Advances in the study of striations in inert gases

We present a review of studies of striations in a dc discharge in inert gases over recent decades. Physical mechanisms of stratification are described for various discharge conditions. Main attention is paid to striations at low pressures and small currents ($S$, $P$, and $R$ striations). The origin of these striations is associated with electron bunching in spatially periodic resonant fields. The idea of this mechanism and qualitative interpretation of the $S$ and $P$ striations based on the analytic theory are described in the pioneering work by L.D. Tsendin (1982). We describe the evolution of these ideas on quantitative level. New ideas concerning nonintegral resonances responsible for the formation of $R$ striations are considered. The theory is compared with experiment.