To deduce the physical properties of cometary dust, some diagnostics are often used: i.e. dynamical models for spatial evolution of brightness of the scattered light and spectral emission features. The linear polarization of the scattered light is another important diagnostic. Polarimetric phase curves have allowed a classification of comets in two classes, although comet C/1995 01 Hale-Bopp has been found to have a higher polarization than the other comets [1,2,3]. Polarimetric maps emphasize variations in the physical properties inside the coma (e.g. [4,5,2,6]). In addition, the wavelength dependence for polarization or brightness provide a complementary diagnostic ([7,8]).
To tentatively interpret the observations some models of light scattering by the particles have been built. The particles are assumed to be compact grains (spheres or irregular particles) or aggregates with a relatively small number of constituent grains (due to computational limitations). For all these models, size is a main parameter.
This paper compares cometary polarimetric phase curves with results obtained for linear polarization of light scattered by large particles (compact and fluffy) studied with the PROGRA2 instrument  which is a type of nephelometer. The compact particles levitate in microgravity, the fluffy ones are lifted in ground-based conditions by an air draft (Hadamcik, 1999). These particles are difficult to model due to their irregular shapes and the high number of grains (hundreds of thousands).
The size distribution of the particles is in the 10 }im to 500 }im range depending on the sample. The irregular compact particles are mainly convex with sharp edges as described by Worms et al. . Two different types of fluffy particles have been used, ground Orgueil meteorite (porosity 30 %; size 1 }im, see ); fluffy samples made of submicron grains (pyrogenic oxides, carbon black and mixtures of them, described in ). The smallest grain size (7 nm) is for a pyrogneic silica sample, the largest one for a carbon black sample (95 nm). These grains are linked together in chains giving aggregates. The size of the aggregates is of the order of ten micrometers and their porosity is higher than 95 %, with a specific surface area in the range of 50-300 m2 g"1. The densities of the grains are given by the company who provides the samples, the densities of the aggregates are estimated by measuring hardly packed powders. These aggregates stick together in agglomerates. When levitated, the particles in the field of view are such agglomerates. The densities of the agglomerates are estimated by measuring sifted powders.
The measurements are performed between 8° and 160° phase angles. Figure 1 gives a summary of typical phase curves in red light. Figure la is for compact particles, figure lb for fluffy particles. We are mainly interested in two regions: the backscattering region at small phase angles and the maximum polarization (Pmax) region.
For phase angles smaller than 20°, three facts can be noticed. First, for compact particles (i.e. basalt), the polarization is negative or close to zero for different sizes of the particles . For levitating fluffy particles (i.e. pyrogenic silica with 16 nm grains, pyrogenic silica with 40 nm grains), the polarization sharply increases near 10°-20° phase angle. This is not the case for mixtures of two fluffy samples; they produce a negative branch at phase angles smaller than 20° (the studied samples are: silica 16 nm and silica 40 nm; silica 40 nm and carbon 14 nm; silica 40 nm and carbon 95 nm; alumina 13 nm and carbon 14 nm).
3.2. Maximum polarization region
In figure 2a three different materials for compact particles are studied. Pma}i increases when the particles size (mean diameter of the single grain) increases, with a different maximum value for each material, as expected from Fresnel laws. In figure 2b two materials of fluffy particles (pyrognenic silica and carbon black) are considered. Pmax decreases when the size of the constituent grains (mean diameter) increases. When the size is smaller than 15 nm (size parameter of about 0.07 in red light) Rayleigh scattering is observed. Pmax for a mixture of two size distributions of pyrogenic silica is smaller than Pmax for the constituent samples. More generally, a lower PmaX than those obtained for the component materials is observed for all the studied mixtures of fluffy particles.
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Figure 1. Polarimetrie measurements (with error bars) and phase curves for different sizes of the grains, a) Compact particles in microgravity. b) Fluffy particles lifted in air.
To study the wavelength dependence, a red and a green laser were used. For all the studied compact samples, the maximum value of polarization is found higher in green light than in red light. The same result was found for Orgueil meteorite . For the other fluffy particles (highly porous and with submicronic grains), Fma* is always found to be higher in red light than in green light .
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