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An initial theoretical attempt to explain the observed decrease of thepolytropic/adiabatic index $\gamma$ in the solar corona has been accomplished.The chemical reactions of the ionization-recombination processes in localthermodynamic equilibrium (LTE) of a solar plasma cocktail containing heavyelements are found to cause $1.1 < \gamma \leq 5/3$ in the quiet solaratmosphere. It is also shown that the quiet solar atmosphere is in LTEjustifying this theoretical study. This result is obtained by numericallysolving the Saha equation and subsequently using a newly derived equation forcalculation of the polytropic index from thermodynamic partial derivatives ofthe enthalpy and pressure with respect to density and temperature. In addition,a comparison between calculated in this way polytropic index and measured fromspectroscopic observations of propagating slow magneto-hydrodynamics (MHD)waves in coronal loops shows that LTE ionization accounts for very small partof the observed decrease of $\gamma$ meaning that the solar plasma in theactive region is not in LTE as expected. However, the observed dependency ofhigher polytropic index at higher temperatures is confirmed by the currenttheoretical approach. It is concluded that to account for the polytropic indexdecrease in the active regions of the solar corona, it is necessary kineticnon-LTE ionization calculations to be performed.

Influence of Ionization on the Polytropic Index of the Solar Atmosphere within Local Thermodynamic Equilibrium Approximation