Complete determination of the magnetic field vector and of the electron density in 14 prominences from linear polarization measurements in the HeI D3 and H-alpha lines

Details Complete determination of the magnetic field vector and of the electron density in 14 prominences from linear polarization measurements in the HeI D3 and H-alpha lines Complete determination of the magnetic field vector and of the electron density in 14 prominences from linear polarization measurements in the HeI D3 and H-alpha lines

Oct 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994soph..154..231b&link_type=abstract

Solar Physics (ISSN 0038-0938), vol. 154, no. 2, p. 231-260

Mathematics

103

Electron Density (Concentration), Line Spectra, Linear Polarization, Solar Magnetic Field, Solar Prominences, Solar Spectra, Vectors (Mathematics), Astronomical Polarimetry, Coronagraphs, Deuterium, Helium, Polarimeters

Scientific paper

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 D3 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 D3, 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 D3 line is insensitive.

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