Structure formation in Saturn's rings

By Henrik Latter

Room T15 (3rd floor) at ENS, 24 rue Lhomond, 11:30 to 12:30

Abstract: Saturn's rings are among the most familiar, beautiful, and puzzling objects in the Solar system, if not all of Space. Their complex, striated structure inspires equal degrees of aethetic pleasure and theoretical agitation. My talk presents an introduction to this multiform phenomena and the physics that is responsible in each case. In particular, I will focus on the fine-scale radial patterns that spontaneously form in dense portions of the disk, as well as the difficult modelling issues posed by a `gas' of infrequently colliding, inelastic particles.

Combining geomagnetic observations, numerical simulations and theory to study dynamo action in Earth's outer core

Par Hagay Amit

Salle T15 (3e étage) à l'ENS, 24 rue Lhomond, 11:30 à 12:30

Flow in the fluid outer core just below the core-mantle boundary is inferred from the historical geomagnetic secular variation (1840-1990), assuming frozen magnetic flux and a new physical assumption termed helical flow, in which tangential divergence correlates with radial vorticity. Flow driven by lower mantle heterogeneity seems responsible for the mid-latitude asymmetry in the zonal core flow; homogeneous dynamo effects are responsible for the zonal flow at high latitudes. A thermal wind model recovers some significant features in the non-zonal time-average core flow. Changes in the core's angular momentum calculated from the time-dependent core flow agree well with decade-scale length-of-day variations. The time-dependent flow is fitted by a torsional oscillations model with periods of 53 and 110 years. Combining geomagnetic secular variation data, time-dependent core flow, and dipole moment time-evolution equations, allows to identify mechanisms of geomagnetic dipole moment change. Meridional advection and radial magnetic diffusion are comparable and account for essentially all the observed dipole intensity decrease.

SIMULATIONS AND EXPERIMENTS OF STELLAR JETS

Par Andrea Ciardi

Salle T15 (3e étage) à l'ENS, 24 rue Lhomond, 11:30 à 12:30

In recent years the development of high-energy density plasma experiments has opened the way to the study of extreme astrophysical phenomena in the laboratory. The validity of the laboratory studies rests, partly, on the condition that the important dimensionless numbers (Reynolds, Peclet, etc) are in the same range as those in the astrophysical environment. Coupled with numerical modelling, much understanding can then be gained by analysing the resulting laboratory phenomena.

Pulsed-power facilities are generally dedicated to the study of intense X-ray sources and high-energy density plasmas with applications to inertial confinement fusion. I will first present a brief review of the development of jet experiments on such facilities and then discuss recent work on (magneto)hydrodynamic jets. In particular I will present experiments addressing the dynamics of curved stellar jets, observed in many star forming regions, and new results to study magnetically launched jets. The implications and relevance to astrophysical models, and future laboratory modelling will be addressed.