Charging of small grains in a space plasma: Application to Jovian stream particles

TL;DR

This paper revises dust charging theory for small grains in space, especially for Jovian dust streams, highlighting the importance of secondary electron emission and its dependence on grain size and material properties.

Contribution

It introduces a new model for charging small dust particles that accounts for size-dependent secondary electron emission effects, improving estimates of their charge in space environments.

Findings

Small grains have size-dependent surface potentials due to secondary electron emission.

The revised model predicts different charging times and charge fluctuations compared to traditional approaches.

Field emission does not limit charging of NaCl grains in the Jovian environment.

Abstract

Most theoretical investigations of dust charging processes in space have treated the current balance condition as independent of grain size. However, for small grains, as they are often observed in space environments, a dependence on grain size is expected due to secondary electron emission (SEE). Here, by the term "small" we mean a particle size comparable to the typical penetration depth for given primary electron energy. The results are relevant for the dynamics of small, charged dust particles emitted by the volcanic moon Io, which form the Jovian dust streams. We revise the theory of charging of small (sub-micron sized) micrometeoroids to take into account a high production of secondary electrons for small grains immersed in an isotropic flux of electrons. We apply our model to obtain an improved estimate for the charge of the dust streams leaving the Jovian system, detected by several spacecraft. For the Jupiter plasma environment we derive the surface potential of grains composed of NaCl (believed to be the major constituent of Jovian dust stream particles) or silicates. For small particles the potential depends on grain size and the secondary electron current induces a sensitivity to material properties. As a result of the small particle effect, the estimates for the charging times and for the fractional charge fluctuations of NaCl grains obtained using our general approach to SEE give results qualitatively different from the analogous estimates derived from the traditional approach to SEE. We find that for the charging environment considered in this paper field emission does not limit the charging of NaCl grains.