Oscillation of the Earth's inner core and its relation to the generation of geomagnetic field

Abstract

The solid inner core of the earth, which is situated in the center of the fluid outer core, may well be excited to a state of oscillation by an external agency such as an earthquake. By using the current representative value of the density contrast of the inner and outer cores, 0.3 g/cm3, and the less certain viscosity of the outer core at the inner core boundary, 0.1–1 poise, the numerical evaluation of the equation of motion for the core system gives a period of oscillation of the inner core of about 7.4 hours and a decay time of oscillation of the order of 1000–10,000 years. Because of the rotation of the earth, the inner core, when it is oscillating, also precesses at a period of about 36 hours and results in three eigenfrequencies of the oscillation of the inner core. By observing this spectral splitting, the density contrast of the inner and outer cores can be uniquely determined. The chance of experimentally verifying the oscillation of the inner core by means of an ultrasensitive tidal gravity meter appears to be promising. Although the amplitude of the oscillation turns out to be extremely small, it is shown here that the oscillation can generate the velocity field within the outer core that is required by present geomagnetic dynamo theory. The decay rate of the main geomagnetic field can also be explained by considering the decay of the inner core oscillation.