CRITERIA FOR THE VALIDITY OF LOCAL THERMODYNAMIC EQUILIBRIUM,

With the use of the Seaton formulae for the collisional excitation and ionization cross sections, a simple equation is obtained giving the relation between the electron temperature T, electron density Ne and the ionization energy, required for a gas to be collision dominated. Then with the use of an approximate equation expressing the ionization energy, in terms of T and Ne, an approximate general equation giving the relation between T and Ne necessary for all elements to be collision dominated is obtained. It is seen that at high temperatures very high electron densities are required for high Z elements to be collision dominated. The additional requirements for a non-optically thin gas to be in local thermodynamic equilibrium LTE and the conditions for an optically thick gas to be in LTE are also briefly discussed. Author