Date of this Version
Transactions of the Nebraska Academy of Sciences, vol. 5 (1978)
Several models of the dynamo that generates the magnetic field of the earth are based on the assumption of a convective flow in the liquid portion of the earth's outer core that possesses non-vanishing helicity (Busse, 1978; Moffatt, 1978). Helicity is a measure of the strength of flow and its twist: the helicity density is the scalar product of the velocity of the flow and its vorticity. The vorticity is the curl of the velocity and thus gives a measure of the twist around the direction of motion. A laboratory model, the polar pattern predictor, that demonstrates several qualitative flow features, including helicity expected to be associated with the flow of the geodynamo, is exhibited here.
Busse (1975 and 1976) performed quasilinear calculations to indicate the probable form of convective motion in the earth's interior that might be associated with the geodynamo. Busse and Carrigan (1976) actually modeled the flow in a rotating annulus and established the presence of the cylindrical convective rolls. These rolls are the dominant feature in the circulation outside a cylinder inscribing the solid inner core. However, such flows do not have net helicity, and secondorder effects, such as Ekrnan suction, are required to give secondary flows capable of dynamo action (Busse, 1970, 1973, and 1976; Moffatt , 1978:313-318).