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Title: Detailed and simplified nonequilibrium helium ionization in the solar atmosphere

Helium ionization plays an important role in the energy balance of the upper chromosphere and transition region. Helium spectral lines are also often used as diagnostics of these regions. We carry out one-dimensional radiation-hydrodynamics simulations of the solar atmosphere and find that the helium ionization is set mostly by photoionization and direct collisional ionization, counteracted by radiative recombination cascades. By introducing an additional recombination rate mimicking the recombination cascades, we construct a simplified three-level helium model atom consisting of only the ground states. This model atom is suitable for modeling nonequilibrium helium ionization in three-dimensional numerical models. We perform a brief investigation of the formation of the He I 10830 and He II 304 spectral lines. Both lines show nonequilibrium features that are not recovered with statistical equilibrium models, and caution should therefore be exercised when such models are used as a basis for interpretating observations.
Authors:
;  [1] ;  [2]
  1. Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo (Norway)
  2. Now at Institutet för Solfysik, Stockholms Universitet (Sweden)
Publication Date:
OSTI Identifier:
22351514
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 784; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CHROMOSPHERE; ENERGY BALANCE; EQUILIBRIUM; GROUND STATES; HELIUM; HYDRODYNAMICS; ONE-DIMENSIONAL CALCULATIONS; PHOTOIONIZATION; RADIANT HEAT TRANSFER; RECOMBINATION; SIMULATION; SUN; THREE-DIMENSIONAL CALCULATIONS