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The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry

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Abstract

The empirical investigation of the magnetic field in the outer solar atmosphere is a very important challenge in astrophysics. To this end, we need to identify, measure and interpret observable quantities sensitive to the magnetism of the upper chromosphere, transition region and corona. This paper provides an overview of the physics and diagnostic potential of spectropolarimetry in permitted spectral lines of the ultraviolet solar spectrum, such as the Mg ii \(h\) and \(k\) lines around 2800 Å, the hydrogen Lyman-\(\alpha\) line at 1216 Å, and the Lyman-\(\alpha\) line of He ii at 304 Å. The outer solar atmosphere is an optically pumped vapor and the linear polarization of such spectral lines is dominated by the atomic level polarization produced by the absorption and scattering of anisotropic radiation. Its modification by the action of the Hanle and Zeeman effects in the inhomogeneous and dynamic solar atmosphere needs to be carefully understood because it encodes the magnetic field information. The circular polarization induced by the Zeeman effect in some ultraviolet lines (e.g., Mg ii \(h\) & \(k\)) is also of diagnostic interest, especially for probing the outer solar atmosphere in plages and more active regions. The few (pioneering) observational attempts carried out so far to measure the ultraviolet spectral line polarization produced by optically pumped atoms in the upper chromosphere, transition region and corona are also discussed. We emphasize that ultraviolet spectropolarimetry is a key gateway to the outer atmosphere of the Sun and of other stars.

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Notes

  1. Quantum mechanics is responsible for introducing in this equation an additional factor \(g\), the level’s Landé factor, so that the general equation reads \(\Delta\varPhi\simeq{ {e_{0} B g \, t _{\mathrm{life}}} / {2 m c} }\).

  2. Note that in this reference frame \(\bar{J}^{0}_{0}\) and \(\bar{J}^{2}_{0}\) are not the only non-vanishing components of the radiation field tensor, even if the incident field is characterized by cylindrical symmetry along the local vertical.

  3. The assumption of isotropic collisions is generally well justified.

  4. This is not the case for optically thick media. When modeling solar disk observations, for instance, interference between different FS levels are fundamental for explaining the polarization profiles in the wings of the components of the multiplet, even when the separation between the levels is much larger than their natural width (see Sect. 4).

  5. Note that summing over \(K\) the scattering phase matrix defined in Eq. (49) one recovers the matrix appearing in the right-hand side of Eq. (9) (multiplied by \(3/4\)).

  6. Note that our choice for the reference direction for linear polarization is the parallel to the nearest solar limb.

  7. Recall that we are always choosing the parallel to the nearest solar limb as the reference direction for linear polarization.

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Acknowledgements

This is the last paper that our friend Egidio Landi Degl’Innocenti wrote with us before he unexpectedly passed away on 12 February 2017. We are deeply grateful to Egidio for the extraordinary scientific legacy that he has left to us and to future generations of scientists. Financial support by the Spanish Ministry of Economy and Competitiveness through projects AYA2014-60476-P and AYA2014-55078-P is gratefully acknowledged, as well as the computing grants provided by the Barcelona Supercomputing Center (National Supercomputing Center, Barcelona, Spain). L.B. gratefully acknowledges the Swiss National Science Foundation through grant 200021-163405.

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Authors and Affiliations

  1. Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain

    Javier Trujillo Bueno

  2. Dipartimento di Fisica e Astronomia, Università di Firenze, 50125, Firenze, Italy

    Egidio Landi Degl’Innocenti

  3. Istituto Ricerche Solari Locarno, 6605, Locarno Monti, Switzerland

    Luca Belluzzi

  4. Kiepenheuer-Institut für Sonnenphysik, 79104, Freiburg, Germany

    Luca Belluzzi

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  1. Javier Trujillo Bueno
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Trujillo Bueno, J., Landi Degl’Innocenti, E. & Belluzzi, L. The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry. Space Sci Rev 210, 183–226 (2017). https://doi.org/10.1007/s11214-016-0306-8

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