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The observation of the non-Keplerian behavior of propeller structures in Saturn's outer A ring (Tiscareno et al. 2010, Seiler et al. 2017, Spahn et al. 2018) raises the question, how the propeller responds to the wandering of the central embedded moonlet. Here, we study numerically how the induced propeller is changing for a librating moonlet. It turns out, that the libration of the moonlet induces an asymmetry in the structural imprint of the propeller, where the asymmetry is depending on the moonlet's libration period and amplitude. Further, we study the dependence of the asymmetry on the libration period and amplitude for a moonlet with 400 m Hill radius, which is located in the outer A ring. In this way, we are able to apply our findings to the largest found propeller structures -- such as Bl\'eriot -- which are expected to be of similar size. For Bl\'eriot, we can conclude that, supposed the moonlet is librating with the largest observed period of 11.1 years and an azimuthal amplitude of about 1845 km (Seiler et al. 2017, Spahn et al. 2018), a small assymmetry should be measurable but depends on the moonlet's libration phase at the observation time. The excess motions of the other giant propellers -- such as Santos Dumont and Earhart -- have similar amplitudes as Bl\'eriot and thus might allow the observation of larger asymmetries due to their smaller azimuthal extent. This would permit to scan the whole gap structure for asymmetries. Although the librational model of the moonlet is a simplification, our results are a first step towards the development of a consistent model for the description of the formation of asymmetric propellers caused by a freely moving moonlet.

The reconstruction of the orbital evolution of the propeller structure Bl\'eriot orbiting in Saturn's A ring from recurrent observations in Cassini ISS images yielded a considerable offset motion from the expected Keplerian orbit (Tiscareno et al., 2010). This offset motion can be composed by three sinusoidal harmonics with amplitudes and periods of 1845, 152, 58 km and 11.1, 3.7 and 2.2 years, respectively (Srem\v{c}evi\'c et al., 2014). In this paper we present results from N-Body simulations, where we integrated the orbital evolution of a moonlet, which is placed at the radial position of Bl\'eriot under the gravitational action of the Saturnian satellites. Our simulations yield, that especially the gravitational interactions with Prometheus, Pandora and Mimas is forcing the moonlet to librate with the right frequencies, but the libration-amplitudes are far too small to explain the observations. Thus, further mechanisms are needed to explain the amplitudes of the forced librations - e.g. moonlet-ring interactions. Here, we develop a model, where the moonlet is allowed to be slightly displaced with respect to its created gaps, resulting in a breaking point-symmetry and in a repulsive force. As a result, the evolution of the moonlet's longitude can be described by a harmonic oscillator. In the presence of external forcings by the outer moons, the libration amplitude gets the more amplified, the more the forcing frequency gets close to the eigenfrequency of the disturbed propeller oscillator. Applying our model to Bl\'eriot, it is possible to reproduce a libration period of 13 years with an amplitude of about 2000 km.

It is found that for objects possessing small surface structures with differing radii of curvature the secondary electron emission (SEE) yield may be significantly higher than for objects with smooth surfaces of the same material. The effect is highly pronounced for surface structures of nanometer scale, often providing a more than 100% increase of the SEE yield. The results also show that the SEE yield from surfaces with structure does not show an universal dependence on the energy of the primary, incident electrons as it is found for flat surfaces in experiments. We derive conditions for the applicability of the conventional formulation of SEE using the simplifying assumption of universal dependence. Our analysis provides a basis for studying low-energy electron emission from nano structured surfaces under a penetrating electron beam important in many technological applications., Comment: 6 pages, 8 figures, Published in Phys. Rev. B, 21 September 2015, http://journals.aps.org/prb/abstract/10.1103/PhysRevB.92.125430

Most theoretical investigations of dust charging processes in space have treated the current balance condition as independent of grain size. However, for small grains, as they are often observed in space environments, a dependence on grain size is expected due to secondary electron emission (SEE). Here, by the term "small" we mean a particle size comparable to the typical penetration depth for given primary electron energy. The results are relevant for the dynamics of small, charged dust particles emitted by the volcanic moon Io, which form the Jovian dust streams. We revise the theory of charging of small (sub-micron sized) micrometeoroids to take into account a high production of secondary electrons for small grains immersed in an isotropic flux of electrons. We apply our model to obtain an improved estimate for the charge of the dust streams leaving the Jovian system, detected by several spacecraft. For the Jupiter plasma environment we derive the surface potential of grains composed of NaCl (believed to be the major constituent of Jovian dust stream particles) or silicates. For small particles the potential depends on grain size and the secondary electron current induces a sensitivity to material properties. As a result of the small particle effect, the estimates for the charging times and for the fractional charge fluctuations of NaCl grains obtained using our general approach to SEE give results qualitatively different from the analogous estimates derived from the traditional approach to SEE. We find that for the charging environment considered in this paper field emission does not limit the charging of NaCl grains., Comment: 7 pages, 9 figures; second revision submitted to A&A

A statistical model of fragmentation of aggregates is proposed, based on the stochastic propagation of cracks through the body. The propagation rules are formulated on a lattice and mimic two important features of the process -- a crack moves against the stress gradient and its energy depletes as it grows. We perform numerical simulations of the model for two-dimensional lattice and reveal that the mass distribution for small and intermediate-size fragments obeys a power-law, F(m)\propto m^(-3/2), in agreement with experimental observations. We develop an analytical theory which explains the detected power-law and demonstrate that the overall fragment mass distribution in our model agrees qualitatively with that, observed in experiments., Comment: 4 pages, submitted to a peer-reviewed journal

We propose a new mechanism which explains the existence of enormously sharp edges in the rings of Saturn. This mechanism is based on the synchronization phenomenon due to which the epicycle rotational phases of particles in the ring, under certain conditions, become synchronized with the phase of external satellite, e.g. with the phase of Mimas in the case of the outer B ring edge. This synchronization eliminates collisions between particles and suppress the diffusion induced by collisions by orders of magnitude. The minimum of the diffusion is reached at the center of the synchronization regime corresponding to the ratio 2:1 between the orbital frequency at the edge of B ring and the orbital frequency of Mimas. The synchronization theory gives the sharpness of the edge in few tens of meters that is in agreement with available observations.