  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.
