Abstract
Partially ionized plasmas are found in many different astrophysical environments. The study of partially ionized plasmas is of great interest for solar physics because some layers of the solar atmosphere (photosphere and chromosphere) as well as solar structures, such as spicules and prominences, are made of these kinds of plasmas. To our knowledge, despite it being known that the adiabatic coefficient,γ, or the first adiabatic exponent, Γ1, depend on the ionization degree, this fact has been disregarded in all the studies related to magnetohydrodynamic waves in solar partially ionized plasmas. However, in other astrophysical areas, the dependence ofγor Γ1on the plasma ionization degree has been taken into account. Therefore, our aim here is to study how, in a plasma with prominence physical properties, the joint action of the temperature, density, and ionization degree modifies the numerical values of the first adiabatic exponent Γ1which affects the adiabatic sound speed and the period of slow waves. In our computations, we have used two different approaches; first of all, we assume local thermodynamic equilibrium (LTE) and, later, we consider a non-local thermodynamic equilibrium (non-LTE) model. When comparing the results in the LTE and non-LTE cases, the numerical values of Γ1are clearly different for both and they are probably strongly dependent on the assumed model which determines how the ionization degree evolves with temperature. Finally, the effect of the ionization degree dependence of Γ1on the period of slow waves has been determined showing that it can be of great importance for seismological studies of partially ionized solar structures.
Highlights
The thermodynamics of partially ionized gas (Ballester et al 2018a) differs from that of fully ionized or fully neutral gas
The adiabatic coefficient, γ, depends on the state of the plasma through the internal energy; it depends on the ionization fraction and its ionization is due to the fact that part of the energy input is invested in ionization, instead of an increasing gas temperature
We have considered a partially ionized plasma composed by neutral and ionized hydrogen as well as neutral helium and, assuming constant density, we have studied the following three different cases: a constant adiabatic coefficient, γ, in the local thermodynamic equilibrium (LTE) case, the first adiabatic exponent, Γ1, in the LTE case, and the first adiabatic exponent, Γ1, in the non-local thermodynamic equilibrium (non-LTE) case
Summary
The thermodynamics of partially ionized gas (Ballester et al 2018a) differs from that of fully ionized or fully neutral gas. A different approach was followed by D’Angelo & Bodenheimer (2013) who made 3D radiation hydrodynamics calculations of the envelopes surrounding young planets They assumed LTE as well, and considered a more complex expression for the specific internal energy involving translational, rotational, and vibrational states of molecular hydrogen, translational energies for hydrogen and helium atoms, dissociation energy of molecular hydrogen, and ionization energies of hydrogen and helium atoms. Vaidya et al (2015) performed astrophysical fluid simulations of thermally ideal gases not having a constant adiabatic index and considered translational, rotational, vibrational, ionization, and dissociation energy terms in the specific internal energy They assumed LTE conditions to compute the ionization degree and the temperature-dependent adiabatic exponent Γ1.
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