The Hanle effect of the coronal Lyman-alpha line of hydrogen: Theoretical investigation

International audienceThis paper is devoted to a computation of the effect of a magnetic field on the linear polarization of the coronal Lα line of hydrogen. Recent works (Gabriel et al., 1971) have shown that the linear polarization of this line is due to resonant scattering of the incident chromospheric Lα line. The Hanle effect is the modification of this linear polarization, due to the magnetic field. After having briefly recalled the main features of this effect and the conditions of the coronal Lα line formation, we present the theoretical formalism to be used for Hanle effect computations. The effect of the hyperfine structure of the line is included. Then the results of our computations are given in terms of linear polarization as a function of the magnetic field. We get that the effect of the hyperfine structure on these results is negligible, although this is not evident a priori. When the hyperfine structure is neglected, the line structure is simplified and the Hanle effect can be expressed with analytical formulae, which we give in the last part of this paper. After integration along the line of sight, these formulae could be used for magnetic field determination in the solar corona from measurements of the linear polarization of the Lα line