The polarimetry group at the Arcetri Observatory has been working for several years, mainly from the theoretical-interpretative side, on the polarization of the radiation emitted by objects of astrophysical interest.

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

E. Landi Degl'Innocenti "The physics of polarization" in "Astrophysical Spectropolarimetry", J. Trujillo Bueno, F. Moreno Insertis, F. Sanchez (eds.), Cambridge University Press, 1.

E. Landi Degl'Innocenti "Magnetic field diagnostic techniques based on the Zeeman and Hanle effects" in "Magnetic fields across the Hertzsprung-Russell diagram", G. Mathys, S.K. Solanki, D.T. Wickramasinghe (eds.), ASP Conference Series 248, 73.

M. Landolfi, S. Bagnulo, M. Landi Degl'Innocenti, E. Landi Degl'Innocenti "The Paschen-Back effect on fine and hyperfine structure: impact on polarized spectra of Ap and Bp stars" in "Magnetic fields across the Hertzsprung-Russell diagram", G. Mathys, S.K. Solanki, D.T. Wickramasinghe (eds.), ASP Conference Series 248, 349.

Publications 2002

J. Trujillo Bueno, E. Landi Degl'Innocenti, M. Collados, L. Merenda, R. Manso Sainz "Selective absorption processes as the origin of puzzling spectral line polarization from the Sun" Nature 415, 403.

J. Trujillo Bueno, R. Casini, M. Landolfi, E. Landi Degl'Innocenti "The physical origin of the scattering polarization of the NaI D-lines in the presence of weak magnetic fields" Ap. J. Lett. 566, L53.

R. Casini, E. Landi Degl'Innocenti, M. Landolfi, J. Trujillo Bueno "On the atomic polarization of the ground level of NaI" Ap. J. 573, 864.

S. Bagnulo, M. Landi Degl'Innocenti, M. Landolfi, G. Mathys "A statistical analysis of the magnetic structure of CP stars" A&A 394, 1023.

E. Landi Degl'Innocenti "Diagnostic of prominence magnetic fields through spectropolarimetric observations" in "THEMIS and the new frontiers of solar atmosphere dynamics", F. Berrilli et al. (eds.), Il Nuovo Cimento B, in press.

R. Manso Sainz, E. Landi Degl'Innocenti "A qualitative interpretation of the second solar spectrum of TiI" A&A 394, 1093.

E. Landi Degl'Innocenti "The Zeeman effect: applications to solar physics" Astron. Nachr., in press.

E. Landi Degl'Innocenti "Old and new applications of the Zeeman effect to the diagnostics of solar magnetic fields" in "Magnetism and activity of the Sun and the stars", J. Arnaud (ed.), Publications de l'Observatoire Midi-Pyrenees, in press.

E. Landi Degl'Innocenti "Resonance polarization in molecular lines" in "Solar polarization 3", J. Trujillo Bueno and J. Sanchez Almeida (eds.), ASP Conference Series, in press.

E. Landi Degl'Innocenti "Theory of polarization: what's next?" in "Solar polarization 3", J. Trujillo Bueno and J. Sanchez Almeida (eds.), ASP Conference Series, in press.

R. Manso Sainz, E. Landi Degl'Innocenti "Understanding the second solar spectrum of TiI" in "Solar polarization 3", J. Trujillo Bueno and J. Sanchez Almeida (eds.), ASP Conference Series, in press.

M. Landolfi, E. Landi Degl'Innocenti "Case studies in resonance polarization and the Hanle effect" in "Solar polarization 3", J. Trujillo Bueno and J. Sanchez Almeida (eds.), ASP Conference Series, in press.

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Last Updated: 11 January, 2003