ABSTRACT
The plasma state, a fully or at least signiÀcantly ionized gas, can occur in many environments. One example is a gas in thermal equilibrium at a high temperature. This may be our intuitive Àrst guess, but it turns out that we seldom meet gases in true thermal equilibrium at relevant temperatures. The problem is interesting nonetheless. The closest we come to thermal equilibrium is probably coronal equilibrium, which is relevant for parts of the solar atmosphere, for instance. The most abundant natural occurrence of plasmas near the Earth is the ionosphere, where the primary source of plasma is ionization by radiation from the Sun. In this chapter we discuss different models for plasma production, focusing mainly on those relevant for the Earth’s ionosphere and magnetosphere. Important topics, such as technological applications of plasmas, are left out.
