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Space Science Reviews

, Volume 210, Issue 1–4, pp 183–226 | Cite as

The Physics and Diagnostic Potential of Ultraviolet Spectropolarimetry

  • Javier Trujillo Bueno
  • Egidio Landi Degl’Innocenti
  • Luca Belluzzi
Article

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.

Keywords

Spectropolarimetry Sun: chromosphere Sun: transition region Sun: corona 

Notes

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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Copyright information

© Springer Science+Business Media Dordrecht 2017

Authors and Affiliations

  • Javier Trujillo Bueno
    • 1
  • Egidio Landi Degl’Innocenti
    • 2
  • Luca Belluzzi
    • 3
    • 4
  1. 1.Instituto de Astrofísica de CanariasLa Laguna, TenerifeSpain
  2. 2.Dipartimento di Fisica e AstronomiaUniversità di FirenzeFirenzeItaly
  3. 3.Istituto Ricerche Solari LocarnoLocarno MontiSwitzerland
  4. 4.Kiepenheuer-Institut für SonnenphysikFreiburgGermany

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