Variations in Earth's rotation (defined in terms of length of day) arise from external tidal torques, or from an exchange of angular momentum between the solid Earth and its fluid components. On short timescales (annual or shorter) the non-tidal component is dominated by the atmosphere, with small contributions from the ocean and hydrological system. On decadal timescales, the dominant contribution is from angular momentum exchange between the solid mantle and fluid outer core. Intradecadal periods have been less clear and have been characterized by signals with a wide range of periods and varying amplitudes, including a peak at about 6 years (refs 2-4). Here, by working in the time domain rather than the frequency domain, we show a clear partition of the non-atmospheric component into only three components: a decadally varying trend, a 5.9-year period oscillation, and jumps at times contemporaneous with geomagnetic jerks. The nature of the jumps in length of day leads to a fundamental change in what class of phenomena may give rise to the jerks, and provides a strong constraint on electrical conductivity of the lower mantle, which can in turn constrain its structure and composition.

In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time‐variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground‐based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial body and Earth attractions, to the mass in the direct vicinity of the instruments, and to vertical displacement of the instrument itself on the ground. In this paper, we review the geophysical questions that can be addressed by ground gravimeters used to monitor time‐variable gravity. This is done in relation to the instrumental characteristics, noise sources, and good practices. We also discuss the next challenges to be met by ground gravimetry, the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation.

International audienceS U M M A R Y The GRACE satellite mission has been measuring the Earth's gravity field and its temporal variations since 2002 April. Although these variations are mainly due to mass transfer within the geofluid envelops, they also result from mass displacements associated with phenomena including glacial isostatic adjustment and earthquakes. However, these last contributions are difficult to isolate because of the presence of noise and of geofluid signals, and because of GRACE's coarse spatial resolution (>400 km half-wavelength). In this paper, we show that a wavelet analysis on the sphere helps to retrieve earthquake signatures from GRACE geoid products. Using a wavelet analysis of GRACE geoids products, we show that the geoid variations caused by the 2004 December (M w = 9.2) and 2005 March (M w = 8.7) Sumatra earthquakes can be detected. At GRACE resolution, the 2004 December earthquake produced a strong coseismic decrease of the gravity field in the Andaman Sea, followed by relaxation in the area affected by both the Andaman 2004 and the Nias 2005 earthquakes. We find two characteristic timescales for the relaxation, with a fast variation occurring in the vicinity of the Central Andaman ridge. We discuss our coseismic observations in terms of density changes of crustal and upper-mantle rocks, and of the vertical displacements in the Andaman Sea. We interpret the post-seismic signal in terms of the viscoelastic response of the Earth's mantle. The transient component of the relaxation may indicate the presence of hot, viscous material beneath the active Central Andaman Basin

Earth's dynamic oblateness (J2) has been decreasing due to postglacial rebound (PGR). However, J2 began to increase in 1997, indicating a pronounced global-scale mass redistribution within Earth's system. We have determined that the observed increases in J2 are caused primarily by a recent surge in subpolar glacial melting and by mass shifts in the Southern, Pacific, and Indian oceans. When these effects are removed, the residual trend in J2 (-2.9 x 10(-11) year-1) becomes consistent with previous estimates of PGR from satellite and eclipse data. The climatic significance of these rapid shifts in glacial and oceanic mass, however, remains to be investigated.

S U M M A R YWe present an inversion of nutation observations in terms of parameters characterizing the Earth's interior properties. We use a Bayesian inversion in the time-domain, allowing us to take fully into account non-linearities in the nutation model and to reduce the loss of information occurring in frequency-domain inversions. Among the parameters we retrieve are two complex parameters, K CMB and K ICB , referred to as 'coupling constants', characterizing the mechanical coupling at the core-mantle boundary (CMB) and the inner core boundary (ICB), respectively. Based on a joint inversion of nutation observations provided by different analysis centres, we find Im(K CMB ) = (−1.78 ± 0.02) 10 −5 , Re(K ICB ) = (1.01 ± 0.02) 10 −3 and Im(K ICB ) = (−1.09 ± 0.03) 10 −3 (where the errors correspond to 99.7 per cent confidence intervals). While our value of Im(K CMB ) is similar to previous estimates, our new values of Re(K ICB ) and Im(K ICB ) are significantly different. This is mainly because of the different inversion strategy that we use and also because of the lengthier record of observation available. We show that, based on existing coupling models, neither viscous nor electromagnetic coupling alone can explain our new values of Re(K ICB ) and Im(K ICB ). A combination of these two mechanisms is required and necessitates a radial magnetic field at the ICB of total rms strength between 6 and 7 mT and a kinematic viscosity of the fluid core at the ICB should be between 10 and 30 m 2 s −1 , depending on the exact partition between viscous and electromagnetic coupling.

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[1] The Martian atmosphere and the CO 2 polar ice caps exchange mass. This exchange, together with the atmospheric response to solar heating, induces variations of the rotation of Mars. Using the angular momentum budget equation of the system solid-Marsatmosphere-polar ice caps, the variations of Mars' rotation can be deduced from the variations of the angular momentum of the superficial layer; this later is associated with the winds, that is, the motion term, and with the mass redistribution, that is, the matter term. For the ''mean'' Martian atmosphere, without global dust storms, total amplitudes of 10 cm on the surface are obtained for both the annual and semiannual polar motion excited by the atmosphere and ice caps. The atmospheric pressure variations are the dominant contribution to these amplitudes. Length-of-day (lod) variations have amplitudes of 0.253 ms for the annual signal and of 0.246 ms for the semiannual signal. The lod variations are mainly associated with changes in the atmospheric contribution to the mass term, partly compensated by the polar ice cap contribution. We computed lod variations and polar motion for three scenarios having different atmospheric dust contents. The differences between the three sets of results for lod variations are about one order of magnitude larger than the expected accuracy of the NEtlander Ionosphere and Geodesy Experiment (NEIGE) for lod. It will thus be possible to constrain the global atmospheric circulation models from the NEIGE measurements.

S U M M A R YBy subtracting computed atmospheric angular momentum from a time-series for length-of-day variations, we obtain a high-resolution time-series that is useful for studying the effects of the core on length-of-day variations. Features in this time-series are closely correlated with the time at which geomagnetic jerks have been observed, suggesting a role for the core in angular momentum exchange within the Earth system on timescales as short as one year, and that jerks are directly related to the processes responsible for changes in core angular momentum.