Geomagnetism

Core surface flow

The core has a radius of 3480 km, which is a little more than half of the radius of the Earth. It consists of a solid inner core (radius of 1215 km) and a liquid outer core. The core is composed of 90% iron. It is a fairly good electrical conductor, with a conductivity estimated at 300,000 Siemens/m. To a good approximation (details) , the flow of the core fluid carries along the magnetic field, as if it were frozen in. This leads to the gradual changes observed in the geomagnetic field at the Earth's surface. These changes are called secular variation of the magnetic field. Under further simplifying conditions, one can invert the observed secular variation for flow at the surface of the outer core. Figure-1 shows a map of the flow inferred from the magnetic field model POMME-3.1 under the assumption that the flow is purely horizontal, meaning that there are no up- and downwellings of the flow.

Fig.1: (click to enlarge) Motion of the liquid iron at the surface of the Earth's core can be inferred from the changes in the main field observed at the Earth's surface. Typical velocities are of the order of tens of kilometers per year

An intriguing application of core flow modeling is that it may help to improve the forecast of the secular variation in geomagnetic field models used for navigation. The idea is to first determine the present flow field and then let the flow carry the field forward in time to predict it at a future time. The possibility of such an approach has been shown by Maus, Silva and Hulot . The main difficulty in this scheme is that the magnetic field experiences occasional sharp changes in its dynamic behavior (so-called jerks), which are not yet well understood.

Available downloads
	Type	Format	Mbyte	Contents
	Article	PDF	0.4	Can core-surface flow models be used to improve the forecast of the Earth's main magnetic field?
	Article	PDF	0.2	On the validity of the frozen flux assumption in core flow modeling

Stefan Maus