Energy Fluxes and Ohmic Dissipation in the Earth’s Core

Simulations have shown that convection-driven geodynamo models can produce reversing magnetic fields of a similar strength to those of the geomagnetic field. These simulations suggest that the ohmic dissipation arising from the geodynamo is not due solely to the largest scale harmonics of the magnetic field, but is dominated by contributions from smaller scale components. It is argued that, in order to produce a vigorous reversing dynamo resembling the geodynamo, ohmic dissipation of between 1 and 2 TW is required. The energy and entropy fluxes through the core are investigated to see under what circumstances the required ohmic dissipation can be produced. The estimates for these fluxes given in Braginsky and Roberts (1995) are revised in the light recent theoretical and experimental advances in high pressure physics. In the absence of radioactive heating in the core, the available thermal and gravitational energy sources can produce more than 1 TW of ohmic dissipation provided the inner core is not significantly older than 1.2 Gyr. If the inner core is much older than this, the thermal and gravitational energy sources are released at too slow a rate to give the required amount of ohmic dissipation. Paleomagnetic evidence suggests that a geomagnetic field of approximately today’s strength has been in existence for over 3 Gyr, perhaps before the inner core came into existence. Estimates of the available energy sources based on current estimates of the heat flux passing through the core-mantle boundary suggest that they may be insufficient to sustain such a field. We consider the possibility that significant radioactivity in the core could resolve this difficulty.