In the present work, the light‐scattering properties of comet Levy 1990XX are studied through simulations using Ballistic Particle‐Cluster Aggregation (BPCA) or Ballistic Cluster–Cluster Aggregation (BCCA) aggregates of up to 128 spherical monomers of different compositions (e.g. silicates, carbonaceous materials etc.) and the best‐fitting theoretical polarization curve is generated using the Superposition T‐matrix code. The best‐fitting refractive indices coming out from the present analysis show silicate behaviour when monomer radius is am= 0.12 μm and provide excellent results on the maximum and negative degrees of linear polarization at a single wavelength λ= 0.485 μm for BCCA aggregates.

Both the in situ measurement of Comet 1P/Halley and the Stardust‐returned samples of Comet Wild 2 showed the presence of a mixture of compact and aggregate particles, with both silicates and organic refractory being in the composition of the cometary dust. Results obtained recently from the Stardust mission suggest that the overall ratio of compact to aggregate particles is 65:35 (or 13:7) for Comet 81P/Wild 2. In the present work, we propose a model that considers cometary dust as a mixture of compact and aggregate particles, with a composition of silicate and organic. We consider compact particles as spheroidal particles and aggregates as both ballistic cluster–cluster aggregate (BCCA) and ballistic agglomeration with two migrations (BAM2) aggregate with a certain size distribution. The mixing ratio of compact to aggregate particles is taken to be 13:7. For modelling Comet 1P/Halley, the power‐law size distribution n(a) ∼a−2.6, obtained from a re‐analysis of the Giotto spacecraft data, for both compact and aggregate particles, is used. We consider a mixture of BAM2 and BCCA aggregates with a lower cut‐off size of about 0.20 μ m and an upper cut‐off of about 1 μ m. We also consider a mixture of prolate, spherical and oblate compact particles with an axial ratio (E) of 0.8–1.2 where a lower cut‐off size of about 0.1 μ m and an upper cut‐off of about 10 μ m are considered. Using a T‐matrix code for polydisperse spheroids (0.1 μ m ≤a≤ 10 μ m) and superposition T‐matrix code for aggregates (0.2 μ m ≤av≤ 1 μ m), the average simulated polarization curves are generated, which can best fit the observed polarization data at the four wavelengths: λ= 0.365, 0.485, 0.670 and 0.684 μ m. The suitable mixing percentages of aggregates obtained from the present modelling are 50 per cent BAM2 and 50 per cent BCCA particles, and the silicate‐to‐organic mixing percentages are 78 per cent silicate and 22 per cent organic, in terms of volume. The present model successfully reproduces the observed polarization data, especially the negative branch, for Comet 1P/Halley at the above four wavelengths, more effectively as compared to other work done in the past. It is found that among the aggregates, the BAM2 aggregate plays a major role in deciding the cross‐over angle and depth of the negative polarization branch.

In our present study, the observed linear polarization data of comet Hyakutake are studied at wavelengths λ = 0.365µm, λ = 0.485µm and 0.684µm through simulations using Ballistic Particle-Cluster Aggregate and Ballistic Cluster-Cluster Aggregate aggregates of 128 spherical monomers. We first investigated that the size parameter of the monomer, x ∼ 1.56 -1.70, turned out to be most suitable which provides the best fits to the observed dust scattering properties at three wavelengths λ = 0.365µm, 0.485µm and 0.684µm. Thus the effective radius of the aggregate (r) lies in the range 0.45µm ≤ r ≤ 0.49µm at λ = 0.365µm; 0.60µm ≤ r ≤ 0.66µm at λ = 0.485µm and 0.88µm ≤ r ≤ 0.94µm at λ = 0.684µm. Now using superposition T-MATRIX code and the power-law size distribution, n(r) ∼ r −3 , the best-fitting values of complex refractive indices are calculated which can best fit the observed polarization data at the above three wavelengths. The best-fitting complex refractive indices (n, k) are found to be (1.745, 0.095) at λ = 0.365 µm, (1.743, 0.100) at λ = 0.485 µm and (1.695, 0.100) at λ = 0.684 µm. The refractive indices coming out from the present analysis correspond to mixture of both silicates and organics, which are in good agreement with the in situ measurement of comets by different spacecraft.