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Ionization of Hydrogen in the Solar Atmosphere

  • Chae, Jongchul (Astronomy Program, Department of Physics and Astronomy, Seoul National University)
  • Received : 2021.02.03
  • Accepted : 2021.03.29
  • Published : 2021.06.15

Abstract

The ionization degree of hydrogen is crucial in the physics of the plasma in the solar chromosphere. It specifically limits the range of plasma temperatures that can be determined from the Hα line. Given that the chromosphere greatly deviates from the local thermodynamic equilibrium (LTE) condition, precise determinations of hydrogen ionization require the solving of the full set of non-LTE radiative transfer equations throughout the atmosphere, which is usually a formidable task. In many cases, it is still necessary to obtain a quick estimate of hydrogen ionization without having to solve for the non-LTE radiative transfer. Here, we present a simple method to meet this need. We adopt the assumption that the photoionizing radiation field changes little over time, even if physical conditions change locally. With this assumption, the photoionization rate can be obtained from a published atmosphere model and can be used to determine the degree of hydrogen ionization when the temperature and electron density are specified. The application of our method indicates that in the chromospheric environment, plasma features contain more than 10% neutral hydrogen at temperatures lower than 17,000 K but less than 1% neutral hydrogen at temperatures higher than 23,000 K, implying that the hydrogen temperature determined from the Hα line is physically plausible if it is lower than 20,000 K, but may not be real, if it is higher than 25,000 K. We conclude that our method can be readily exploited to obtain a quick estimate of hydrogen ionization in plasma features in the solar chromosphere.

Keywords

Acknowledgement

We sincerely appreciate the referees' constructive comments, which greatly helped to improve this paper. This research was supported by the National Research Foundation of Korea (NRF-2020R1A2C2004616).

References

  1. Ahn K, Chae J, Cho KS, Song D, Yang H, et al., Active region coronal rain event observed by the fast imaging solar spectrograph on the NST. Sol. Phys. 289, 4117-4136 (2014). https://doi.org/10.1007/s11207-014-0559-x
  2. Carlsson M, Stein RF, Non-LTE radiating acoustic shocks and CA II K2V bright points. Astrophys. J. 397, L59 (1992). https://doi.org/10.1086/186544
  3. Carlsson M, Stein RF, Dynamic hydrogen ionization. Astrophys. J. 572, 626-635 (2002). https://doi.org/10.1086/340293
  4. Chae J, Park HM, Ahn K, Yang H, Park YD, et al., Fast imaging solar spectrograph of the 1.6 meter new solar telescope at big bear solar observatory. Sol. Phys. 288, 1-22 (2013). https://doi.org/10.1007/s11207-012-0147-x
  5. Chae J, Madjarska MS, Kwak H, Cho K, Inference of chromospheric plasma parameters on the Sun. Multilayer spectral inversion of strong absorption lines. Astron. Astrophys. 640, A45 (2020). https://doi.org/10.1051/0004-6361/202038141
  6. Cox AN, Allen's Astrophysical Quantities, 4th ed. (AIP Press, Springer, New York, NY, 2000).
  7. Fontenla JM, Avrett EH, Loeser R, Energy balance in the solar transition region. III. Helium emission in hydrostatic, constant-abundance models with diffusion. Astrophys. J. 406, 319 (1993). https://doi.org/10.1086/172443
  8. Kwak H, Impulsive wave excitation in the solar atmosphere, PhD Dissertation, Seoul National University (2021).
  9. Park H, Chae J, Song D, Maurya RA, Yang H, et al., Temperature of solar prominences obtained with the fast imaging solar spectrograph on the 1.6 m new solar telescope at the big bear solar observatory. Sol. Phys. 288, 105-116 (2013). https://doi.org/10.1007/s11207-013-0271-2
  10. Pereira TMD, Uitenbroek H, RH 1.5D: a massively parallel code for multi-level radiative transfer with partial frequency redistribution and Zeeman polarisation. Astron Astrophys 574, A3 (2015). https://doi.org/10.1051/0004-6361/201424785
  11. Uitenbroek H, Multilevel radiative transfer with partial frequency redistribution. Astrophys. J. 557, 389-398 (2001). https://doi.org/10.1086/321659
  12. Vernazza JE, Avrett EH, Loeser R, Structure of the solar chromosphere. III. models of the EUV brightness components of the quiet sun. Astrophys. J. Suppl. Ser. 45, 635-725 (1981). https://doi.org/10.1086/190731
  13. Yang H, Chae J, Lim EK, Park H, Cho K, et al., Velocities and temperatures of an Ellerman bomb and its associated features. Sol. Phys. 288, 39-53 (2013). https://doi.org/10.1007/s11207-013-0354-0