Seismic Studies of the Earth’s Core

Izvestiya, Physics of the Solid Earth - Abstract—The paper presents the review of the conceptually most important results of seismological studies of the Earth’s core and their...     

Three distinct types of hotspots in the Earth’s mantle

The origin of mantle hotspots is a controversial topic. Only seven (‘primary’) out of 49 hotspots meet criteria aimed at detecting a very deep origin (three in the Pacific, four in the Indo-Atlantic hemisphere). In each hemisphere these move slowly, whereas there has been up to 50 mm/a motion between the two hemispheres prior to 50 Ma ago. This correlates with latitudinal shifts in the Hawaiian and Reunion hotspots, and with a change in true polar wander. We propose that hotspots may come from distinct mantle boundary layers, and that the primary ones trace shifts in quadrupolar convection in the lower mantle.     

Water and partial melting of Earth’s mantle

Water plays a crucial role in the melting of Earth’s mantle. Mantle magmatisms mostly occur at plate boundaries (including subduction zones and mid-ocean ridges) and in some intraplate regions with thermal anomaly. At oceanic subduction zones, water released by the subducted slab may induce melting of the overlying mantle wedge or even the slab itself, giving rise to arc magmatism, or may evolve into a supercritical fluid. The physicochemical conditions for the formation of slab melt and supercritical fluid are still under debate. At mid-ocean ridges and intraplate hot zones, water and CO₂ cause melting of the upwelling mantle to occur at greater depths and in greater extents. Low degree melting of the mantle may occur at boundaries between Earth’s internal spheres, including the lithosphere-asthenosphere boundary (LAB), the upper mantletransition zone boundary, and the transition zone-lower mantle boundary, usually attributed to contrasting water storage capacity across the boundary. The origin for the stimulating effect of water on melting lies in that water as an incompatible component has a strong tendency to be enriched in the melt (i.e., with a mineral-melt partition coefficient much smaller than unity), thereby lowering the Gibbs free energy of the melt. The partitioning of water between melt and mantle minerals such as olivine, pyroxenes and garnet has been investigated extensively, but the effects of hydration on the density and transport properties of silicate melts require further assessments by experimental and computational approaches.     

Impedances for induction soundings of the Earth’s mantle

Determination of impedances is necessary in order to eliminate some shortcoming of our knowledge about structures of the exciting source fields and their fickleness. The experimental impedances for induction soundings result from the impedance boundary conditions or heuristic models. The simplified models give just a rough idea of their domain of applicability. Impedances can depend on many factors, including the exciting field structures of several source types which are present in the period range of the mantle soundings (10⁴−4×10⁸ s). The problem in the mantle investigations arises if impedances measured by different methods have to be jointly inverted in order to essentially prolongate the analyzed period range and hence to increase the reliability and depth of induction soundings on land. The subject of our work is an analysis of the known magnetotelluric and magnetovariation impedances to suggest a physically substantiated approach for their joint inversions.     

Identification of low-frequency kinetic wave modes in the Earth’s ion foreshock

In this work we use ion and magnetic field data from the AMPTE-UKS mission to study the characteristics of low frequency (_{r p}) waves observed upstream of the Earths bow shock. We test the application of various plasma-field correlations and magnetic ratios derived from linear Vlasov theory to identify the modes in this region. We evaluate (for a parameter space consistent with the ion foreshock) the Alfvén ratio, the parallel compressibility, the crosshelicity, the noncoplanar ratio, the magnetic compression and the polarization for the two kinetic instabilities that can be generated in the foreshock by the interaction of hot diffuse ions with the solar wind: the left-hand resonant and the right-hand resonant ion beam instabilities. Comparison of these quantities with the observed plasma-field correlations and various magnetic properties of the waves observed during 10 intervals on 30 October 1984, where the waves are associated with diffuse ions, allows us to identify regions with Alfvénic waves and regions where the predominant mode is the right-hand resonant instability. In all the cases the waves are transverse, propagating at angles 33° and are elliptically polarized. Our results suggest that while the observed Alfvén waves are generated locally by hot diffuse ions, the right-handed waves may result from the superposition of waves generated by two different types of beam distribution (i.e. cold beam and diffuse ions). Even when there was good agreement between the values of observed transport ratios and the values given by the theory, some discrepancies were found. This shows that the observed waves are different from the theoretical modes and that mode identification based only on polarization quantities does not give a complete picture of the waves characteristics and can lead to mode identification of waves whose polarization may agree with theoretical predictions even when other properties can diverge from those of the theoretical modes.     

Seismic response to electromagnetic sounding of the Earth’s lithosphere

The seismic catalogues of 1967?C2008 for the Bishkek geodynamical test site are analyzed for the purpose of studying the response of seismic activity to the electromagnetic sounding of the Earth??s crust during two series of field experiments with high-power controlled sources. The first series of the experiments, which were carried out in 1982?C1990, utilized the pulses provided by a magnetohydrodynamic (MHD) generator. The sounding signals in the second series of the experiments (2000?C2005) were generated by the capacitor-thyristor source ERGU 600-2. In these experiments, temporal variations of the set of statistical parameters characterizing the seismicity, which are typically used in the studies of the background and transient modes of seismicity, were investigated in a selected spatial domain within 150 km from the current electrodes. In terms of time, the analysis was conducted on two levels of detail. The study on a temporal scale of a few years was focused on the variations that preceded, accompanied, and followed the series of the experiments, while the day-scale analysis considered variations that were observed within 10 days after each sounding event. The day-scale analysis yields the following results. The slope of the frequency-magnitude diagram of the earthquakes (b value) during the sounding events is substantially larger than its background value. The slope of the graph gradually becomes gentler within about a day and a half after termination of sounding. The seismic activity slightly enhances during the interval of sounding and abates after its termination to a minimum, which corresponds to the interval of decreasing b value. This character of variations in seismicity differs from the scenario previously established for other transitional seismic regimes. The analysis on a temporal scale of a few years revealed variations in the studied parameters of the seismicity, some of which fall in both sounding intervals of 1983?C1990 and 2000?C2005. However, these variations are not unique; their character and durations suggest their being associated with the processes of preparation and after effects of the strong earthquakes that occurred in the vicinity of the sounding dipole.     

Certain spectral properties of cyclonic turbulence in the Earth’s liquid core

Rapid diurnal rotation of planets results in the appearance of cyclonic thermal turbulence in liquid cores, which is the source of generation of the observed magnetic fields. The model that makes it possible to reproduce characteristic features of small-scale geostrophic flows in the physical and wave spaces is considered in the work. The flows of energy and hydrodynamic helicity as a function of the wavenumber have been estimated. Joint existence of direct and inverse cascades has been indicated. The consequences for the Earth’s core and geodynamo problems have been considered.     

The effect of wind on the gravity wave propagation in the Earth’s ionosphere

The effect of ionospheric wind on the gravity wave propagation is studied. These waves arise in the ionosphere due to intensification of their sources near the Earth’s surface during enhanced seismic activity. The influence of the wind on these waves is connected with the Ampere’s force that produces the ion-drag force acting on the atmosphere. This results in the occurrence of the discrete wave spectrum the maximum of which increases in proportion to the numbers of the natural scale. Furthermore, these waves are amplified during propagation from the source region in the direction perpendicular to the wind direction. These peculiarities of the gravity waves can be used for monitoring of seismic activity based on the ionosphere sounding.     

On mechanical Q parameters of the lower mantle inferred from data on the Earth’s free oscillations and nutation

A new method for the interpretation of recent tidal and astrometry data based on integral relations connecting the mantle Q parameters to variations in frequencies of the main Earth’s spheroidal oscillation, nearly diurnal free nutation, Chandler wobble, and Love numbers is proposed in this paper. The normalized weighting functions in the aforementioned integral relations are shown to be very close to each other in a wide range of oscillation periods (from 54 minutes to 14 months) for the actual Earth’s model and therefore the detection of the dependence of the mantle Q parameters averaged with these weighting functions on frequency does not need to be solved for the inverse problem of the Q parameters as a function of the oscillation frequency and depth: instead, it is sufficient to determine the dependence of the adequate integral characteristics of the mantle’s inelasticity only on frequency. This makes it possible to significantly simplify the solution of the inverse problem and improve the rheologic models of the lower mantle.     

THEMIS statistical study on the plasma properties of high-speed flows in Earth’s magnetotail

Using Time History of Events and Macroscale Interactions during Substorms(THEMIS) observations from 2007 to 2011 tail seasons, we study the plasma properties of high speed flows(HSFs) and background plasma sheet events(BPSs) in Earth's magnetotail(|Y_(GSM)|13R_E, |Z_(GSM)|5R_E, –30R_EX_(GSM)–6R_E), and their correlations with solar wind parameters. Statistical results show that the closer the HSFs and BPSs are to the Earth, the hotter they become, and the temperature increase of HSFs is larger than that of BPSs. The density and temperature ratios between HSFs and BPSs are also larger when events are closer to Earth. We also find that the best correlations between the HSFs(BPSs) density and the solar wind density occur when the solar wind density is averaged 2(3.5) hours prior to the onset of HSFs(BPSs). The normalized densities of both HSFs and BPSs are correlated with the interplanetary magnetic field(IMF) θ angles(θ=arctan(B_Z/((B_x~2)+(B_y~2))~(1/2) which are averaged 3 hours before the observation time. Further analysis indicates that both HSFs and BPSs become denser during the northward IMF period.     

The Relationship between Multifractal and Entropy Properties of Seismic Noise in Kamchatka and Irregularity of the Earth’s Rotation

Izvestiya, Physics of the Solid Earth - Abstract—The connection between the properties of seismic noise continuously recorded by the network of 21 broadband seismic stations in...     

Ranges of AGW propagation in the Earth’s atmosphere

The propagation of atmospheric gravity waves (AGWs) is studied in the context of geometrical optics in the nonisothermal, viscous, and thermal-conductive atmosphere of Earth in the presence of wind shifts. Parametric diagrams are plotted, determining the regions of allowed frequencies and horizontal phase velocities of AGWs depending on the altitude. It is shown that a part of the spectrum of AGWs propagates in stationary air in an altitude range from the Earth’s surface through the ionospheric F1 layer. AGW from nearearth sources attenuate below 250 km, while waves generated at altitudes of about 300 km and higher do not reach the Earth’s surface because of the inner reflection from the thermosphere base. The pattern changes under strong thermospheric winds. AGW dissipation decreases with an adverse wind shift and, hence, a part of the wave spectrum penetrated from the lower atmosphere to the altitudes of F2 layer.     

Model of muon generation in the Earth’s atmosphere

The muon fluxes on the Earth’s surface and at depths of 7, 20, and 40 m of water equivalent are calculated based on a simple model of pion generation by primary particles with different energies. This generation model is based on the known concepts of multiple pion production. The model parameters are compared with the data obtained using accelerating machines.     

The ONAD method for solving the SH-wave equation and simulation of the SH-wave propagation in the Earth’s mantle

The optimal nearly-analytic discrete(ONAD) method is a new numerical method developed in recent years for solving the wave equation.Compared with other methods,such as popularly-used finite-difference methods,the ONAD method can effectively suppress the numerical dispersion when coarse grids are used.In this paper,the ONAD method is extended to solve the 2-dimensional SH-wave equation in the spherical coordinates.To investigate the accuracy and the efficiency of the ONAD method,we compare the numerical results calculated by the ONAD method and other methods for both the homogeneous model and the inhomogeneous IASP91 model.The comparisons indicate that the ONAD method for solving the SH-wave equation in the spherical coordinates has the advantages of less numerical dispersion,small memory requirement for computer codes,and fast calculation.As an application,we use the ONAD method to simulate the SH-wave propagating between the Earth’s surface and the core-mantle boundary(CMB).Meanwhile,we investigate the SH-wave propagating in the mantle through analyzing the wave-field snapshots in different times and synthetic seismograms.     

Convection in anelastic models of the Earth’s liquid core

The effects of adiabatic cooling on the convection in the anelastic model of the liquid core of the Earth are considered. It is shown that even minor adiabatic cooling causes significant changes in the pattern of the convection, shifting the peak in the convection intensity to the inner part of the core. Just as in the Boussinesq model, both direct and inverse kinetic energy cascades are simultaneously present, and the direct cascade of entropy is observed.     

Plume Mode of Thermal Convection in the Earth’s Mantle

In our previous works, based on numerical models, it was shown that under certain conditions a hot material can rise in portions in the tails of thermal mantle plumes. The spectrum of these pulsations can correspond to the observed spectra of catastrophic hotspot eruptions. Since most of the existing numerical models of thermal convection for the mantle of the present Earth do not reveal these pulsations, in this work, we analyze the physical cause and initiation conditions of pulsations of thermal plumes. The results of a numerical solution of the thermal convection equations for a material with varying parameters in the extended Boussinesq approximation are presented. It is shown how the structure of the convection is transformed with the increase of convection intensity. At the Rayleigh numbers Ra > 10⁶, convection becomes unsteady, and the configuration of the ascending and descending flows changes. The new flow emerging at the mantle bottom acquires a mushroom shape with a head and a tail. After the rise of the plume’s head to the surface, the tail remains in the mantle in the form of a quasi-stationary hot steam. It turns out that at Ra ~ 5 × 10⁷, the thermal mantle plume becomes pulsating and its tail is in fact a heated channel through which the hot material rises in successive portions. At the Rayleigh numbers Ra > 5 × 10⁸, the tail of the thermal plume breaks and the plume becomes a regular conveyor of separate ascending portions of the hot material, which are referred to as thermals. Thus, thermal convection with pulsating plumes takes place at the transitional stage from the regime of quasi-stationary plumes to the regime of thermals.     

The Role of Water Vapour in Earth’s Energy Flows

Water vapour modulates energy flows in Earth's climate system through transfer of latent heat by evaporation and condensation and by modifying the flows of radiative energy both in the longwave and shortwave portions of the electromagnetic spectrum. This article summarizes the role of water vapour in Earth's energy flows with particular emphasis on (1) the powerful thermodynamic constraint of the Clausius Clapeyron equation, (2) dynamical controls on humidity above the boundary layer (or free-troposphere), (3) uncertainty in continuum absorption in the relatively transparent "window" regions of the radiative spectrum and (4) implications for changes in the atmospheric hydrological cycle.     

Detection of Antarctic oscillation signals in Earth’s oblateness variations

Variations of Earth’s oblateness (J ₂) reflect a large scale mass redistribution within the Earth system. The climate effect causing J ₂ interannual variations is still not clear, though previous studies indicated it may be related to EI Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). However, we have a new discovery of the significant Antarctic oscillation (AAO) signals in J ₂ interannual variations, especially on 4–6 year scales based on cross wavelet and wavelet coherence analysis with 95% confidence test during 1979–2012. The results additionally indicate that the close phase relationship between J ₂ and AAO (AAO leading J ₂ variations by 3 ± 2 months in phase) is far superior to that between J ₂ and ENSO/PDO on 4–6 year scales. In this work, we discuss, for the first time, a possible geophysical mechanism of AAO effecting J ₂ variations. The investigations are based on the definition of AAO and its spatial–temporal behavior influencing the large-scale mass movement. Finally, an approximate quantitative estimate of the AAO imprint on J ₂ with an emphasis on the atmospheric contribution is made.