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Filtering of interstellar dust in the heliosphere


Sterken,  Veerle Jasmin
Ralf Srama - Heidelberg Dust Group, Research Groups, MPI for Nuclear Physics, Max Planck Society;

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Sterken, V. J. (2012). Filtering of interstellar dust in the heliosphere. PhD Thesis, Technische Universität Carolo-Wilhelmina, Braunschweig.

The space between the stars is filled with gas and interstellar dust (ISD). The dust grains cause extinction of the starlight that we observe and they play an important role in the evolution of galaxies and in the formation of stellar and planetary systems. The ISD has been long observed by astronomical methods, the grains were measured by in-situ measurements of spacecraft in the solar system and even samples of interstellar material have been brought back to Earth by the Stardust mission in 2006. Many questions on the composition, morphology and size distribution of ISD still exist today which require more and improved measurements. Modeling and understanding the ISD flow through the solar system is needed to interpret these observations and to fully extract the information on ISD grains carried by these measurements. Also, the modelling is necessary for designing and optimizing future ISD missions. The modeling in this thesis follows this flow of ISD through the solar system taking into account three main forces: solar gravity, solar radiation pressure force and Lorentz forces resulting from the motion of the charged grains through the interplanetary magnetic field (IMF). Simulations of dust trajectories over a large range of grain parameters β and Q/m were performed. β is the ratio of solar radiation pressure force to gravity and depends on the grain size, morphology and material. Q/m is the charge to mass ratio. The influence of solar radiation pressure force and Lorentz forces on the trajectories, densities and fluxes were systematically studied. Radiation pressure reduces gravitational attraction and can become even dominant (β > 1) for particles of about 0.2 μm radius, leading to a void region downstream from the Sun: the β -cone. The ISD flow under the influence of solar radiation pressure and gravity only is axi-symmetric, stationary and can even be calculated analytically. Lorentz force becomes stronger for smaller grains having higher Q=m and dominates for grains < 0.15–0.2μm. The azimuthal component of the IMF causes the grains to deflect towards or away from the solar equatorial plane depending on the polarity of the IMF. This focusing and defocusing effect occurs in a 22-year cycle, which makes the stream non-stationary and the flow pattern much more complicated. If not filtered at the termination shock, then very small grains of 0.1–0.15 _m would still reach the solar system at favorable times and would be reflected back upstream (called ‘mirroring’ in this thesis) causing locally enhanced concentrations. Since small grains are more abundant in the size distribution of the ISD, this may still play an important role in future observations like for Cassini between 2010 and 2017 and may teach us about filtering processes at the termination shock. The size distribution of ISD in the solar system is strongly modified from the incoming ISD size distribution and varies with grain properties, location in the solar system and time in the solar cycle. These modified size distributions are discussed for a fixed position along the ISD flow axis and for moving objects like Saturn, Jupiter and the main-belt asteroid Ceres. Implications for specific missions (Cassini, JUICE and Stardust) are studied too. The ISD flux predicted for Cassini at Saturn reveals that in 2010, the flux of 0.25 μm grains is maximum and that between 2010 and the end of the mission (2017) the flux of different smaller sizes of the grains is maximum at different times. For the JUpiter ICy moons Explorer (JUICE) the simulations showed that there is an optimum opportunity for ISD observations just before and at arrival of the spacecraft at Jupiter in 2030. Finally, the ISD conditions during the Stardust sample return mission are studied and the results of the simulations are compared to the preliminary findings (ISD samples) of the Stardust Team. The simulations indicate that the observations are compatible with an interstellar origin of the identified ISD candidates but cannot be taken as a proof. Insights and techniques acquired and used in this thesis will open doors for future ISD research. Combining the knowledge derived from astronomical observations, sample return missions, and from comparing the ISD modeling to in-situ spacecraft measurements will constrain the grain properties and teach us about their size distributions before and after filtering on their way through the solar system. From this, we can learn about the immediate galactic environment of the Sun and the heliosphere.