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Solar Physics

, Volume 154, Issue 2, pp 231–260 | Cite as

Complete determination of the magnetic field vector and of the electron density in 14 prominences from linear polarizaton measurements in the HeI D3 and Hα lines

  • Véronique Bommier
  • Egidio Landi Degl'Innocenti
  • Jean-Louis Leroy
  • Sylvie Sahal-Bréchot
Article

Abstract

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 Hei D3 and 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 Hα 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 Hei D3, which is optically thin, and Hα, which is optically thick (τ = 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 Hα line is sensitive, and the optically thin Hei D3 line is insensitive.

For the Inverse prominences, the average field strength is 7.5±1.2 G, the average angle,α, between the field vector and the prominence long axis is 36° ± 15°, the average angle, ζ, between the outgoing field lines and the solar surface at the prominence boundary is 29° ± 20°, and the average electron density is 2.1 × 1010 ± 0.7 × 1010 cm−3. For the Normal prominences, the average field strength is 13.2±2.0 G, the average angle,α, between the field vector and the prominence long axis is 53° ± 15°, the average angle, ζ, between the outgoing field lines and the solar surface at the prominence boundary is 0° ± 20° (horizontal field), and the average electron density is 8.7 × 109 ± 3.0 × 109 cm−3.

Keywords

Field Vector Linear Polarization Polarization Data Solar Surface Normal Polarity 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Kluwer Academic Publishers 1994

Authors and Affiliations

  • Véronique Bommier
    • 1
  • Egidio Landi Degl'Innocenti
    • 2
  • Jean-Louis Leroy
    • 3
  • Sylvie Sahal-Bréchot
    • 4
  1. 1.Laboratoire ‘Astrophysique, Atomes et Molécules’, CNRS URA 812 - DAMApObservatoire de Paris, Section de MeudonMeudon CedexFrance
  2. 2.Dipartimento di Astronomia e Scienza dello SpazioUniversità di FirenzeFirenzeItalia
  3. 3.Observatoire Midi-PyrénéesToulouseFrance
  4. 4.Laboratoire ‘Astrophysique, Atomes et Molécules’, CNRS URA 812 - DAMApObservatoire de Paris, Section de MeudonMeudon CedexFrance

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