Polarization studies in electromagnetic scattering by small Solar-system particles

In remote-sensing studies, particles that are comparable to the wavelength exhibit characteristic features in electromagnetic scattering, especially in the degree of linear polarization. These features vary with the physical properties of the particles, such as shape, size, refractive index, and orientation. In the thesis, the direct problem of computing the unknown scattered quantities using the known properties of the particles and the incident radiation is solved at both optical and radar spectral regions in a unique way. The internal electromagnetic fields of wavelength-scale particles are analyzed by using both novel and established methods to show how the internal fields are related to the scattered fields in the far zone. This is achieved by using the tools and methods that were developed specifically to reveal the internal field structure of particles and to study the mechanisms that relate the structure to the scattering characteristics of those particles. It is shown that, for spherical particles, the internal field is a combination of a forward propagating wave with the apparent wavelength determined by the refractive index of the particle, and a standing wave pattern with the apparent wavelength the same as for the incident wave. Due to the surface curvature and dielectric nature of the particle, the incident wave front undergoes a phase shift, and the resulting internal wave is focused mostly at the forward part of the particle similar to an optical lens. This focusing is also seen for irregular particles. It is concluded that, for both spherical and nonspherical particles, the interference at the far field between the partial waves that originate from these concentrated areas in the particle interior, is responsible for the specific polarization features that are common for wavelength-scale particles, such as negative values and local extrema in the degree of linear polarization, asymmetry of the phase function, and enhancement of intensity near the backscattering direction. The papers presented in this thesis solve the direct problem for particles with both simple and irregular shapes to demonstrate that these interference mechanisms are common for all dielectric wavelength-scale particles. Furthermore, it is shown that these mechanisms can be applied to both regolith particles in the optical wavelengths and hydrometeors at microwave frequencies. An advantage from this kind of study is that it does not matter whether the observation is active (e.g., polarimetric radar) or passive (e.g., optical telescope). In both cases, the internal field is computed for two mutually perpendicular incident polarizations, so that the polarization characteristics can then be analyzed according to the relation between these fields and the scattered far field. Acknowledgements During the years I have been working on the thesis, there have been several people that provided guidance and support for me. First, I want to thank professor Karri Muinonen who convinced me that there might actually be something interesting in the internal fields of particles. Over the years, he has been the one to push me deeper into light scattering studies and showed me, that the deeper I go, the less I actually know. Another person who has had a great influence on me is doctor Timo Nousiainen. When I expanded my research interest into atmospheric sciences, he helped me to find my place in this field, which at first seemed unfamiliar to me. I have also enjoyed our endless discussions on topics that may not be relevant for this study, but that have kept me partly sane from staying long hours in front of the computer screen. Doctor Evgenij Zubko introduced me to the discrete-dipole approximation, which I have used regurarly ever since. Doctor Dmitri Moisseev has been my backbone in radar-related studies. I have been lucky to have four excellent advisors in my thesis studies. I am grateful for the Academy of Finland, which has funded most of my studies, and the Center for Scientific Computing, which has provided me access to their superclusters. I have probably used more CPU-hours than done real working hours. I want to thank my other colleagues in the planetary system and the atmospheric radiation research groups for constant support. Especially Antti Penttila and Jussi Leinonen, who I consider to be my fellow computation accomplishers. There are many former colleagues who have contributed to this study in small parts. Thank you all! Lastly, I thank my family for helping me to get my mind out of work-related stuff. It has been valuable during more difficult times. List of papers I Muinonen, K., Zubko, E., Tyynela, J., Shkuratov, Yu. G., and Videen, G., 2007. Light scattering by Gaussian random particles with discrete-dipole approximation. Journal of Quantitative Spectroscopy and Radiative Transfer 106, 360–377 II Tyynela, J., Zubko, E., Videen, G., and Muinonen, K., 2007. Interrelating angular scattering characteristics to internal electric fields for wavelength-scale spherical particles. Journal of Quantitative Spectroscopy and Radiative Transfer 106, 520– 534 III Tyynela, J., Muinonen, K., Zubko, E., and Videen, G., 2008. Interrelating scattering characteristics to internal electric fields for Gaussian-random-sphere particles. Journal of Quantitative Spectroscopy and Radiative Transfer 109, 2207–2218 IV Tyynela, J., Zubko, E., Muinonen, K., and Videen, G., 2010. Interpretation of single-particle negative polarization at intermediate scattering angles. Applied Optics 49, 5284–5296 V Muinonen, K., Tyynela, J., Zubko, E., Lindqvist, H., Penttila, A., and Videen, G., 2011. Polarization of light backscattered by small particles. Journal of Quantitative Spectroscopy and Radiative Transfer, 112, 2193–2212 VI Tyynela, J., Nousiainen, T., Goke, S., and Muinonen, K., 2009. Modeling C-band single scattering properties of hydrometeors using discrete-dipole approximation and T -matrix method. Journal of Quantitative Spectroscopy and Radiative Transfer 110, 1654–1664 VII Tyynela, J., Leinonen, J., Moisseev, D., and Nousiainen, T., 2011. Radar backscattering from snowflakes: comparison of fractal, aggregate, and soft-spheroid models. Journal of Atmospheric and Oceanic Technology, in press