The experiments described in Sect. 2 dealt with phenomena on short time scales, where the ion component could be con- sidered immobile. Similar vortical structures were found nu- merically (Sakanaka, 1972; P´ecseli et al., 1984) and experi- mentally (P´ecseli et al., 1981, 1984) also in ion phase space, where now both electron and ion components participate in the plasma dynamics. In the experiments on ion phase space vortices, the evolution of the ion distribution function, and thereby, the phase space dynamics, could be measured di- rectly by an ion energy analyzer (see also a summary by P´ecseli, 1984). Other experiments later confirmed parts of these observations (e.g. the works by Chan et al., 1984), but investigated only space-time variations of plasma den- sity and potential. Recent and more modern techniques have shown promise in detailed investigations of the ion phase space dynamics associated with the formation of ion holes (Skiff et al., 2001). Recent observations by Nakamura et al. (1999) also point out the formation and propagation of “ion holes”, i.e. ion phase space vortices. These experiments were carried out under conditions similar to those in the studies by P´ecseli et al. (1981) or P´ecseli et al. (1984), but with larger electron to ion temperature ratios. All of these investiga- tions were dealing with externally excited coherent phenom- ena. Evidence for the formation, propagation and subsequent decay of phase space vortices was also obtained in experi- ments carried out in an unmagnetized double-plasma device (Johnsen et al., 1985, 1987), where an ion beam was injected into a background plasma. A conditional sampling method was used for analyzing the space-time variation of the spon- taneously generated fluctuations in plasma density. These studies differ from those previously mentioned by making a statistical analysis of the spontaneously generated fluctua- tions.