with the only intensity, and the state of atoms with a non-diagonal density matrix rather than with the only level populations.
The theory underlying this topic is presented in the book "Polarization in spectral lines" by E. Landi Degl'Innocenti and M. Landolfi, that will be published in the near future. The book contains the derivation of the fundamental equations from the principles of Quantum Electrodynamics, their adaptation to different situations of astrophysical interest, and several applications to specific problems, with special emphasis on the measurement of solar magnetic fields. The book is largely based on original research work of the Arcetri polarimetry group.
The group actively collaborates with researchers at different Institutions (Observatoire de Meudon, Instituto de Astrofisica de Canarias, High Altitude Observatory, European Southern Observatory).
Publications 2001
Publications 2002
Last Updated: 11 January, 2003