Terms of Reference

SC 3.1 addresses the entire range of tidal phenomena, non-tidal loading and dynamics of the Earth, both at the theoretical as well as at the observational level. The Earth tide affects many types of high precision instrumentation, be it measurements of position, deformation, potential field or acceleration. The tidal phenomena influence both terrestrial and satellite-borne acquisitions. The tidal potential is a driving force that is accurately calculated. The tidal response observable as deformation and variations in Earth orientation and rotation parameters, gives information on Earth’s rheology. Instruments sensitive enough to detect the tidal signal, record a large range of periodic and aperiodic phenomena such as ocean and atmospheric tidal loading, ocean, atmospheric and hydrospheric non-tidal effects, mass redistributions and deformation related to the earthquake cycle and even to gravitational waves, as well as plate tectonics and intraplate deformation. The periods range from seismic normal modes over to the Earth tides and the Chandler Wobble and beyond, ending at the long periods of nutation-precession. Thus, the time scales range from seconds to decennia, and for the spatial scales from local, continental to global dimensions. As tidal friction is affecting Earth’s rotation, all the physical properties of the Earth contribute to the explanation of this phenomenon. Therefore, the research on tidal deformation due to changes of the tidal potential as well as ocean and atmospheric loading are a prerequisite to constrain Earth’s rheological properties. Further, direct and indirect tidal phenomena affect the position of fiducial sites and have to be corrected to provide accurate spatial referencing. Such referencing is needed for the observation and monitoring of changes of the Earth’s surface at global, regional and local scales. Therefore, there is a considerable contribution of tidal research to global geodynamics and climate change by providing important constraints to geophysical models. Modern instrumental developments for which tidal phenomena are relevant are gravimeters and gradiometers based on superconductivity, atom interferometry, micro-electromechanical-system (MEMS) gravimeters, Inertial Measurement Units, gravitational wave antennas, satellite gravimetry and atomic clocks. The improvements in gravimetric instrumentation leads to the use of gravimetry as a tool to detect underground mass changes, as naturally occurring hydrologic draughts or fluids injected into the underground for the purpose of temporary storage for instance. The Earth is a complex dynamic system interacting with its fluid surficial layers. The Earth is then studied in terms of its global gravity field and its temporal variations, as well as its surface deformation in order to characterize the Earth’s internal structure and dynamics. In the next few years, instrumental developments in portable absolute gravimeters are expected, and further innovations can be envisaged from the atomic interferometry technology. The SC 3.1 will follow the instrumental developments and infer innovative applications. These geophysical observations together with other geodetic observations and geological information provide the means to better understand the structure, dynamics and evolution of the Earth system. The existence of a network of superconducting gravimeters (SGs) allows continuous monitoring of the gravity signal at selected stations with a precision of better than 10-10 m/s2. The range of applications of SG measurements has become very wide and applicable not only to Earth tides investigations, but also to support studies on Earth’s seismic cycle and hydrological mass estimates. The SG network has had scientifically close relation to the SC 3.1 and IGETS (International Geodynamics and Earth Tide Service), which distributes the data. Therefore, the Chair of SC 3.1 is responsible for the close cooperation with the IGETS to provide effective service-with science coupling.