Geophysical and Astrophysical Fluid Dynamics

Abstract:

We test the feasibility of two scenarios that may drive the broad, prograde, equatorial jets in the Jovian atmosphere within the shallow "weather layer". The first idea attempts to explain the flat-headed jet as a consequence of a hydrodynamic instability along an equatorially trapped primarily jet. The strong primary jet is induced by a 300 in s(-1) Kelvin wave. The second idea is a bridging of a pair of off-equatorial jets due to horizontal eddy diffusion (the so-called Gierasch mechanism). The primary jets can be induced by a Hadley circulation, and might then be interconnected by subsequent hydrodynamic instabilities between them. We test the two scenarios using a general circulation model, but have so far been unable to obtain an equatorial jet that resembles observations. It appears, therefore, that the previously proposed model of combining Kelvin and Hadley forcing, is more plausible under the shallow hypothesis. (c) 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.

Abstract:

Using 2D magnetohydrodynamic (MHD) numerical simulations performed with two different finite-difference Eulerian codes, we analyse the effect that a toroidal magnetic field has on low-mass planet migration in non-turbulent protoplanetary discs. The presence of the magnetic field modifies the waves that can propagate in the disc. In agreement with a recent linear analysis, we find that two magnetic resonances develop on both sides of the planet orbit, which contribute to a significant global torque. In order to measure the torque exerted by the disc on the planet, we perform simulations in which the latter is either fixed on a circular orbit or allowed to migrate. For a 5-M⊕ planet, when the ratio β between the square of the sound speed and that of the Alfven speed at the location of the planet is equal to 2, we find inward migration when the magnetic field Bφ is uniform in the disc, reduced migration when Bφ decreases as r-1 and outward migration when Bφ decreases as r-2. These results are in agreement with predictions from the linear analysis. Taken as a whole, our results confirm that even a subthermal stable field can stop inward migration of an earth-like planet. © 2005 RAS.