The Geophysical Interpretation of Changes in the Length of the Day and Polar Motion

Abstract

The data on the irregular fluctuations in the length of the day and the motion of the pole is of great significance in the geophysicist’s task of constructing a model of the earth’s interior.

Short term instabilities in the dynamo generating the earth’s magnetic field produce what is observed at the surface as the secular variation. These changes induce currents in the lower mantle and the resulting torques appear to be the cause of the irregular fluctuations in the length of the day, although some quantitative problems remain.

The excitation of the Chandler wobble could result from impulsive torques applied to the mantle by very short period (a year or less) local magnetic field disturbances coming to the surface of the core. The alternative mechanism by earthquakes has been much investigated and the possibility of this is still obscure. A test however is available in the polar motion data: a disturbance in its path displaces the subsequent centre of its Chandler motion in the latter theory, but only its amplitude on the former theory. The behaviour of the pole around 1968 supports the core theory, but much more analysis of the polar motion is required.