Ion Hole Equilibrium and Dynamics in One Dimension

TL;DR

This paper provides a theoretical analysis of ion holes in one dimension, challenging previous criteria for their existence, and explores their stability, dynamics, and observational longevity.

Contribution

It generalizes the theory of ion holes, corrects misconceptions about their existence criteria, and analyzes their stability and dynamics in detail.

Findings

Ion holes can exist for a wide range of ion to electron temperature ratios.

Ion holes are usually unstably accelerated by electron reflection forces.

Ion holes can survive longer than typical satellite transit times, making them observable.

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.