Jean-Louis Leroy

The magnetic field in the prominences of the polar crown

The Hanle effect method has been applied to the determination of the magnetic field in 120 prominences of the polar crown observed during the 1974–1980 period, which is the ascending phase of cycle XXI. The average field strength which was about 6 G at the beginning of the cycle reached twice this value just before the maximum. There is also a clear trend for a increase of the prominence field with the altitude. We confirm the fact that the magnetic vector makes a small angle (25 °) with the long axis of the prominence. As to the field orientation, we show that the most striking feature lies in the regular pattern of the component which is parallel to the axis of the filament; its direction seems to depend closely on the polarities of the high latitude photospheric field.

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

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 andHα 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.

Emissions ‘froides’ dans la couronne solaire

Prominences have been photographed through a coronagraph and an Hα Lyot filter with long exposure times. Faint Hα emissions are often detected down to the threshold 2 × 10-6 times of the Suns brightness; they show definite structures but their relations to the low-level ordinary prominences are not very clear. Estimates are given for the density and thickness of such cool regions.

Simultaneous measurements of the polarization in Hα and D3 prominence emissions

Simultaneous observations of the polarization of radiations Hα and D3 have been performed in 1979 at Pic du Midi on 60 solar prominences (300 pairs of measurements). For the essential part of the sample, the degree of polarization p(D3) does not depend strongly upon the intensity E(D3); on the contrary p(Hα) decreases steeply when E(Hα) goes beyond the brightness which corresponds to τ(Hα) = 1 (Figure 4); the deviation of the polarization direction ϑ(Hα) does not show such a strong dependance upon E(Hα) (Figure 6). A crude estimate of the possible role of multiple scattering in an optically thick prominence is in general agreement with observational data but a detailed comparison will be possible only when a complete theory of the Hanle effect in non optically thin layers will be available.Therefore, Hα polarization measurements are presently useful mainly when τ(Hα) < 1 and, in connexion with D3 measurements (Figure 8), they should make possible to determine the three components of the prominence magnetic vector. Prospects are given for the observation of other lines (Hβ; λ 10 830 Å) which do not suffer heavy self-absorption effects and would be well-suited to this type of study.

Un programme de photographie en lumire monochromatique des missions coronales pour la priode 1973?1976

A Cooperative international program is suggested for monochromatic coronal photography by means of interference filters.