Comparison between PROGRA2 polarizatio measurements and numerical simulations

The PROGRA2 experiment is a laboratory light scattering facility providing measurements on levitating particles (either during microgravity flights or on the ground with air-draught for low-density particles) [1]. This enables the realistic simulation of clouds of particles with large sizes and complex structures. Experimental samples can present a variety of size distributions (nm to hundreds of microns), different particles' shapes and structures (e.g. spheres, aggregates, plates, etc.) and mixtures of particles with different compositions can also be studied. To validate the experimental system and its results, systematic comparisons with numerical simulations of light scattering by dust particles have been performed. This presentation reports on the results obtained so far for compact particles and small aggregates and presents tentative interpretations of the results measured and calculated for aggregates of dust particles. Initial validations and results used spherical particles with different absorption properties, including homogeneous particles and particles covered with different materials. Numerical simulation results compare very well with the measured signals and properties such as optical indices, or layers' thicknesses can be determined [2]. During levitation small grains tend to form aggregates. The measured polarimetric properties vary strongly with the properties of the constituent particles as well as the structure and size of the aggregate. Numerical simulations of such systems are challenging, but provide insights into the light scattering properties of small aggregates and the influence of the different parameters [3, 4]. PROGRA2 measurements and related simulations are presented to further interpret the transition from single grain polarization phase curves to larger aggregates scattering properties that is relevant to interpret cometary dust light scattering observations [e.g. 5]. CNES and ESA support for the microgravity flights and experimental work is acknowledged References [1] E. Hadamcik, J.-B. Renard, A.C. Levasseur-Regourd, J. Lasue. NATO 137 (2011) [2] J. Lasue, A.C. Levasseur-Regourd, E. Hadamcik, J.-B. Renard. JQSRT 106, 212 (2007) [3] Hadamcik et al. JQSRT 106, 74 (2007) [4] Hadamcik et al. JQSRT 110, 1755 (2009