Dynamic Hydrogen Ionization

By M. Carlsson, Robert F. Stein. 2002

225 citations recorded by the source corpus.

We investigate the ionization of hydrogen in a dynamic solar atmosphere. The simulations include a detailed non-LTE treatment of hydrogen, calcium, and helium but lack other important elements. Furthermore, the omission of magnetic fields and the one-dimensional approach make the modeling unrealistic in the upper chromosphere and higher. We discuss these limitations and show that the main results remain valid for any reasonable chromospheric conditions. As in the static case, we find that the ionization of hydrogen in the chromosphere is dominated by collisional excitation in the Lyα transition followed by photoionization by Balmer continuum photons—the Lyman continuum does not play any significant role. In the transition region, collisional ionization from the ground state becomes the primary process. We show that the timescale for ionization/recombination can be estimated from the eigenvalues of a modified rate matrix where the optically thick Lyman transitions that are in detailed balance have been excluded. We find that the timescale for ionization/recombination is dominated by the slow collisional leakage from the ground state to the first excited state. Throughout the chromosphere the timescale is long (103-105 s), except in shocks where the increased temperature and density shorten the timescale for ionization/recombination, especially in the upper chromosphere. Because the relaxation timescale is much longer than dynamic timescales, hydrogen ionization does not have time to reach its equilibrium value and its fluctuations are much smaller than the variation of its statistical equilibrium value appropriate for the instantaneous conditions. Because the ionization and recombination rates increase with increasing temperature and density, ionization in shocks is more rapid than recombination behind them. Therefore, the ionization state tends to represent the higher temperature of the shocks, and the mean electron density is up to a factor of 6 higher than the electron density calculated in statistical equilibrium from the mean atmosphere. The simulations show that a static picture and a dynamic picture of the chromosphere are fundamentally different and that time variations are crucial for our understanding of the chromosphere itself and the spectral features formed there.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/3b04490ed5f93b5b15eeaa7e4ad67d87e6655e64

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Citations in the corpus

THE FORMATION OF IRIS DIAGNOSTICS. II. THE FORMATION OF THE Mg ii h&k LINES IN THE SOLAR ATMOSPHERE (2013, methodology): “Computations without the assumption of statistical equilibrium are currently only possible using 1D simulations (Carlsson & Stein 2002; Rammacher & Ulmschneider 2003; Kašparová et al. 2009), and therefore lack realism in other areas.”
A publicly available simulation of an enhanced network region of the Sun (2015, methodology): “Chromospheric radiative losses are calculated in nonLTE using simplified recipes (Carlsson & Leenaarts 2012) based on detailed 1D full non-LTE radiative transfer simulations using the RADYN code (Carlsson & Stein 1992, 1995, 1997, 2002).”
Waves in the Magnetized Solar Atmosphere. II. Waves from Localized Sources in Magnetic Flux Concentrations (2003, result): “But it must be stated at the outset that the definitive treatment would require that the simulation determines the wave heating of the background atmosphere in a manner consistent with the governing primitive equations (Carlsson & Stein 1992, 1995, 1997, 2002).”
Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence (2007, background): “These relations also are valid for a plasma with a constant ionization state across the shock, and Carlsson & Stein (2002) found that shock trains in the solar chromosphere often approach this nearly steady-state condition.”
An Ab Initio Approach to the Solar Coronal Heating Problem (2004, methodology): “The second was inspired by the work of Carlsson & Stein (see, for instance, Carlsson & Stein 2002), who argue for a highly time-dependent, low mean temperature chromosphere with no average temperature increase.”
THE FORMATION OF THE Hα LINE IN THE SOLAR CHROMOSPHERE (2012, methodology): “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).”
The FIP and Inverse FIP Effects in Solar and Stellar Coronae (2015, background): “This appears increasingly at odds with the modern view of the solar chromosphere as a dynamic environment, continually being perturbed by the passage of shocks (Carlsson and Stein, 2002) or reconnection at chromospheric layers (Isobe et al., 2008). It also conflicts with the argument that in the abs”
Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere (2007, background): “The degree of ionization of hydrogen in the chromosphere does not follow the local temperature, as described already by Carlsson & Stein (2002) and Leenaarts & Wedemeyer-Böhm (2006).”
Models of the Solar Chromosphere and Transition Region from SUMER and HRTS Observations: Formation of the Extreme-Ultraviolet Spectrum of Hydrogen, Carbon, and Oxygen (2008, background): “Kneer (1980), Carlsson & Stein (2002), Leenaarts &WedemeyerBöhm (2006), and Leenaarts et al. (2007) have shown that ionization/recombination timescales in hydrogen can be longer than typical hydrodynamical timescales in the solar chromosphere.”
Approximations for radiative cooling and heating in the solar chromosphere (2012, methodology): “…effort that has been accomplished in one dimensional simulations of wave propagation in the quiet solar atmosphere (Klein et al. 1976, 1978; Carlsson & Stein 1992, 1994, 1995, 1997, 2002; Rammacher & Ulmschneider 2003; Fossum & Carlsson 2005, 2006), sunspots (Bard & Carlsson 2010), solar…”
Emergence of magnetic flux from the convection zone into the corona (2004, unknown): “…simultaneously present, the latter being created by the dissipation of vertically running waves generated mainly through the ongoing convection in the (sub)photospheric levels and steepening to yield shocks (Ayres 1981; Asensio Ramos et al. 2003; Carlsson & Stein 2002; Wedemeyer et al. 2004).”