We aim at developing novel plasma diagnostic tools for deciphering the complex magnetic activity of our nearest star, from the smallest to the largest spatial scales. To this end we focus on the following subjects:
- Scattering polarization and the Hanle effect
- The Zeeman and Paschen-Back effects in atomic and molecular lines
Here you can find some of the computer codes developed to solve these complex problems.
Scattering polarization of the Ca II IR triplet for probing the quiet solar chromosphere
The chromosphere of the quiet Sun is a very important stellar atmospheric region whose thermal and magnetic structure we need to decipher in order to unlock new discoveries in solar and stellar physics. To this end, we need to identify and exploit observables sensitive to weak magnetic ﬁelds (B <= 100 G) and to the presence of cool and hot gas in the bulk of the solar chromosphere. Here, we report on an investigation of the Hanle effect in two semi-empirical models of the quiet solar atmosphere with different chromospheric thermal structures. Our study reveals that the linear polarization proﬁles produced by scattering in the Ca II IR triplet have thermal and magnetic sensitivities potentially of great diagnostic value. The linear polarization in the 8498 Å line shows a strong sensitivity to inclined magnetic ﬁelds with strengths between 0.001 and 10 G, while the emergent linear polarization in the 8542 Å and 8662 Å lines is mainly sensitive to magnetic ﬁelds with strengths between 0.001 and 0.1 G. The reason for this is that the scattering polarization of the 8542 Å and 8662 Å lines, unlike the 8498 Å line, is controlled mainly by the Hanle effect in their (metastable) lower levels. Therefore, in regions with magnetic strengths noticeably larger than 1 G, their Stokes Q and U proﬁles are sensitive only to the orientation of the magnetic ﬁeld vector. We also ﬁnd that for given magnetic ﬁeld conﬁgurations the sign of the Q/I and U/I proﬁles of the 8542 Å and 8662 Å lines is the same in both atmospheric models, while the sign of the linear polarization proﬁle of the 8498 Å line turns out to be very sensitive to the thermal structure of the lower chromosphere. We suggest that spectropolarimetric observations providing information on the relative scattering polarization amplitudes of the Ca II IR triplet will be very useful to improve our empirical understanding of the thermal and magnetic structure of the quiet chromosphere.
Hanle diagrams showing the Q/I and U/I signal at the line center of the Ca II infrared triplet lines for several LOS and magnetic ﬁeld conﬁgurations in the FAL-C atmospheric model. The observation is always close to the limb (μ = 0.1). Thick lines correspond to constant strength magnetic ﬁelds of 1.8 × 10−3 ,5.6 × 10−3 , 1.8 × 10−2 , and 18 G when observed with 0◦ < χ − χB < 180◦ (solid line) and 180◦ < χ − χB < 360◦ (dotted line), where χ − χB is the azimuth between the LOS and the magnetic ﬁeld. (These two tracks do not coincide in the θB = 90◦ case because we are not in the 90◦ scattering or tangential observation limit, but at μ = 0.1.) The thin solid lines correspond to χ − χB = 0◦ , 30◦ , 60◦ , 90◦ , 120◦ , 150◦ , and 180◦ . Upper panels correspond to a magnetic ﬁeld inclined θB = 90◦ with respect to the vertical (i.e., horizontal magnetic ﬁeld), central and, lower panels correspond to inclinations of θB = 60◦ and θB = 30◦ , respectively.
On the sensitivity of partial redistribution scattering polarization profiles to various atmospheric parameters
This paper presents a detailed study of the scattering polarization proﬁles formed under partial frequency redistribution (PRD) in two thermal models of the solar atmosphere. Particular attention is given to understanding the inﬂuence of several atmospheric parameters on the emergent fractional linear polarization proﬁles. The shapes of these Q/I proﬁles are interpreted in terms of the anisotropy of the radiation ﬁeld, which in turn depends on the source function gradient that sets the angular variation of the speciﬁc intensity. We deﬁne a suitable frequency integrated anisotropy factor for PRD that can be directly related to the emergent linear polarization. We show that complete frequency redistribution is a good approximation to model weak resonance lines. We also show that the emergent linear polarization proﬁles can be very sensitive to the thermal structure of the solar atmosphere and, in particular, to spatial variations of the damping parameter.
Sensitivity of the PRD solutions in the VALC model to the line strength parameter r. The different line types are the following. Solid: r = 10−6 ; dotted: r = 10−5 ; dashed: r = 10−4 ; dot-dashed: r = 10−3 ; dash-triple-dotted: r = 10−2 ; long-dashed lines: r = 0.7. Other model parameters are λ = 5000 Å, eps= 10−4 , a = 10−3 , and ΓE /ΓR = D(2) /ΓR = 0. The top solid line in the SI panel is the Planck function B for the temperature stratiﬁcation of the VALC model. The vertical lines in the same panel show the height at which τ0μ = 1 for an LOS with μ = 0.11. The inset in the −Q/I panel shows the line core region in more detail. The symbol Icont denotes the intensity at very large distance from line center.