LIGHT-SCATTERING BY POROUS DUST PARTICLES IN THE DISCRETE-DIPOLE APPROXIMATION

Electromagnetic scattering by irregular dust grains and aggregates has long been a challenging problem in various astronomical applications. Only the quite recent advancement in the discrete-dipole approximation (DDA) have made the problem trackable. We have applied a new iterative method, the quasi-minimal residual algorithm, to the system of linear equations arising in the DDA applications. This method is approximately 3 times faster than the standard iterative scheme, conjugate gradient, used with DDA. We apply DDA both to pseudospherical objects, of which we randomly remove different numbers of small scattering elements (dipoles) to model porous grains, and to aggregates of these grains which we believe to properly model fluffy particles. We study both the single particle intensity and linear polarization as a function of the grain size, packing density and the refractive index. Surprisingly, some results for inhomogeneous particles can be matched by the classical Mie theory using a generalized version of the effective medium theory. Some well-known photometric and polarimetric features of solar system objects seem to follow naturally from the scattering properties of fluffy particles.