Solar Spectral Lines with Special Polarization Properties for the Calibration of Instrument Polarization Wenxian Li, Roberto Casini, T. del Pino Alemán and P. G. Judge High Altitude Observatory National Center for Atmospheric Research NSO #30, Aug. 8, 2017

Motivations Spectral Lines Polarization Modeling Classification of spectral lines with special polarization properties Conclusion

Motivations: The measurement of solar magnetic fields is critical to our understanding of most unsolved problems of solar physics Spectro-polarimetry is the most powerful tool for the accurate determination of solar magnetic fields the precise polarization calibration of the instruments

Motivations: Close calibration pairs Sanchez Almeida, J, & Vela Villahoz, E. 1993, A&A,280,688 Judge, P.G. 2017, Atomic Physics and modern Solar spectr-polarimetry, CaJPh, (in press) Close calibration pairs

Spectral Lines “weak-field” approximation J2, g2 J1, g1 --------circular polarization --------linear polarization

Spectral Lines LS-coupling scheme:

Polarization Modeling Spectral Lines Sources: LS-coupling, Non-LTE, Zeeman effect realistic Landé g-factors Zeeman effect Hanle effect Atomic polarization Effect of dichroism

Polarization Modeling HANLE.J code: solves the statistical equilibrium for a multi-level atom illuminated by anisotropic radiation limited by the assumption of complete frequency redistribution of the anisotropic incident radiation Slab model: optical thickness τ = 1.0 located at the surface of the photosphere(h = 0) LOS is inclined at 𝝁=0.1 from the local vertical magnetic field B = 100 G, θB = 45◦ ,φB = 0◦ local plasma temperature T= 4500K

spectral lines with special polarization properties “small”: |S/Imax|≲5e-4 lines with zero polarization; lines with small polarization; lines with zero linear polarization; lines with small linear polarization; lines with small circular polarization; lines with zero Stokes U and V. the difference between the Landé g-factors of the transition levels with respect to their LS-coupling values the possibility of those levels to harbor atomic polarization.

Fe I 5F1-5F1 at 7389.398 Å Landé g-factor

Ti II 4P1/2 – 4D1/2 at 4314.975 Å Zeeman effect

Scattering polarization, Zeeman effect and Hanle effect Cr I 5D0-5F1 at 4339.710 Å Scattering polarization, Zeeman effect and Hanle effect

Close calibration pairs

Conclusion We have investigated the polarization of atomic transitions that could be used for the calibration of instrumental polarization. Slab model + realistic Landé g-factors + Zeeman effect + Hanle effect + Atomic polarization + Effect of dichroism Taking account multi-level, collisions and optical thick atmosphere, the effects of atomic polarization are expected to be even smaller. Offering practical solutions to the difficult problem of instrumental polarization calibration

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