*Earth Planets Space,* Vol. 50 (Nos. 6, 7), pp. 577-585, 1998

## Light scattering by irregular interplanetary dust particles

Hajime Okamoto^{1} and Yu-lin Xu^{2}

^{1}Kashima Space Research Center, Communications Research Laboratory, Japan

^{2}Department of Astronomy, University of Florida, U.S.A.

(Received December 17, 1997; Revised March 13, 1998; Accepted March 24, 1998)

**Abstract: **
We review recent progresses in light scattering for non-spherical particles. Special attention is paid to cluster of spheres in order to improve our understanding of interplanetary dust particles. For scattering by non-spherical particles the discrete-dipole approximation (DDA) has widely been used in many scientific fields. However mainly due to the requirements of large computing memory and long computing time, the applicability of this theory is practically limited for rather small particle compared with wavelength. In order to overcome this practical problem, i.e., the large particle can not be calculated by the DDA, we recently developed the *a*_{1}-term method, which is a modification version of the DDA where the dipole polarizability is determined by the first term of scattering coefficient in Mie theory. Accuracy of this method is tested by comparing the solutions by the a1-term method with those by modal analysis, which gives the analytical solutions for cluster of spherical monomers. According to the error analysis mentioned above, the applicabilities of the *a*_{1}-term method are established as follows. The maximum size parameter of the monomer in the cluster is 1 and the total size parameter of the cluster can exceed *X*~ 100 when the *N*~ 10^{6} dipoles are used. We show the extinction efficiencies and asymmetry factors for cluster of spheres whose size parameter is larger than the wavelength, e.g., the volume equivalent size parameter *X*is larger than 30. Finally we summarize the applicabilities of DDA, T-Matrix, modal analysis and the *a*_{1}-term method. The *a*_{1}-term method can partly fulfill a gap where both DDA and the ray tracing technique based on geometrical optics can not be applied when the target is cluster. However for the target which has edges remains to be problematic. This would be the topic which should be focused on future research.

Corresponding author E-mail: hokamoto@crl.go.jp

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