Two-state ion heating at quasi-parallel shocks

This paper documents the alternating occurrence of two different ion states of the ion distributions downstream from several quasi-parallel shocks: a cooler (and denser) core/shoulder type and a hotter (less dense) and more Maxwellian type. Three separate lines of evidence are presented to show that the two states are not related in an evolutionary sense, but that both are produced alternately at the shock: (1) the asymptotic downstream plasma parameters (density, ion temperautre, 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 distributions are found during transitions from one state to the other; and (3) examples of both types of distributions are found at actual crossings of the shock ramp.