Degree-2 spherical harmonics of the Earth's gravity field from Earth rotation parameters and SLR measurements to LAGEOS

Abstract

The gravity field of the Earth is fundamental to subjects such as geodesy and geophysics. Many observations within geodesy refer directly or indirectly to gravity. Geodetic techniques provide information regarding the Earth and the processes that act on it. Mass and angular momentum are, according to physics, conserved in a closed system. The Earth interacts very little with components outside of it and can be thought of as a closed system. Mass components in one reservoir of the Earth system are exchanged with others. Mass redistribution within the Earth system is caused by geophysical processes. This movement of geophysical fluid (mass) causes variations in the Earth’s rotation, gravity field and geocentre. The improvement of geodetic techniques over the last few decades allows us to measure the effects of these processes on the Earth to an unprecedented accuracy. Earth rotation parameters (ERPs) are excited by variations in the mass distribution on the Earth’s surface and the exchange of angular momentum between the atmosphere and oceans and the solid Earth. The same mass redistribution causes temporal changes in the gravity field coefficients with the second degree harmonics related to the rotational deformation and hence to changes in the Earth’s inertial tensor. If precise models of the atmospheric and oceanic angular momentum are available solution for polar motion and degree-2 Stokes harmonics can be unified. In this study we utilise SLR tracking of LAGEOS to compare (i) degree-2 harmonics from ERPs and gravitation, and (ii) LAGEOS excitation functions and geophysical data (mass + motion). To what extent a unified approach is possible with current models for AM data and gravity mass change estimated from ERP within orbit determinations is investigated. Finally, the ability of SLR to calculate the motion of the Earth’s geocentre is also investigated.