From stable dipolar towards reversing numerical dynamos

By Carsten Kutzner, U. Christensen. 2002

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We are using a three-dimensional convection-driven numerical dynamo model without hyperdiffusivity to study the characteristic structure and time variability of the magnetic field in dependence of the Rayleigh number (Ra) for values up to 40 times supercritical. We also compare a variety of ways to drive the convection and basically find two dynamo regimes. At low Ra, the magnetic field at the surface of the model is dominated by the non-reversing axial dipole component. At high Ra, the dipole part becomes small in comparison to higher multipole components. At transitional values of Ra, the dynamo vacillates between the dipole-dominated and the multipolar regime, which includes excursions and reversals of the dipole axis. We discuss, in particular, one model of chemically driven convection, where for a suitable value of Ra, the mean dipole moment and the temporal evolution of the magnetic field resemble the known properties of the Earth’s field from paleomagnetic data.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Molecular Biology

View Paper: https://www.semanticscholar.org/paper/e865f992341eca9c324237535f9a241260b96e63

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Citations in the corpus

The influence of potassium on core and geodynamo evolution (2003, background): “Further numerical simulations of geodynamo activity (e.g. Kuang & Bloxham 1997; Glatzmaier 2002; Kutzner & Christensen 2002) may be helpful in resolving these questions.”
Scaling properties of convection-driven dynamos in rotating spherical shells and application to planetary magnetic fields (2006, background): “Kutzner & Christensen (2002) found that the dipolar dynamo regime gives way to a class of dynamos that generate small-scale magnetic fields when the Rayleigh number is sufficiently increased with all other parameters held constant.”
Conditions for Earth-like geodynamo models (2010, background): “…from within (or secular cooling, which is equivalent) tends to generate less dipolar fields than are found inmodels that are driven from below (Kutzner and Christensen, 2002; Olson and Christensen, 2006), although the choice of a fixed flux condition instead of a fixed temperature condition…”
Modelling the palaeo-evolution of the geodynamo (2009, methodology): “This can be accounted for within our framework by adding a volumetric correction term ST/ξ in (4), such that the mass anomaly budget of the spherical shell vanishes (Braginsky & Roberts 1995; Kutzner & Christensen 2002).”
The time-averaged magnetic field in numerical dynamos with non-uniform boundary heat flow (2002, methodology): “Magnetic reversals have been obtained with the model we use here, but at significantly larger Rayleigh numbers, where the dipole field is more time variable (Kutzner & Christensen 2002).”
The magnetic structure of convection-driven numerical dynamos (2008, methodology): “The control parameters in models C and T have been selected in the literature (respectively from Kutzner & Christensen 2002; Wicht 2005) for the similarity of some of the model outputs with what is known of the geomagnetic and palaeomagnetic field: a reasonably Earth-like magnetic Reynolds number…”
Dipole moment scaling for convection-driven planetary dynamos (2005, background): “The transition may be abrupt in some cases and gradual in others, as depicted by the solid and dashed lines in Fig. 4. Dynamo modeling studies have shown that dipole reversals are frequent in the multipolar regime, and in the dipolar regime they often occur near this transition [ 27 ,28].”
Mean-field concept and direct numerical simulations of rotating magnetoconvection and the geodynamo (2006, unknown): “In addition, the time-dependence of the dipole moment, including secular variations, excursions and reversals, resembles the observed Earth’s magnetic field (Glatzmaier and Roberts 1995b, Kutzner and Christensen 2002, Wicht and Olson 2004).”
Steady zonal flows in spherical shell dynamos (2005, background): “The zonal flow structure is robust against variations of Ra until the regime identified by Kutzner & Christensen (2002) where the magnetic dipole collapses.”