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Polarized Forbidden Coronal Line Emission in the Presence of Active Regions

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Abstract

Photoexcited forbidden lines at visible and infrared wavelengths provide important diagnostics for the coronal magnetic field via scattering induced polarization and the Zeeman effect. In forward models, the polarized formation of these lines is often treated assuming a simplified exciting radiation field consisting only of the photospheric quiet-Sun continuum, which is both cylindrically-symmetric relative to the solar vertical and unpolarized. In particular, this assumption breaks down near active regions, especially due to the presence of sunspots and other surface features that modify the strength and anisotropy of the continuum radiation field. Here we investigate the role of symmetry-breaking on the emergent polarized emission in high resolution models of the active corona simulated with the MURaM code. We treat the full 3D unpolarized continuum radiation field of the photosphere that excites the coronal ions and compare the cases where the symmetry-breaking effects of the photospheric features are included or ignored. Our discussion focuses on the key observables soon to be available by the National Science Foundation’s Daniel K Inouye Solar Telescope. The results indicate that while symmetry breaking can in principle have a large effect, its role is relatively minor for the simulated active region, largely due to the low inherent polarization fraction emitted by forbidden lines in denser active region plasmas.

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Notes

  1. pyCELP refers to the python package for Coronal Emission Line Polarization, which is an updated version of the code introduced in Paper I. This code is publicly available at https://github.com/tschad/pycelp. See the Appendix for more details.

  2. See Paper I for further discussion of the influence of the number of included levels on the calculated atomic level polarization.

  3. We point out that there is a scale error in the top left panel of Figure 10 in Paper I which shows the magnetic field amplitude at the \(\langle\tau_{5000} \rangle\sim1\) plane. Those values need to be multiplied by a factor of \(\sqrt{4\pi}\).

  4. We used the version of the Wittman opacity package ported to Python by Jaime de la Cruz Rodriguez and available at https://github.com/jaimedelacruz/witt/.

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Acknowledgements

The National Solar Observatory (NSO) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation. The authors extend our thanks to Matthias Rempel for providing and helping use the MURaM simulation and to Serena Criscuoli for providing the RH generated opacities. Thanks also to Jaime de la Cruz Rodriguez for making Python versions of the Wittmann opacity routines publicly available. Chianti is a collaborative project involving George Mason University, the University of Michigan (USA), University of Cambridge (UK) and NASA Goddard Space Flight Center (USA). This research has made use of NASA’s Astrophysics Data System.

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  1. National Solar Observatory, 22 Ohi’a Ku Street, Makawao, HI, 96768, USA

    Thomas Schad

  2. High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO, 80307-3000, USA

    Gabriel Dima

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Appendix: pyCELP: Software Updates and Improvements

Appendix: pyCELP: Software Updates and Improvements

Schad and Dima (2020) introduced a code referred to as pyCLE, which is capable of multilevel atomic density calculations for the purpose of calculating the atomic level polarization of forbidden emission lines in the no-coherence hypothesis. This code has since been reformulated and updated from a Fortran code wrapped in Python to a Python-only code. The new code is called pyCELP for the python package for Coronal Emission Line Polarization. It provides an extensible class for model ion calculations and takes advantage of the Numba package for just-in-time compilation (Lam, Pitrou, and Seibert, 2015). By pre-computing all static factors in the statistical equilibrium equations, we have significantly accelerated the code. It is now primarily limited in execution speed by the time required to solve the system of linear equations using the libraries available in the Numpy package (Harris et al., 2020). The code is publicly available at https://github.com/tschad/pycelp.

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Schad, T., Dima, G. Polarized Forbidden Coronal Line Emission in the Presence of Active Regions. Sol Phys 296, 166 (2021). https://doi.org/10.1007/s11207-021-01917-y

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