COMPLETE DETERMINATION OF THE MAGNETIC-FIELD VECTOR AND OF THE ELECTRON-DENSITY IN 14 PROMINENCES FROM LINEAR POLARIZATON MEASUREMENTS IN THE HE-I-D-3 AND H-ALPHA LINES

The present paper is devoted to the interpretation of linear polarization data obtained in 14 quiescent prominences with the Pic-du-Midi coronagraph-polarimeter by J. L. Leroy, in the two lines He I D-3 and H alpha quasi-simultaneously. The linear polarization of the lines is due to scattering of the anisotropic photospheric radiation, modified by the Hanle effect due to the local magnetic field. The interpretation of the polarization data in the two lines is able to provide the 3 components of the magnetic field vector, and one extra parameter, namely the electron density, because the linear polarization of H alpha is also sensitive to the depolarizing effect of collisions with the electrons and protons of the medium. Moreover, by using two lines with different optical thicknesses, namely He I D-3, which is optically thin, and H alpha, which is optically thick (tau = 1), it is possible to solve the fundamental ambiguity, each line providing two field vector solutions that are symmetrical in direction with respect to the line of sight in the case of the optically thin line, and which have a different symmetry in the case of the optically thick line. It is then possible to determine without ambiguity the polarity of the prominence magnetic field with respect to that of the photospheric field: 12 prominences are found to be Inverse polarity prominences, whereas 2 prominences are found to be Normal polarity prominences. It must be noticed that in 12 of the 14 cases, the line-of-sight component of the magnetic field vector has a Normal polarity (to the extent that the notion of polarity of a vector component is meaningful; no polarity can be derived in the 2 remaining cases); this may explain the controversy between the results obtained with methods based on the Hanle effect with results obtained through the Zeeman effect. A dip of the magnetic field lines across the prominence has been assumed, to which the optically thick H alpha line is sensitive, and the optically thin He I D-3 line is insensitive. For the Inverse prominences, the average field strength is 7.5+/-1.2 G, the average angle, alpha, between the field vector and the prominence long axis is 36 degrees+/-15 degrees, the average angle, zeta, between the outgoing field lines and the solar surface at the prominence boundary is 29 degrees+/-20 degrees, and the average electron density is 2.1 x 10(10)+/-0.7 x 10(10) cm(-3). For the Normal prominences, the average field strength is 13.2+/-2.0 G, the average angle, alpha, between the field vector and the prominence long axis is 53 degrees+/-15 degrees, the average angle, zeta, between the outgoing field lines and the solar surface at the prominence boundary is 0 degrees+/-20 degrees (horizontal field), and the average electron density is 8.7 x 10(9)+/-3.0 x 10(9) cm(-3). The two Normal prominences of our sample are both found in the vicinity of an active region; the 12 Inverse prominences of our sample are all found to be located in the weak field regions, not in the vicinity of an active region; 10 of them are Polar Crown prominences. All the wide objects (high and broad, with visible apparent structures) are found in the Inverse prominences class; the two Normal prominences of our sample are lower and narrower and seem to be more compact.