Polar Motion Research Articles

Earth rotation variations observed by VLBI and the Wettzell “G” ring laser during the CONT17 campaign

Abstract. The VLBI (Very Long Baseline Interferometry) technique can provide the full set of parameters needed for the transformation between celestial and terrestrial reference frames with high accuracy. Yet it has some limitations regarding temporal resolution and continuity, and the accuracy of the resulting Earth Orientation Parameters (EOP) varies depending on the network geometry. In this work we explore the benefit of combining VLBI observations with the measurements of the large ring laser gyroscope “G” in Wettzell for deriving highly resolved ERP (Earth Rotation Parameters, i.e. polar motion and universal time variations, δUT1). We examine the observations collected by two simultaneously operating VLBI networks during the 15 d of the CONT17 campaign. These two networks, of 14 globally distributed telescopes each, were designed for the estimation of Earth rotation variations, for which reason the resulting hourly ERP are appointed as benchmark in this investigation. To evaluate the advantage of a VLBI and ring laser combined solution, we create degraded versions of the original networks, containing only six stations. The ERP derived from those sparse networks and from the VLBI sparse plus ring laser solutions are then compared in terms of differences to the reference values. It should certainly be considered that these are relative numbers, since they are also determined by the number and selection of the stations remaining in the sparse networks. The root mean square of the difference to the benchmark is reduced by 24 % in case of δUT1 from one network. The polar motion yp component from the same network moves 14 % closer to the reference value due to the inclusion of the ring laser data. The impact on xp and on all ERP from the other network ranges between 2 % and 9 %. The research again confirms the feasibility and also the potential gain of a combined evaluation of VLBI and ring laser observations, but the full capacity of such a sensor fusion will emerge once the ring laser gyroscopes reach a level of accuracy similar to VLBI.

Selecting data for autoregressive modeling in polar motion prediction

The Least-squares extrapolation of harmonic models and autoregressive (LS + AR) prediction is currently considered to be one of the best prediction model for polar motion parameters. In this method, LS fitting residuals are treated as data to train an AR model. But it is readily known that using too many data will result in learning a badly relevant AR model, implying increasing the model bias. It can also be possible that using too few data will result in a lower estimation accuracy of the AR model, implying increasing the model variance. So selecting data is a critical issue to compromise between bias and variance, and hence to obtain a model with optimized prediction performance. In this paper, an experimental study is conducted to check the effect of different data volume on the final prediction performance and hence to select an optimal data portion for AR model. The earth orientation parameters products released by the International Earth Rotation and Reference Systems Service were used as primary data to predict changes in polar motion parameters over spans of 1–500 days for 800 experiments. The experimental results showed that although the short term prediction were not ameliorated, but the method that the AR model parameters calculated by appropriate data volume can effectively improve the accuracy of long-term prediction of polar motion.

Nutation terms adjustment to VLBI and implication for the Earth rotation resonance parameters

SUMMARY The nutation harmonic terms are commonly determined from celestial pole offset series produced from very long baseline interferometry (VLBI) time delay analysis. This approach is called an indirect approach. As VLBI observations are treated independently for every session, this approach has some deficiencies such as a lack of consistency in the geometry of the session. To tackle this problem, we propose to directly estimate nutation terms from the whole set of VLBI time delays, hereafter referred as a direct approach, in which the nutation amplitudes are taken as global parameters. This approach allows us to reduce the correlations and the formal errors and gives significant discrepancies for the amplitude of some nutation terms. This paper is also dedicated to the determination of the Earth resonance parameters, named polar motion, free core nutation, and free inner core nutation. No statistically significant difference has been found between the estimates of resonance parameters based upon ‘direct’ and ‘indirect’ nutation terms. The inclusion of a complete atmospheric-oceanic non-tidal correction to the nutation amplitudes significantly affected the estimates of the free core nutation and the free inner core nutation resonant frequencies. Finally, we analyzed the frequency sensitivity of polar motion resonance and found that this resonance is mostly determined by the prograde nutation terms of period smaller than 386 d.

The radioscience LaRa instrument onboard ExoMars 2020 to investigate the rotation and interior of mars

LaRa (Lander Radioscience) is an experiment on the ExoMars 2020 mission that uses the Doppler shift on the radio link due to the motion of the ExoMars platform tied to the surface of Mars with respect to the Earth ground stations (e.g. the deep space network stations of NASA), in order to precisely measure the relative velocity of the lander on Mars with respect to the Earth. The LaRa measurements shall improve the understanding of the structure and processes in the deep interior of Mars by obtaining the rotation and orientation of Mars with a better precision compared to the previous missions. In this paper, we provide the analysis done until now for the best realization of these objectives. We explain the geophysical observation that will be reached with LaRa (Length-of-day variations, precession, nutation, and possibly polar motion). We develop the experiment set up, which includes the ground stations on Earth (so-called ground segment). We describe the instrument, i.e. the transponder and its three antennas. We further detail the link budget and the expected noise level that will be reached. Finally, we detail the expected results, which encompasses the explanation of how we shall determine Mars' orientation parameters, and the way we shall deduce Mars' interior structure and Mars’ atmosphere from them. Lastly, we explain briefly how we will be able to determine the Surface platform position.

Combination of geo-magnetic jerks with updated ESMGFZ effective angular momentum functions for the modelling of polar motion excitation

Combination of geo-magnetic jerks with updated ESMGFZ effective angular momentum functions for the modelling of polar motion excitation

Attractiveness of thermally different, uniformly black targets to horseflies: Tabanus tergestinus prefers sunlit warm shiny dark targets.

From a large distance tabanid flies may find their host animal by means of its shape, size, motion, odour, radiance and degree of polarization of host-reflected light. After alighting on the host, tabanids may use their mechano-, thermo-, hygro- and chemoreceptors to sense the substrate characteristics. Female tabanids prefer to attack sunlit against shady dark host animals, or dark against bright hosts for a blood meal, the exact reasons for which are unknown. Since sunlit darker surfaces are warmer than shady ones or sunlit/shady brighter surfaces, the differences in surface temperatures of dark and bright as well as sunlit and shady hosts may partly explain their different attractiveness to tabanids. We tested this observed warmth preference in field experiments, where we compared the attractiveness to tabanids (Tabanus tergestinus) of a warm and a cold shiny black barrel imitating dark hosts with the same optical characteristics. Using imaging polarimetry, thermography and Schlieren imaging, we measured the optical and thermal characteristics of both barrels and their small-scale models. We recorded the number of landings on these targets and measured the time periods spent on them. Our study revealed that T. tergestinus tabanid flies prefer sunlit warm shiny black targets against sunlit or shady cold ones with the same optical characteristics. These results support our new hypothesis that a blood-seeking female tabanid prefers elevated temperatures, partly because her wing muscles are more rapid and her nervous system functions better (due to faster conduction velocities and synaptic transmission of signals) in a warmer microclimate, and thus, she can avoid the parasite-repelling reactions of host animals by a prompt take-off.

Modeling and analysis of the Earth pole motion with nonstationary perturbations

Using a numerical-analytical approach, the Earth pole motion is considered under nonstationary perturbations. A method of refined modeling of the Earth pole motion is proposed, which allows to improve the accuracy of the short-term forecast during the occurrence of irregular effects due to changes in the amplitudes of the fundamental harmonics of the oscillatory process.

Correcting surface loading at the observation level: impact on global GNSS and VLBI station networks

Time-dependent mass variations of the near-surface geophysical fluids in atmosphere, oceans and the continental hydrosphere lead to systematic and significant load-induced deformations of the Earth’s crust. The Earth System Modeling group of Deutsches GeoForschungsZentrum (ESMGFZ) provides vertical and horizontal surface deformations based on numerical models of the global geophysical fluids in atmosphere, oceans and the continental hydrosphere with a spatial resolution of $$0.5^\circ $$ and a temporal sampling of down to 3 h (Dill and Dobslaw in J Geophys Res 118(9):5008–5017, 2013. https://doi.org/10.1002/jgrb.50353). The assessment of conventionally—i.e. without consideration of non-tidal loading models—processed global GNSS datasets reveals that large parts of the residual station coordinates are indeed related to surface loading effects. Residuals explained by the models often have a pronounced annual component, but variability at other periodicities also contributes to generally high correlations for 7-day averages. More than 10 years of observations from about 400 GNSS and 33 VLBI stations were specifically reprocessed for this study to incorporate non-tidal loading correction models at the observation level. Comparisons with the corresponding conventional processing schemes indicate that the coordinate repeatabilities and residual annual amplitudes decrease by up to 13 mm and 7 mm, respectively, when ESMGFZ’s loading models are applied. In addition, the standard deviation of the daily estimated vertical coordinate is reduced by up to 6.8 mm. The network solutions also allow for an assessment of surface loading effects on GNSS satellite orbits, resulting in radial translations of up to 4 mm and Earth orientation parameters (EOP). In particular, the VLBI-based EOP estimates are critically susceptible to surface loading effects, with root-mean-squared differences reaching of up to 0.2 mas for polar motion, and 10 µs for UT1-UTC.

Determining and Evaluating the Hydrological Signal in Polar Motion Excitation from Gravity Field Models Obtained from Kinematic Orbits of LEO Satellites

This study evaluates the gravity field solutions based on high-low satellite-to-satellite tracking (hl-SST) of low-Earth-orbit (LEO) satellites: GRACE, Swarm, TerraSAR-X, TanDEM-X, MetOp-A, MetOp-B, and Jason 2, by converting them into hydrological polar motion excitation functions (or hydrological angular momentum (HAM)). The resulting HAM series are compared with the residuals of observed polar motion excitation (geodetic residuals, GAO) derived from precise geodetic measurements, and the HAM obtained from the GRACE ITSG 2018 solution. The findings indicate a large impact of orbital altitude and inclination on the accuracy of derived HAM. The HAM series obtained from Swarm data are found to be the most consistent with GAO. Visible differences are found in HAM obtained from GRACE and Swarm orbits and provided by different processing centres. The main reasons for such differences are likely to be different processing approaches and background models. The findings of this study provide important information on alternative data sets that may be used to provide continuous polar motion excitation observations, of which the Swarm solution provided by the Astronomical Institute, Czech Academy of Sciences, is the most accurate. However, further analysis is needed to determine which processing algorithms are most appropriate to obtain the best correspondence with GAO.

Reassessment of electromagnetic core-mantle coupling and its implications to the Earth's decadal polar motion

The observed Earth's polar motion on decadal time scales has long been conjectured to be excited by the exchange of equatorial angular momentum between the solid mantle and the fluid outer core, via the mechanism of electromagnetic (EM) core-mantle coupling. However, past estimations of the EM coupling torque from surface geomagnetic observations is too weak to account for the observed decadal polar motion. Our recent estimations from numerical geodynamo simulations have shown the opposite. In this paper, we re-examine in detail the EM coupling mechanism and the properties of the magnetic field in the electrically conducting lower mantle (characterized by a thin D″-layer at the base of the mantle). Our simulations find that the toroidal field in the D″-layer from the induction and convection of the toroidal field in the outer core could be potentially much stronger than that from the advection of the poloidal field in the outer core. The former, however, cannot be inferred from geomagnetic observations at the Earth's surface, and is missing in previous EM torque estimated from geomagnetic observations. Our deduction suggests further that this field could make the actual EM coupling torque sufficiently strong, at approximately 5 × 1019 Nm, to excite, and hence explain, the decadal polar motion to magnitude of approximately 10 mas.

A Model of the Motion of the Earth Pole Taking Into Account Lunar-Solar Perturbations

In the framework of the celestial-mechanical approach, a long-period forecasting model of the pole motion is developed, taking into account perturbations from the Sun and the Moon with periods of one year and 18.61 years, respectively. The refined model takes into account the dynamic effects of the spatial motion of the Earth-Moon system in the oscillatory process of the Earth’s pole. Using numerical-analytical modeling, the possibilities of identifying the parameters of the Earth’s pole oscillation and approximating the developed refined model to real trajectory measurements are considered. Estimates of the accuracy of forecasting the motion of the pole are given taking into account additional terms caused by the lunar perturbation.

Seismic Effects on the Secular Drift of the Earth's Rotational Pole

AbstractEarthquake‐induced mass redistribution in the Earth excites the polar motion; its cumulative coseismic effect has been found to cause a secular polar drift (SPD) toward ~140°E longitude with strong statistical tendency. Here we find numerically the cumulatively coseismic effect in SPD since 1952 to be at the rate of ~0.75 mas/year (or ~2.3 cm/year), amounting to nearly 20% of the observed SPD that points to the opposite geographical direction and hence is significant in the pursuit of understanding the source budget of SPD. We further argue on theoretical and observational ground that such behavior reflects that of the overall plate tectonic motion and in fact accounts for a fraction of the latter over long term. The exact amount of the fraction is indeterminate until mass transport models of plate tectonics prove adequate. This viewpoint is in contrast to that of Cambiotti et al. (2016, https://doi.org/10.1093/gji/ggw077) which required the coseismic effect to get annihilated completely by the interseismic effect under their earthquake cycle decomposition of the velocity field at the faulting system.

Jurassic Monster Polar Shift Confirmed by Sequential Paleopoles From Adria, Promontory of Africa

AbstractJurassic paleomagnetic data from North America have long been contentious, generating ambiguities in the shape of the global‐composite apparent polar wander path. Here we show from a restudy of two subdivisions of the Late Jurassic Morrison Formation at the classic locality at Norwood on the Colorado Plateau that the derived paleopoles reflect variable overprinting probably in the Cretaceous and are of limited value for apparent polar wander determination. We instead assembled an updated set of Jurassic paleopoles from parauthocthonous Adria, the African promontory, using primary paleomagnetic component directions derived from stratigraphically superposed intervals and corrected for sedimentary inclination error. These paleopoles are found to be in superb agreement with independent igneous paleopoles from the literature across the so‐called Jurassic monster polar shift, which in North American coordinates is a jump of ~30° arc distance from the 190‐ to 160‐Ma stillstand pole at 79.5°N 104.8°E to a 148 ± 3.5‐Ma pole at 60.8°N 200.6°E defined by four Adria sedimentary paleopoles and the published Ithaca, Hinlopenstretet, and Swartsruggens‐Bumbeni igneous paleopoles. The implied high rate of polar motion of ~2.5°/Myr across the monster shift is compatible with maximum theoretical estimates for true polar wander. We include a critique of published Jurassic paleomagnetic data that have been variably used in reference APWPs but that as a result of their low quality muted the real magnitude of the Jurassic monster shift. Finally, we provide paleocontinental reconstructions to describe examples of the bold signature that the monster polar shift left in the distribution of climate‐sensitive sedimentary facies worldwide.

Chandler Wobble and Free Core Nutation: Theory and Features

Being a torque free motion of the rotating Earth, Chandler wobble is the major component in the Earth’s polar motion with amplitude about 0.05-0.2 arcsec and period about 430-435 days. Free core nutation, also called nearly diurnal free wobble, exists due to the elliptical core-mantle boundary in the Earth and takes almost the whole part of un-modelled variation of the Earth’s pole in the celestial sphere beside precession and nutation. We hereby present a brief summary of their theories and report their recent features acquired from updated datasets (EOP C04 and ECMWF) by using Fourier transform, modelling, and wavelet analysis. Our new findings include (1) period-instability of free core nutation between 420 and 450 days as well as its large amplitude-variation, (2) re-determined Chandler period and its quality factor, (3) fast decrease in Chandler amplitude after 2010.

Coupling between the spin precession and polar motion of a synchronously rotating satellite: application to Titan

We here develop, in an angular momentum approach, a consistent model that integrates all rotation variables and considers forcing both by the central planet and a potential atmosphere. Existing angular momentum approaches for studying the polar motion, precession, and libration of synchronously rotating satellites, with or without an internal global fluid layer (e.g., a subsurface ocean) usually focus on one aspect of rotation and neglect coupling with the other rotation phenomena. The model variables chosen correspond most naturally with the free modes, although they differ from those of Earth rotation studies, and facilitate a comparison with existing decoupled rotation models that break the link between the rotation motions. The decoupled models perform well in reproducing the free modes, except for the Free Ocean Nutation in the decoupled polar motion model. We also demonstrate the high accuracy of the analytical forced solutions of decoupled models, which are easier to use to interpret observations from past and future space missions. We show that the effective decoupling between the polar motion and precession implies that the spin precession and its associated mean obliquity are mainly governed by the external gravitational torque by the parent planet, whereas the polar motion of the solid layers is mainly governed by the angular momentum exchanges between the atmosphere (e.g., for Titan) and the surface. To put into perspective the difference between rotation models for a synchronously rotating icy moon with a thin ice shell and classical Earth rotation models, we also consider the case of the Moon, which has a thick outer layer above a liquid core. We also show that for non-synchronous rotators, the free precession of the outer layer in space degenerates into the tilt-over mode.

Nonchaotic and globally synchronized short-term climatic variations and their origin

Careful computations of atmospheric power spectra are done. These computations reveal that these spectra are continuous in the range of timescales from 2 days to 1 year, and so they confirm that weather variations are chaotic; however, the continuity of a part of the atmospheric power spectra, corresponding to the periods from 2 years to one decade is questioned. This part is prominent by the existence of several narrow bands of increased spectral density centered at the subharmonics 2:1, 3:1, and 4:1 of the Chandler wobble in the Earth’s pole motion (~ 1.2 years); the superharmonics 1:2, 1:3, and 1:4 of the Luni-Solar nutation of the Earth’s rotation axis (~ 18.6 years) as well as the superharmonics 1:2, 1:3, and 1:4 of the 11-year cycle of the Sun spots. The existence of similar bands in the El Nino–Southern Oscillation (ENSO) power spectra was recognized many years ago; however, it turns out that the above spectral bands also are seen in spectra of the atmospheric characteristics outside of tropics. Moreover, the respective climatic variations are globally synchronized. The synchronization takes place because the above-mentioned external periodicities must influence short-term climatic variations everywhere on the Earth. It is very probable that the periods of the external periodicities indicated are incommensurable with each other. Therefore, if these periodicities actually influence short-term climatic variations, they would have to do it discordantly. As a result, no resonances can exist which could make the affected climatic variations to be chaotic. A linear dependence of logarithms of serial numbers of the spectral bands on logarithms of the band magnitudes as well as a linear decrease of the accumulated sum of the squared autocorrelations of the respective atmospheric characteristics confirm that the dynamics of the interannual to decadal climatic variations are not chaotic.

Hydrological signals in polar motion excitation – Evidence after fifteen years of the GRACE mission

We investigate the contribution of continental hydrology signals to polar motion excitation resulting from changes in mass distribution associated with terrestrial water and snow/ice based on a variety of datasets. The impact of the former is less well known than that of other geophysical fluids, namely atmospheric and oceanic angular momentum signals, especially for oscillations over shorter-than-annual periods. The geodetic excitation function of polar motion can be determined from observations, while the hydrological excitation function can be estimated either from global models of the land-based hydrosphere or from indirect observations of variations in the Earth’s gravity field. With more than 15 years of data, the pair of GRACE satellites has provided valid datasets for studying interannual polar motion excitation. Recent results are promising and indicate agreement between GRACE-derived interannual hydrological excitations and the corresponding geodetic observations. Our work focuses on the non-seasonal spectral range, and compares hydrological excitations based on recent GRACE solutions from different centres with signals from climate models, and as a residual from geodetic observations.

Assessing hydrological signal in polar motion from observations and geophysical models

Changes in Terrestrial Water Storage (TWS) due to seasonal changes in soil moisture, ice and snow loading and melting influence the Earth’s inertia tensor. Quantitative assessment of hydrological effects of polar motion remains unclear because of the lack of the observations and differences between various atmospheric and ocean models. We compare the effects of several hydrological excitation functions computed as the difference between the excitation function of polar motion Geodetic Angular Momentum (GAM) and joint atmospheric plus oceanic excitation functions, called geodetic residuals. Geodetic residuals are computed for different Atmospheric Angular Momentum (AAM) and Oceanic Angular Momentum (OAM) models and are analyzed and compared with the hydrological excitation function determined from the Land Surface Discharge Model. They are analyzed on decadal, interannual, seasonal and non-seasonal time scales. The equatorial components of hydrological geodetic excitation functions χ1 and χ2 are decomposed into prograde and retrograde time series by applying Complex Fourier Transform Models. The agreement between hydrological geodetic residuals and excitation functions is validated using Taylor diagrams. This shows that agreement is highly dependent on AAM and OAM models. Errors in these models affect the resulting geodetic residuals and have a strong impact on the Earth’s angular momentum budget.

Terrestrial water storage variations and their effect on polar motion

The role of continental water in polar motion excitation can be illustrated by determining Hydrological Angular Momentum calculated from terrestrial water storage (TWS). In this paper we compare global and regional changes in TWS computed using Coupled Model Intercomparison Project Phase 5 climate models, Global Land Data Assimilation System (GLDAS) land hydrology models and observations from the Gravity Recovery and Climate Experiment (GRACE) satellite mission. We also compare hydrological excitation functions derived from models with those obtained from the GRACE mission and the hydrological signal in observed polar motion excitation (the so-called geodetic residuals). The results confirm that GLDAS models of seasonal and non-seasonal TWS change are more consistent with GRACE data than climate models; on the other hand, none of the considered models are fully consistent with GRACE data or geodetic residuals. In turn, GRACE observations are most consistent with the non-seasonal hydrological signal in observed excitation. A detailed study of the contribution of different TWS components to the hydrological excitation function shows that soil moisture dominates.

Mass-related excitation of polar motion: an assessment of the new RL06 GRACE gravity field models

The new Release-06 (RL06) Gravity Recovery and Climate Experiment (GRACE) gravity field solutions are evaluated by converting them into equatorial effective angular momentum functions (so-called excitation functions) for polar motion and comparing these to respective time series based on space-geodetic observations (geodetic excitation). The same is performed for the older RL05 solutions using identical processing. Maps of equivalent water heights derived from both releases show that the signal-to-noise ratio is significantly improved in RL06. The derived polar motion excitation functions from RL05 and RL06 differ by about 15%. An analysis of the contributions of different Earth subsystems revealed that the release update mainly influenced the hydrological (12%) and oceanic excitations (17%), but it has a relatively small impact on the cryospheric excitations related to Antarctica (4%) and Greenland (1%). The RL06 data from different GRACE processing centers are more consistent among each other than the previous RL05 data. Comparisons of the GRACE-based excitation functions with the geodetic and model-based oceanic excitations show that the latest release update improved the agreement by about 2 to 15 percentage points.