@phdthesis{Seiler2020,
  author    = {Seiler, Michael},
  title     = {The Non-Keplerian Motion of Propeller Moons in the Saturnian Ring System},
  school      = {Universit{\"a}t Potsdam},
  pages     = {127},
  year      = {2020},
  abstract  = {One of the tremendous discoveries by the Cassini spacecraft has been the detection of propeller structures in Saturn's A ring. Although the generating moonlet is too small to be resolved by the cameras aboard Cassini, its produced density structure within the rings, caused by its gravity can be well observed. The largest observed propeller is called Bl{\´e}riot and has an azimuthal extent over several thousand kilometers. Thanks to its large size, Bl{\´e}riot could be identified in different images over a time span of over 10 years, allowing the reconstruction of its orbital evolution. It turns out that Bl{\´e}riot deviates considerably from its expected Keplerian orbit in azimuthal direction by several thousand kilometers. This excess motion can be well reconstructed by a superposition of three harmonics, and therefore resembles the typical fingerprint of a resonantly perturbed body. This PhD thesis is directed to the excess motion of Bl{\´e}riot. Resonant perturbations are a known for some of the outer satellites of Saturn. Thus, in the first part of this thesis, we seek for suiting resonance candidates nearby the propeller, which might explain the observed periods and amplitudes. In numeric simulations, we show that indeed resonances by Prometheus, Pandora and Mimas can explain the libration periods in good agreement, but not the amplitudes. The amplitude problem is solved by the introduction of a propeller-moonlet interaction model, where we assume a broken symmetry of the propeller by a small displacement of the moonlet. This results in a librating motion the moonlet around the propeller's symmetry center due to the non-vanishing accelerations. The retardation of the reaction of the propeller structure to the motion of the moonlet causes the propeller to become asymmetric. Hydrodynamic simulations to test our analytical model confirm our predictions. In the second part of this thesis, we consider a stochastic migration of the moonlet, which is an alternative hypothesis to explain the observed excess motion of Bl{\´e}riot. The mean-longitude is a time-integrated quantity and thus introduces a correlation between the independent kicks of a random walk, smoothing the noise and thus makes the residual look similar to the observed one for Bl{\´e}riot. We apply a diagonalization test to decorrelated the observed residuals for the propellers Bl{\´e}riot and Earhart and the ring-moon Daphnis. It turns out that the decorrelated distributions do not strictly follow the expected Gaussian distribution. The decorrelation method fails to distinguish a correlated random walk from a noisy libration and thus we provide an alternative study. Assuming the three-harmonic fit to be a valid representation of the excess motion for Bl{\´e}riot, independently from its origin, we test the likelihood that this excess motion can be created by a random walk. It turns out that a non-correlated and correlated random walk is unlikely to explain the observed excess motion.},
  language  = {en}
}
@article{SeissAlbersSremčevićetal.2019,
  author    = {Seiß, Martin and Albers, Nicole and Sremčević, Miodrag and Schmidt, J{\"u}rgen and Salo, Heikki and Seiler, Michael and Hoffmann, Holger and Spahn, Frank},
  title     = {Hydrodynamic Simulations of Moonlet-induced Propellers in Saturn's Rings},
  series = {The astronomical journal},
  volume    = {157},
  journal   = {The astronomical journal},
  number    = {1},
  publisher = {IOP Publishing Ltd.},
  address   = {Bristol},
  issn      = {0004-6256},
  doi       = {10.3847/1538-3881/aaed44},
  pages     = {11},
  year      = {2019},
  abstract  = {One of the biggest successes of the Cassini mission is the detection of small moons (moonlets) embedded in Saturns rings that cause S-shaped density structures in their close vicinity, called propellers. Here, we present isothermal hydrodynamic simulations of moonlet-induced propellers in Saturn's A ring that denote a further development of the original model. We find excellent agreement between these new hydrodynamic and corresponding N-body simulations. Furthermore, the hydrodynamic simulations confirm the predicted scaling laws and the analytical solution for the density in the propeller gaps. Finally, this mean field approach allows us to simulate the pattern of the giant propeller Bl{\´e}riot, which is too large to be modeled by direct N-body simulations. Our results are compared to two stellar occultation observations by the Cassini Ultraviolet Imaging Spectrometer (UVIS), which intersect the propeller Bl{\´e}riot. Best fits to the UVIS optical depth profiles are achieved for a Hill radius of 590 m, which implies a moonlet diameter of about 860 m. Furthermore, the model favors a kinematic shear viscosity of the surrounding ring material of ν0 = 340 cm2 s-1, a dispersion velocity in the range of 0.3 cm s-1 < c0 < 1.5 cm s-1, and a fairly high bulk viscosity 7 < ξ0/ν0 < 17. These large transport values might be overestimated by our isothermal ring model and should be reviewed by an extended model including thermal fluctuations.},
  language  = {en}
}
@article{SeilerSeissHoffmannetal.2019,
  author    = {Seiler, Michael and Seiß, Martin and Hoffmann, Holger and Spahn, Frank},
  title     = {Hydrodynamic Simulations of Asymmetric Propeller Structures in Saturn's Rings},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Supplement series},
  volume    = {243},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Supplement series},
  number    = {2},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0067-0049},
  doi       = {10.3847/1538-4365/ab26b0},
  pages     = {16},
  year      = {2019},
  abstract  = {The observation of the non-Keplerian behavior of propeller structures in Saturn's outer A ring raises the question: how does the propeller respond to the wandering of the central embedded moonlet? Here, we study numerically how the structural imprint of the propeller changes for a libration of the moonlet. It turns out that the libration induces an asymmetry in the propeller, which depends on the libration period and amplitude of the moonlet. Further, we study the dependence of the asymmetry on the libration period and amplitude for a moonlet with a 400 m Hill radius, which is located in the outer A ring. This allows us to apply our findings to the largest known propeller Bl{\´e}riot, which is expected to be of a similar size. For Bl{\´e}riot, we can conclude that, supposing the moonlet is librating with the largest observed period of 11.1 yr and an azimuthal amplitude of about 1845 km, a small asymmetry should be measurable but depends on the moonlet's libration phase at the observation time. The longitude residuals of other trans-Encke propellers (e.g., Earhart) show amplitudes similar to Bl{\´e}riot, which might allow us to observe larger asymmetries due to their smaller azimuthal extent, allowing us to scan the whole gap structure for asymmetries in one observation. Although the librational model of the moonlet is a simplification, our results are a first step toward the development of a consistent model for the description of the formation of asymmetric propellers caused by a freely moving moonlet.},
  language  = {en}
}