In a previous study of ion heating at quasi-parallel shocks, we showed a case in which the ion distributions downstream from the shock alternated between a cooler, denser, core/shoulder type and a hotter, less dense, more Maxwellian type. In this paper we further document the alternating occurrence of two different ion states downstream from several quasi-parallel shocks. Three separate lines of evidence are presented to show that the two states are not related in an evolutionary sense (i.e., the hotter distributions do not evolve from the cooler ones), but rather both are produced alternately at the shock: (1) The asymptotic downstream plasma parameters (density, ion temperature, and flow speed) are intermediate between those characterizing the two different states closer to the shock, suggesting that the asymptotic state is produced by a mixing of the two initial states; (2) examples of apparently interpenetrating (i.e., mixing) distributions can be found during transitions from one state to the other; and (3) examples of both types of distributions can be found at actual crossing of the shock ramp.

The alternation between the two different types of ion distribution provides direct observational support for the idea that the dissipative dynamics of at least some quasi-parallel shocks is non-stationary and cyclic in nature, as demonstrated by recent numerical simulations. Typical cycle times between intervals of similar ion heating states are ~2 upstream ion gyroperiods. Both the simulations and the in situ observations indicate that a process of coherent ion reflection is commonly an important part of the dissipation at quasi-parallel shocks; we suggest that cyclic variations in the fraction of reflected ions could account for the alternating regions of cooler and hotter downstream ion distributions documented in this paper. ¿ Copyright 1990 by the American Geophysical Union