Datensatz

Zusammenfassung

Stream particles are nanometer-sized dust particles ejected with speeds greater than 100km/s from the Jovian and Saturnian system. Due to their tiny size, the dynamics of the charged grains is dominated by electromagnetic forces. The strong correlation between the stream particle flux and the strength of the interplanetary magnetic field (IMF) was observed first by the dust detectors on the Ulysses and Galileo spacecrafts. The Cosmic Dust Analyser (CDA) onboard the Cassini spacecraft offers the unique opportunity achieve a deeper understanding of the stream particle phenomenon due to the detector’s improved sensitivity and its capability to determine the particles' composition. Furthermore, CDA is so far the only detector which observed stream particles from both source planets. The direct comparison between the properties of grains from different source planets provide deep insights into the physics of the dust-magnetosphere interaction. Because the observations of Jovian stream particles by three spacecrafts covers a very long time span, also the long term evolution of the stream particle flux can be studied. This study finds that the Jovian stream flux in the interplanetary space monitored by CDA during Cassinis flyby at Jupiter in 2000 to 2001 follows a similar trend as the stream particle flux in the inner Jovian system simultaneously observed by the Galileo detector. By employing a plasma model based on the observed ultraviolet emission of Io's plasma torus, it is shown that the charging conditions in the vicinity of Io are consistent with the enhancement of the stream particle production rate derived from Galileo measurements. This finding is indicative of a complex dust-moon-magnetosphere interaction, which has not yet been understood. An important focus of this work are the Saturnian stream particles discovered by the Cassini dust detector. The dynamical evolution of the particles in the interplanetary space as well as in Saturn's magnetosphere is investigated in depth. During Cassini's approach to Jupiter in 2004 the interplanetary magnetic field showed a recurrent two-sector structure associated with Corotation Interaction Regions (CIRs). CDA observations during this period clearly show a drastic change of the particles' dynamical properties during their passage from solar wind rarefaction regions into compressed, high magnetic field strength solar wind regions. This implies that the "dust stream" phenomenon stems from "local" stream particle-IMF interactions. By means of backward tracing simulations using Cassini insitu solar wind data it is shown that Saturnian stream particles have sizes ranging between 2 to 8 nm and are escaping from the Saturnian system at speeds between 50 and 200km/s�. The newly developed ejection model, which includes stochastic charging and employes the latest Cassini plasma data, matches the dynamical properties derived from backward tracing simulations. This allows us to identify the source region of the particles in the inner Saturnian system. A analysis of CDA mass spectra shows that the grain composition of the source region (water ice in E ring particles) is different from the composition of Saturnian stream particles, which have a drastically enhanced siliceous compound. The pronounced difference between the secondary electron emission yield and the sputtering efficiency of water ice and siliceous material suggests that plasma sputtering not only governs the lifetime of the E ring particles but also provides an compositional selection mechanism responsible for the observed compositional discrepancy between icy E ring grains and Saturnian stream particles. The high sputtering yield of water ice suggest that siliceous impurities released via sputtering from the dynamically evolved E ring particles are the most probable source of Saturnian stream particles. This work also indicates that the radiolysis of icy E ring grains may be responsible for the observed atomic and molecular oxygen ion features in Saturn's magnetosphere.