In this introduction, we present the motivation for an expansion of activities of the International Earth Rotation Service (IERS) as discussed at a workshop held at the Paris Observatory in October, 1996 (Reigber and Feissel, 1997). The proposed new activities include the monitoring of geodetically important properties of the oceans, the liquid core, and terrestrial water, to supplement present efforts related to atmospheric angular momentum (AAM) variation. The IERS, founded in 1988, coordinates the observations and estimates of Earth Orientation Parameters (EOP), including Precession and Nutation, Polar Motion (PM), and variations in the Length Of Day (LOD) or variations in Universal Time UT1. IERS activities continue a century-old tradition of international cooperation in the determination of the EOP, relying, in the absence of formal funding, on a variety of national agencies responsible for time keeping, positioning, and scientific research. A common theme in IERS activities is the use of the techniques of space geodesy, including Very Long Baseline Interferometry (VLBI), radio frequency ranging to satellites (e.g., Global Positioning System GPS, Doppler Orbite determination and Radio positioning Integrated on Satellite DORIS), and laser ranging to artificial satellites (SLR) and to the moon (LLR). This has required that the IERS be engaged in both the computation of the EOP time series, and improvements in the fundamental space geodetic observations. Consequently, the IERS is presently involved in the publication of geodetic standards, the realization of international terrestrial and celestial reference frames, and the monitoring of AAM changes, a major cause of Earth rotation variations.
The EOP observations supplied by the IERS provide fundamental information about the physical properties of the Earth's interior, and about variability of its fluids, including the atmosphere, oceans, water stored in various forms on land, and the liquid core. Given the success of the SBAAM, (Special Bureau for the AAM) expansion of IERS activities into other areas is not only natural as a means of understanding the observed EOP, but essential to fulfill the IERS mission of improving the underlying space geodetic observations. The October 1996 IERS workshop developed a recommendation, later ratified by the IERS directing board, that the IERS should coordinate monitoring of the angular momentum of various geophysical fluids, in addition to the atmosphere. A call for participation (deadline: 30 June 1997) in this activity has been recently issued, and requests, for a trial period, the participation of research groups involved in the modeling and monitoring of the various geophysical fluids discussed above (for more information, visit the ftp site at Central Bureau of the IERS).
At present, geophysical fluids, aside from the atmosphere, lack operational centers analogous to weather forecast centers, but the IERS wants nevertheless, to be active, rather than passive in seeking collaborations and coordination in these new areas. Collaboration is bound to benefit both the studies of the EOP and of the various geophysical fluids. Routine calculation of geodetic quantities in the process of numerical modeling of the core, mantle, hydrologic cycle, oceans, and atmosphere should enrich and improve numerical model assessment and performance, and lead to improved understanding of variations in the orientation of the Earth in space.
The Special Bureau for the Core (this web-site) is in that frame and is part of the Product Center for Global Geophysical Fluids (CGGF). The particular motivations of this Special Bureau are related to the fact that, for instance, at decade time scales, LOD and geomagnetic field variations appear to be correlated, probably reflecting Core Angular Momentum (CAM) transfer to the mantle. It has been suggested that the main mechanism for this transfer is either a topographic coupling (the pressure or mountain torque due to the dynamic pressure acting on bumps on the core-mantle boundary) or an electromagnetic coupling (advective torque related to the Lorentz force at the core-mantle boundary); gravitational and viscous torques are usually considered as minor actors in the core-mantle coupling at long time scale. In any case, fluid core variability is an active area of investigation within the geophysical community, as indicated by recently published numerical simulations of the geodynamo and the seismological discovery of an inner core super rotation. Studies such as these have natural linkages to the observational programs of the IERS.