CHAMP, Mass Displacements, and the Earth’s Rotation

Abstract

The angular momentum of the atmosphere and oceans changes as both the distribution of mass within the atmosphere and oceans changes and as the direction and speed of the winds and currents change. Since, in the absence of external torques, the angular momentum of the solid Earth-atmosphere-ocean system is conserved, the changing atmospheric and oceanic angular momentum will cause the solid Earth’s angular momentum to change, or, in other words, will cause the Earth’s rotation to change. The changing distribution of mass within the atmosphere and oceans also causes the Earth’s gravitational field to change, an effect being measured by the CHAMP satellite mission. By measuring changes in the second-degree spherical harmonic coefficients of the Earth’s gravitational field, which are related to the elements of the Earth’s inertia tensor, CHAMP is, in effect, directly measuring changes in the Earth’s rotation caused by mass redistribution. Thus, independent measurements of the Earth’s rotation can, in principle, be used to validate the time-varying second-degree coefficients of the Earth’s gravitational field measured by CHAMP after the effects of winds and currents on the Earth’s rotation are removed. In addition, and perhaps surprisingly, the effects of sufficiently large earthquakes on the trace of the Earth’s inertia tensor must also be removed from length of day measurements before comparing them to observed changes in the second-degree zonal gravitational field coefficient.