Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere
By J. Leenaarts, M. Carlsson, V. Hansteen, R. J. Rutten. 2007
135 citations recorded by the source corpus.
Context: The ionization of hydrogen in the solar chromosphere and transition region does not obey LTE or instantaneous statistical equilibrium because the timescale is long compared with important hydrodynamical timescales, especially of magneto-acoustic shocks. Since the pressure, temperature, and electron density depend sensitively on hydrogen ionization, numerical simulation of the solar atmosphere requires non-equilibrium treatment of all pertinent hydrogen transitions. The same holds for any diagnostic application employing hydrogen lines. Aims: To demonstrate the importance and to quantify the effects of non-equilibrium hydrogen ionization, both on the dynamical structure of the solar atmosphere and on hydrogen line formation, in particular Hα. Methods: We implement an algorithm to compute non-equilibrium hydrogen ionization and its coupling into the MHD equations within an existing radiation MHD code, and perform a two-dimensional simulation of the solar atmosphere from the convection zone to the corona. Results: Analysis of the simulation results and comparison to a companion simulation assuming LTE shows that: a) non-equilibrium computation delivers much smaller variations of the chromospheric hydrogen ionization than for LTE. The ionization is smaller within shocks but subsequently remains high in the cool intershock phases. As a result, the chromospheric temperature variations are much larger than for LTE because in non-equilibrium, hydrogen ionization is a less effective internal energy buffer. The actual shock temperatures are therefore higher and the intershock temperatures lower. b) The chromospheric populations of the hydrogen n = 2 level, which governs the opacity of Hα, are coupled to the ion populations. They are set by the high temperature in shocks and subsequently remain high in the cool intershock phases. c) The temperature structure and the hydrogen level populations differ much between the chromosphere above photospheric magnetic elements and above quiet internetwork. d) The hydrogen n = 2 population and column density are persistently high in dynamic fibrils, suggesting that these obtain their visibility from being optically thick in Hα also at low temperature.
Subfields: Astronomy and Astrophysics
View Paper: https://www.semanticscholar.org/paper/ff9e0bf7caab1c068218e07ae02d5ac1753f6455
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In Astronomy and Astrophysics
- J. Leenaarts
- M. Carlsson
- Bart De Pontieu
- L. R. V. Voort
- V. Hansteen
- B. Gudiksen
- T. Pereira
- H. Uitenbroek
Citations in the corpus
- A publicly available simulation of an enhanced network region of the Sun (2015, methodology): “For the simulation described in this paper we have included non-equilibrium ionization of hydrogen following the description by Leenaarts et al. (2007) based on the approximations by Sollum (1999).”
- THE FORMATION OF THE Hα LINE IN THE SOLAR CHROMOSPHERE (2012, background): “The latter is essential: assuming statistical equilibrium when calculating the atmosphere leads to temperatures that can be more than 2000K off and electron densities that can be even orders of magnitude off (Carlsson & Stein 2002; Leenaarts et al. 2007).”
- Approximations for radiative cooling and heating in the solar chromosphere (2012, methodology): “This simulation was performed with the radiation-magnetohydrodynamics code Bifrost (Gudiksen et al. 2011) and includes the effects of non-equilibrium ionization of hydrogen using the methods described in Leenaarts et al. (2007).”
- THE FORMATION OF IRIS DIAGNOSTICS. II. THE FORMATION OF THE Mg ii h&k LINES IN THE SOLAR ATMOSPHERE (2013, background): “…radiative losses in the upper chromosphere, transition region and corona (Carlsson & Leenaarts 2012), thermal conduction along magnetic field lines (Gudiksen et al. 2011) and an equation of state that includes the effects of non-equilibrium ionization of hydrogen (Leenaarts et al. 2007).”