Hanle effect from a dipolar magnetic structure: the case of the solar corona and the case of a star

Abstract

Context. The context is the magnetic field measurement in external solar or stellar layers by interpreting line polarization measurements and the Hanle effect. Aims. The aim is to model the Hanle effect depolarization by integrating upon a star on the one hand, and by integrating along a line-of-sight through the solar corona on the other hand. Methods. The formalism of the atomic density matrix is recalled. Particular attention was devoted to the four axis rotations necessary to transform the magnetic field reference frame into that of the line-of-sight. Conclusions. In the stellar case, we confirm that the effect integrated on a star leads to a non-vanishing magnetic depolarization due to the high non-linearity of the Hanle effect. In the solar case, we find that the Hanle sensitivity of hydrogen Lyβ and Lyγ could be better adapted to the measurement of the coronal background magnetic field. They form a pair of lines of different and complementary sensitivity, which makes it possible to determine the full vector. Lyα would be instead adapted to the coronal loop magnetic field measurement, because this field is stronger and suited to the Lyα Hanle sensitivity.

Ultraviolet Spectropolarimetry With Polstar: Hot Star Magnetospheres

“…Applications to stars other than the Sun have been little addressed. López Ariste et al ( 2011); Ignace et al (2011);Bommier (2012); Manso Sainz & Martínez González (2012) have explored the influence of the Hanle effect in spectropolarimetry of the photospheric lines of unresolved stellar atmospheres. In particular, Manso Sainz & Martínez González (2012) calculated the total line polarization in Stokes Q and U for stars with either dipole or quadrupole magnetic fields.…”

Study of the Effect of Active Regions on the Scattering Polarization in the Solar Corona

“…Derouich et al (2010) studied the formation of the polarization of the Lα line and in their model the photoexcitation was considered to be inhomogeneous. Bommier (2012) revisited the calculations of the scattering polarization in the coronal Lα line and found results different from Derouich et al (2010). Bommier (2012) explained these differences by considering that the results of Derouich et al (2010) do not have the required symmetry.…”

“…However, this does not seem to be a proper explanation. The real explanation lies in the fact that the models used by Bommier (2012) and Derouich et al (2010) are quite different and consequently the results are different. In Bommier (2012), the incident radiation field is assumed to be CS about the local vertical axis.…”

“…The real explanation lies in the fact that the models used by Bommier (2012) and Derouich et al (2010) are quite different and consequently the results are different. In Bommier (2012), the incident radiation field is assumed to be CS about the local vertical axis. On the contrary, Derouich et al (2010) modeled the formation of the coronal Lα line by using inhomogeneous photoexcitation, i.e., non-CS incident radiation.…”

Faraday Rotation Distributions From Stellar Magnetism in Wind-Blown Bubbles

“…The effect is sensitive to magnetic fields in the 1-100 G range (depending on the Einstein A-value of the lines being measured) and has been successfully used in studies of solar magnetism for decades (e.g., Sahal-Brechot, Bommier, & Leroy 1977;Stenflo 1982;Berdyugina & Fluri 2004;Trujillo Bueno et al 2005). There is a small but growing literature on its potential application to other stars (Ignace, Nordsieck, & Cassinelli 1997;Lopez Ariste, Asensio Ramos, Gonzalez Fernandez 2011;Ignace et al 2011;Bommier 2012;Manso Sainz & Martinez Gonzalez 2012).…”