Ion Hole Equilibrium and Dynamics in One Dimension

Abstract

Electrostatic solitary waves with negative potential (ion holes) are analyzed theoretically using a generalization of the treatment recently developed for slow electron holes. It is shown that an often-cited criterion for their existence is mistaken and they can in fact exist for a wide range of ion to electron temperature ratios. Shifts of the hole velocity $v_h$ relative to the ion distributions systematically decrease the permitted hole depths, which become extremely small by $v_h/v_{ti}\sim 2$. Ion holes are usually unstably accelerated by electron reflection forces which are calculated numerically and analytically for the resulting asymmetric potential structure. The timescale of this acceleration is proportional to the ion plasma period, and generally longer than the ion bounce time in the potential well. Thus, ion holes behave like approximately rigid entities and even when unstable can survive much longer than the typical transit time of a satellite, so as to be observable.