Laboratory Observations of Elliptical Rotational Parametric Instability Confirm a Dynamo Mechanism
Abstract
Periodic forcing of planetary core fluid, through straining of otherwise circular streamlines, can give rise to large scale flows on millennial time scales. Tidal perturbations of the rotating contained fluid core couple pairs of normal modes resulting in repetitive sequential growth, collapse and decay, provided viscous and ohmic dissipations are weak. This phenomenon as been studied in a laboratory setting through a traveling elliptical perturbation of a flexible spheroidal cavity, and a spheroidal shell of fluid. Slow growth and decay, as predicted by linear stability theory, are measured in our experiments, confirming the existence of parametric fluid instability in both geometries. Our results are shown to be consistent with Earth's fluctuating paleomagnetic field. Furthermore, parametric instabilities can drive a dynamo throughout Earth's history: with and without the presence on an inner core, and thus escape the problem of the geodynamo predating a compositional convective process. Finally, elliptical instability may also provide a means to explain the ancient Martian dynamo through periodic tidal strain from a co-orbiting body.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSMGP11D..04B
- Keywords:
-
- 1213 Earth's interior: dynamics (1507;
- 7207;
- 7208;
- 8115;
- 8120);
- 1507 Core processes (1213;
- 8115);
- 1510 Dynamo: theories and simulations;
- 1535 Reversals: process;
- timescale;
- magnetostratigraphy;
- 1595 Planetary magnetism: all frequencies and wavelengths