ABSTRACT
The plasma state, a fully or at least significantly ionized gas, can occur in many environments. One example is a gas in thermal equilibrium at a high temperature. This may be our intuitive first guess, but it turns out that we seldom meet gases in true thermal equilibrium at relevant temperatures. The most abundant natural occurrence of plasmas near the Earth is the ionosphere, where the primary source of plasma is ionization by ultra-violet (UV) radiation from the Sun. This chapter discusses different models for plasma production, focusing mainly on those relevant for the Earth’s ionosphere and magnetosphere. The simplest expression for degree of ionization in thermal equilibrium, with the more general result known as Saha’s equation. The approximation consists essentially of considering only a mixture of bound states and free ions and electrons, ignoring excited states, internal degrees of freedom, etc.
