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Understanding the source of sub-stellar polarimetric observations in theoptical and near-infrared is key to characterizing sub-stellar objects anddeveloping potential diagnostics for determining properties of theiratmospheres. Differential scattering from a population of aligned,non-spherical dust grains is a potential source of polarization that could beused to determine geometric properties of the dust clouds. This paper addressesthe problem of the spheroidal growth of dust grains in electrically activatedsub-stellar atmospheres. It presents the novel application of a mechanismwhereby non-spherical, elongated dust grains can be grown via plasma depositionas a consequence of the surface electric field effects of charged dust grains.We numerically solve the differential equations governing the spheroidal growthof charged dust grains via plasma deposition as a result of surface electricfield effects in order to determine how the dust eccentricity and the dustparticle eccentricity distribution function evolve with time. From theseresults, we determine the effect of spheroidal dust on the observed linearpolarization. Numerical solutions show that $e\approx 0.94$ defines a watershedeccentricity, where the eccentricity of grains with an initial eccentricityless than (greater than) this value decreases (increases) and spherical(spheroidal) growth occurs. This produces a characteristic bimodal eccentricitydistribution function yielding a fractional change in the observed linearpolarization of up to $\approx0.1$ corresponding to dust grains of maximaleccentricity at wavelengths of $\approx1 \mu$m, consistent with the nearinfrared observational window. The results presented here are relevant to thegrowth of non-spherical, irregularly shaped dust grains of general geometrywhere non-uniform surface electric field effects of charged dust grains aresignificant.

Evolution of spheroidal dust in electrically active sub-stellar atmospheres