Magneto-optical Effects in the Scattering Polarization Wings of the Ca i 4227 Å Resonance Line

Abstract

The linear polarization pattern produced by scattering processes in the Ca i 4227Å resonance line is a valuable observable for probing the solar atmosphere. Via the Hanle effect, the very significant Q/I and U/I line-center signals are sensitive to the presence of magnetic fields in the lower chromosphere with strengths between 5 and 125 G, approximately. On the other hand, partial frequency redistribution (PRD) produces sizable signals in the wings of the Q/I profile, which have always been thought to be insensitive to the presence of magnetic fields. Interestingly, novel observations of this line revealed a surprising behavior: fully unexpected signals in the wings of the U/I profile and spatial variability in the wings of both Q/I and U/I. Here we show that the magneto-optical (MO) terms of the Stokes-vector transfer equation produce sizable signals in the wings of U/I and a clear sensitivity of the Q/I and U/I wings to the presence of photospheric magnetic fields with strengths similar to those that produce the Hanle effect in the line core. This radiative transfer investigation on the joint action of scattering processes and the Hanle and Zeeman effects in the Ca i 4227Å line should facilitate the development of more reliable techniques for exploring the magnetism of stellar atmospheres. To this end, we can now exploit the circular polarization produced by the Zeeman effect, the magnetic sensitivity caused by the above-mentioned MO effects in the Q/I and U/I wings, and the Hanle effect in the line core. 6 The Hanle critical field for the onset of the Hanle effect is given by Bc = 1.137 · 10 −7 Γ R /gu, where Γ R is the radiative line broadening parameter and gu is the upper level's Landé factor.

Modeling Mg ii h, k and Triplet Lines at Solar Flare Ribbons

“…Magnetic fields: The magnetic field can affect the Mg II energy levels through Zeeman effect. Previous studies have shown that the Mg II h&k lines are sensitive to Zeeman effect even for weak fields (e.g., Alsina Ballester et al 2016). By manually introducing a magnetic field with a strength up to 1000 Gauss in the RH code, we find that the Mg II Zeeman broadening in the Stokes I profiles is negligible.…”

Magnetic Sensitivity in the Wing Scattering Polarization Signals of the Hydrogen Lyman-α Line of the Solar Disk Radiation

“…At spectral distances from the line center that are much greater than both the Doppler width of the line and the magnetic splitting of the energy levels, the line emissivity is insensitive to both the Hanle and Zeeman effects, provided that the collisional broadening is significantly smaller than the natural width of the line (see LL04; also Appendix B of Alsina Ballester et al 2018). For illustrative purposes, throughout this work we will focus on a wing wavelength around which the linear polarization maximizes.…”

“…In turn, such large U/I wing signals allow the ρ V U term to introduce a magnetic sensitivity in the Q/I wing signals. This mechanism causes both a rotation of the plane of linear polarization as the radiation travels through the solar atmosphere (see Alsina Ballester et al 2017) and an effective decrease of the degree of total linear polarization (see Alsina Ballester et al 2018). Recent RT investigations have indicated that such MO effects should play an important role in the wings of many strong chromospheric lines, such as the Mg ii k line (Alsina Ballester et al 2016), the Mg ii h & k lines (del Pino Alemán et al 2016), the Sr ii 407.8 nm line (Alsina Ballester et al 2017), and the Ca i 422.7 nm line (Alsina Ballester et al 2018).…”

Rayleigh Scattering in Spectral Series with L-term Interference

“…Recently, Alsina Ballester, Belluzzi, & Trujillo Bueno (2016) and del Pino Alemán, Casini, & Manso Sainz (2016; see also Manso Sainz, del Pino Alemán, & Casini 2017) have shown how PRD effects can combine with magneto-optical effects in an optically thick plasma to give rise to a significant Q U "rotation" of the scattering polarization in the near and far wings of deep resonance lines, even for magnetic strengths as small as a few gauss. Similar effects can be expected to manifest also for the model atom discussed in this paper.…”

“…The results of this experiment confirmed important predictions from theoretical modeling of the Hanle effect in this line, but also opened new questions about the structure and magnetic topology of the upper solar chromosphere (Kano et al 2017). Motivated by the CLASP success, and by ongoing efforts in the modeling of the Mg II h-k doublet in the solar transition region spectrum at 280 nm (Belluzzi & Trujillo Bueno 2012;Alsina Ballester, Belluzzi, & Trujillo Bueno 2016;del Pino Alemán, Casini, & Manso Sainz 2016), the CLASP-2 mission (Narukage et al 2016) was proposed in order to measure the polarization produced by the joint action of scattering processes and the Hanle and Zeeman effects in these lines.…”