M. Landolfi

Magneto-optical effects and the determination of vector magnetic fields from stokes profiles

The analysis procedure proposed by Auer et al. (1977) for deducing magnetic field vectors from Stokes profiles has been tested to investigate the influence of magneto-optical effects on the deduced field parameters. The quality of the fit between synthetic profiles generated with the inclusion of magneto-optical effects and the profiles returned by the inversion routine is also investigated. The results show that magneto-optical effects should be included in the inversion routine especially to increase the accuracy of the deduced, azimuth of the magnetic field.

UNNOFIT inversion of spectro-polarimetric maps observed with THEMIS

This paper presents the inversion of a spectro-polarimetric scan (240 arcsec×340 arcsec) of an active region and a filament, obtained with THEMIS, on December 7, 2003, in the two Fe I lines at 6302.5 A and 6301.5 A. The inversion was applied to each line separately, using the UNNOFIT code of tet{b13 La84}, improved by the introduction of a magnetic filling factor parameter. The magnetic and non-magnetic theoretical atmospheres, mixed in the proportion given by the filling factor, are derived from the same set of parameters, except for the presence (or absence) of a magnetic field. The fundamental ambiguity is not solved. The tests run with UNNOFIT show that the magnetic field strength, B, and the magnetic filling factor, α, cannot be separately recovered by the inversion of Fe I 6302.5 A, but that their product, α B, which is the local average magnetic field, is recovered. The magnetic flux is only its longitudinal component. Our results also reveal two distinct regimes, corresponding to two different ranges of local average magnetic field strength as inferred from Fe I 6302.5 A: a) The network, having a field inclined of about 20°-30° from the vertical in Fe I 6302.5 A (more spreaded, but non-horizontal in Fe I 6301.5 A), with a homogeneous azimuth. In this zone, the local average field strength in Fe I 6302.5 A is larger than 45 G. b) The internetwork, where the field is turbulent (with a horizontal trend, more spreaded at lower altitudes), and with local average field strength in Fe I 6302.5 A smaller than 45 G. The two lines gives coherent results, in particular in magnetic field azimuth. From this we conclude that the turbulence of the internetwork field is of solar origin. This work has been presented in detail by tet{b13 Bo06}.

A statistical analysis of the magnetic structure of CP stars

We present the results of a statistical study of the magnetic structure of upper main sequence chemically peculiar stars. We have modelled a sample of 34 stars, assuming that the magnetic morphology is described by the superposition of a dipole and a quadrupole field, arbitrarily oriented. In order to interpret the modelling results, we have introduced a novel set of angles that provides one with a convenient way to represent the mutual orientation of the quadrupolar component, the dipolar component, and the rotation axis. Some of our results are similar to what has already been found in previous studies, e.g., that the inclination of the dipole axis to the rotation axis is usually large for short-period stars and small for long-period ones - see Landstreet & Mathys (2000). We also found that for short-period stars (approximately P< 10 days) the plane containing the two unit vectors that characterise the quadrupole is almost coincident with the plane containing the stellar rotation axis and the dipole axis. Long-period stars seem to be preferentially characterised by a quadrupole orientation such that the planes just mentioned are perpendicular. There is also some loose indication of a continuous transition between the two classes of stars with increasing rotational period.

On the diagnostic of magnetic fields in sunspots through the interpretation of stokes parameters profiles

The inversion routine proposed by Aueret al. (1977), for the determination of vector magnetic fields from Stokes profiles, has been generalized to include magneto-optical and damping effects. Synthetic profiles have then been generated from a sunspot model atmosphere accounting for the depth variation of the relevant physical parameters such as the magnetic field amplitude, inclination angle, etc...., each variation being considered one at a time. Alfvén waves and magnetic inhomogeneities over the field of view have also been considered. These synthetic profiles have been presented to the inversion routine. The results of the fits show that the magnetic field amplitude and direction are always recovered with good accuracy when these quantities are constant in the model atmosphere, and, in those cases where te magnetic field vector is supposed to vary monotonically with optical depth, the values recovered are always intermediate between the values corresponding to the top and bottom of the atmosphere. Moreover, we found that the differences between synthetic and best-fit profiles are able to characterize, in many cases, the particular physical situation considered.

Net circular polarization in magnetic spectral lines produced by velocity gradients: Some analytical results

The net circular polarization in a spectral line due to the combined effect of magnetic fields and velocity gradients is analyzed for a few schematic situations. In some particular cases, its dependence on the magnetic field, velocity field and line parameters can be expressed analytically.

FREQUENCY REDISTRIBUTION FUNCTION FOR THE POLARIZED TWO-TERM ATOM*

We present a generalized frequency redistribution function for the polarized two-term atom in an arbitrary magnetic field. This result is derived within a new formulation of the quantum problem of coherent scattering of polarized radiation by atoms in the collisionless regime. The general theory, which is based on a diagrammatic treatment of the atom-photon interaction, is still a work in progress. However, the results anticipated here are relevant enough for the study of the magnetism of the solar chromosphere and of interest for astrophysics in general.

Polarization properties of a ‘Zeiss-type’ coelostat: The case of the solar tower in Arcetri

A theoretical model of the polarization properties of a ‘Zeiss-type’ coelostat is presented and discussed in detail. The Muller matrix describing the modification of the Stokes vector of the incident radiation as a result of the multiple reflections on the coelostat mirrors is derived as a function of the solar coordinates, the geometrical configuration of the coelostat, and the parameters defining the optical properties of the mirrors. These parameters, or more particularly, the index of refraction n and the extinction coefficient k, have been evaluated by means of laboratory measurements performed on a series of specimens having characteristics similar to those of the coelostat mirrors. The geometry of the coelostat configuration is described in full detail. The theoretical model has been then particularized to the case of the Donati Solar Tower in Arcetri, and some experimental measurements have been performed to check the correctness of the model. These measurements show the basic adequacy of the mathematical model, although some offset terms are found in the Stokes parameters U and V.

Thermodynamical properties of unresolved magnetic flux tubes: I: A diagnostic method based on circular polarization ratios in line pairs

We propose a diagnostic method, based on the observation of circular polarization signals in line pairs, to derive the thermodynamical properties of unresolved magnetic elements in the solar atmosphere. The concept of response function for the ratio of circular polarization signals in two lines is introduced and its main properties are analyzed. Some detailed calculations for suitably selected line pairs are presented.

Asymmetries in stokes profiles of magnetic lines: A linear analysis in terms of velocity gradients

AbstractA linear analysis of the asymmetries in Stokes profiles of magnetic lines is performed. The asymmetries in the linear and circular polarization profiles are characterized by suitable quantities,