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Chen L.,Wuhan University | Yi F.,Key Laboratory of Geospace Environment and Geodesy
Annales Geophysicae | Year: 2011

We report the average properties and small-scale variation features of the mesospheric Na and Fe layers at 30° N from extensive simultaneous and common-volume Na and Fe lidar measurements at Wuhan, China. The annual mean Na and Fe density profiles are derived in terms of an averaging method taken from an early literature. The mean Na and Fe profiles preserve the sharp gradients present in most individual density profiles near the layer bottom. Near the bottommost of the layers the mean Na and Fe scale heights are respectively -0.42 and -0.30 km. The mean layer parameters coincide well with the previous report. The Na and Fe densities in the lowest several kilometers of the layers consistently exhibit nearly the same time variations. A clear- cut distinction between the Na and Fe time variations always appears in an altitude range near 90 km. A relatively weak positive correlation between them persistently occurs also in an altitude range near 100 km. The mean increase and decrease rates for both Na and Fe are altitude dependent and have a single-peak structure. The time constant of the layer variation is ~0.07-2.0hforNaand ~0.02-1.7hforFe, suggesting that the variability is dominated by small-scale processes. However, there is also a slow net increase in each of the annual mean column abundances (Na and Fe) during night. Keywords. Atmospheric composition and structure (Middle atmosphere - composition and chemistry). © European Geosciences Union 2011.

Ma Z.,Wuhan University | Ma Z.,Key Laboratory of Geospace Environment and Geodesy | Ma Z.,State Observatory for Atmospheric Remote Sensing | Yi F.,Wuhan University | And 2 more authors.
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2010

The seasonal/annual characteristics of the high-altitude sporadic metal atom layers are presented on the basis of extensive Na and Fe lidar measurements at 30°N during the past several years. It is found that the extremely high sporadic Na (Nas) and Fe (Fes) layers above 105km occurred mostly during summer. They had long durations (a few hours) and broad layer widths (much larger than 2km). Their absolute peak densities could be comparable to or even larger than those of the corresponding main layers on a few nights. By using all the raw data profiles including sporadic layers, we have constructed the contour plots of Na and Fe densities versus month and altitude at 30°N. The Na and Fe layers both exhibit evidence for summer topside extension, which is consistent with the earlier observations for K and Ca at different latitudes. The summer topside extension of mean metal atom layers might represent a universal phenomenon that is alike for different atom species, different geographic locations and different measurement years. The extremely high sporadic metal atom layers above 105km occurring during summer give rise to the phenomenon. © 2010 Elsevier Ltd.

Chen W.,Wuhan University | Chen W.,State Key Laboratory of Information Engineering in Surveying | Shen W.,Wuhan University | Shen W.,State Key Laboratory of Information Engineering in Surveying | Shen W.,Key Laboratory of Geospace Environment and Geodesy
Journal of Geophysical Research: Solid Earth | Year: 2010

The Earth's rotation is perturbed by mass redistributions and relative motions within the Earth system, as well as by the torques from both the internal Earth and celestial bodies. The present study aims to establish a theory to incorporate all these factors perturbing the rotation state of the triaxial Earth, just like the traditional rotation theory of the axial-symmetric Earth. First of all, we reestimate the Earth's inertia tensor on the basis of two new gravity models, EIGEN-GL05C and EGM2008. Then we formulate the dynamic equations and obtain their normal modes for an Earth model with a triaxial anelastic mantle, a triaxial fluid core, and dissipative oceans. The periods of the Chandler wobble and the free core nutation are successfully recovered, being ∼433 and ∼430 mean solar days, respectively. Further, the Liouville equations and their general solutions for that triaxial nonrigid Earth are deduced. The Liouville equations are characterized by the complex frequency-dependent transfer functions, which incorporate the effects of triaxialities and deformations of both the mantle and the core, as well as the effects of the mantle anelasticity, the equilibrium, and dissipative ocean tides. Complex transfer functions just reflect the fact that decays and phase lags exist in the Earth's response to the periodic forcing. Our theory reduces to the traditional rotation theory of the axial-symmetric Earth when assuming rotational symmetry of the inertia tensor. Finally, the present theory is applied to the case of atmospheric-oceanic excitation. The effective atmospheric-oceanic angular momentum function (AMF) χeff = χeff1 + iχeff2 for the present theory is compared with the AMF χeff sym = χeff1 sym + iχeff2 sym for the traditional theory and the observed AMF χobs = χ1 obs + iχ2 obs; we find that the difference between χeff and χeff sym is of a few milliseconds of arc (mas) and can sometimes exceed 10 mas. In addition, spectrum analyses indicate that χeff is in good agreement with χeff sym and, further, show an increase of coherency with χobs especially in the low-frequency band. The obvious advantage of χeff in the low-frequency band with respect to χeff sym is the critical support of the present theory. However, still better performance of our theory can be expected if the models of the mantle anelasticity and oceanic dynamics were improved. Thus we conclude that the traditional Earth rotation theory should be revised and upgraded to include the effects of the Earth's triaxiality, the mantle anelasticity, and oceanic dynamics. The theory presented in this study might be more appropriate to describe the rotation of the triaxial Earth (or other triaxial celestial bodies such as Mars), though further studies are needed to incorporate the effects of the solid inner core and other possible influences. Copyright 2010 by the American Geophysical Union.

Wen Y.,Hubei University | Wen Y.,Key Laboratory of Geospace Environment and Geodesy | Wen Y.,University of Glasgow | Li Z.,University of Glasgow | And 4 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2012

On November 14th 2001, a Mw 7.8 earthquake occurred in the Kokoxili region of northern Tibet. The earthquake ruptured more than 400 km along the western part of the Kunlun fault with a maximum of 8 m left-lateral slip. In this paper, we use a multitemporal Interferometric SAR (InSAR) time series technique to map the postseismic motion following the large Kokoxili event. SAR data from Envisat descending orbits along five adjacent tracks covering almost the entire ruptured fault length are used to calculate the displacement time series for a period between 2 and 6 years after the earthquake. A peak-to-trough signal of 8 cm in the radar line of sight is observed during the period between 2003 and 2008. Two different mechanisms are employed to explain the observed surface displacements, namely afterslip and viscoelastic relaxation. The observations inverted for afterslip on and below the coseismic rupture plane shows that the maximum slip in the afterslip model is 0.6 m. The position of the maximum postseismic slip is located in the middle of two relatively high coseismic slip patches, which suggests that afterslip is a plausible mechanism. Models of viscoelastic stress relaxation in a Maxwell half-space give a best fitting viscosity for the mid-to-lower crust of 2-5 × 1019 Pa s, and the principal postseismic relaxation process is due to viscous flow in the lower crust to upper mantle. However, the InSAR observations are incapable of distinguishing between localized (afterslip) and distributed (viscoelastic relaxation) deformation. And the lowest misfits are produced by mixed models of viscoelastic relaxation in the mantle below 70 km and afterslip in the crust. Modeling of viscoelastic relaxation in a Maxwell half-space, and also a mixed mechanism model, enables us to place an effective viscosity of 2 × 10 19 Pa s on the lower crust to mantle of northern Tibet. © 2012. American Geophysical Union. All Rights Reserved.

Ping P.,Wuhan University | Zhang Y.,Wuhan University | Zhang Y.,Key Laboratory of Geospace Environment and Geodesy | Xu Y.,Wuhan University
Journal of Applied Geophysics | Year: 2014

In order to conquer the spurious reflections from the truncated edges and maintain the stability in the long-time simulation of elastic wave propagation, several perfectly matched layer (PML) methods have been proposed in the first-order (e.g., velocity-stress equations) and the second-order (e.g., energy equation with displacement unknown only) formulations. The multiaxial perfectly matched layer (M-PML) holds the excellent stability for the long-time simulation of wave propagation, even though it is not perfectly matched in the discretized M-PML equation system. This absorbing boundary approach can offer an alternative way to solve the problem of the late-time instability, especially for anisotropic media, which is also suffered by the convolutional perfectly matched layer (C-PML) that is supposed to be competent to handle most stable problems. The M-PML termination implementation in the first-order formulations is well proposed. The common drawback of the implementation of the first-order M-PML formulations is that it necessitates fundamental reconstruction of the existing codes of the second-order spectral element method (SEM) or finite element method (FEM). Therefore, we propose a nonconvolutional second-order M-PML absorbing boundary condition approach for the wave propagation simulation in elastic media that has not yet been developed before. Two-dimensional numerical simulation validations demonstrate that the proposed second-order M-PML has good performances: 1) superior efficiency and stability of absorbing the spurious elastic wavefields, both the surface waves and body waves, reflected on the boundaries; 2) superior stability in the long-time simulation even in the isotropic medium with a high Poisson's ratio; 3) superior efficiency and stability in the long-time simulation for anisotropic media. This method hence makes the SEM and FEM in the second-order wave equation formulation more efficient and stable for the long-time simulation. © 2013 Elsevier B.V.

Gan Q.,Hubei University | Gan Q.,Key Laboratory of Geospace Environment and Geodesy | Gan Q.,State Observatory for Atmospheric Remote Sensing | Zhang S.D.,Hubei University | And 6 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2012

We present the global distribution, seasonal, and interannual variations of the lower mesospheric inversion layers (MILs) using SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) temperature data. We show that both the characteristics and the formation mechanisms of large spatiotemporal-scale lower MILs are latitude dependent. At low latitudes, the monthly zonal mean amplitude of the lower MILs exhibits a semi-annual cycle and reaches a maximum of ∼40 K in spring and a secondary maximum of ∼30 K in autumn. On the equator, the semi-annual oscillations in the background and diurnal-migrating-tide temperatures could contribute more than 12 and 25 K, respectively, suggesting they are the key causes of large spatiotemporal-scale lower MILs at low latitudes. At middle latitudes, the monthly zonal mean amplitude of the lower MILs exhibits an annual cycle with its maximum in the range 24-33 K in winter. In addition, their longitudinal distribution and daily variation in winter are closely correlated with the transient structure of a composite wave composed of stationary and westward-propagating quasi-16-day planetary waves with zonal wave number 1. The correlation coefficient between the lower MILs and the composite wave can sometimes reach unity. The composite planetary wave could contribute temperature enhancements of at least 15-20 K to the lower MILs. Thus, we believe that the transient structure of planetary waves is also an important cause of the large spatiotemporal-scale lower MILs in winter at middle latitudes, in addition to previously proposed mechanisms. Copyright 2012 by the American Geophysical Union.

Zhang Y.,Wuhan University | Zhang Y.,Key Laboratory of Geospace Environment and Geodesy | Zhang S.,Wuhan University | Zhang S.,Key Laboratory of Geospace Environment and Geodesy | Xia J.,Wuhan University
International Journal of Solids and Structures | Year: 2014

Three-dimensional transient responses of porous media under moving surface impulses of finite frequency components are theoretically studied. We discuss three free-surface stiffness conditions, such as fully permeable - 'open pore', fully impermeable - 'closed pore', and partially permeable boundaries, that are not explicitly discussed before. The transient responses of the solid vertical displacement and the pore fluid pressure triggered by the moving impulses on the surface are particularly investigated in different typical surface stiffness, moving impulse velocities, material permeabilities and impulse peak frequencies. It is concluded that the R1 surface wave carries the strongest energy as that for stationary source configurations. Moreover, it is more sensitive to surface stiffness condition than body waves represented in the responses of the corresponding wave forms of obvious different amplitudes and arrival time. Furthermore, the apparent velocity of the moving impulse pointing toward the fixed receiver may cause 'blue shift' in frequency. The higher velocity triggers more obvious frequency shift. For the moving impulse of low peak frequency, this shift becomes much serious. The lateral velocity of the moving impulse to the receiver may also twist the received wave forms, especially for the impulse of low peak frequency. © 2013 Elsevier Ltd. All rights reserved.

Luo Z.C.,Wuhan University | Luo Z.C.,Key Laboratory of Geospace Environment and Geodesy | Luo Z.C.,State Key Laboratory of Information Engineering in Surveying | Li Q.,Wuhan University | And 3 more authors.
Science China Earth Sciences | Year: 2012

It is important to quantify mass variations in the Antarctic ice sheet to study the global sea-level rise and climate change. A hybrid filtering scheme employing a combination of the decorrelated filter P3M6 and 300 km Fan filter was used, and the surface mass variations over the Antarctic are recovered from GRACE CSR RL04 monthly gravity field models from August 2002 to June 2010. After deduction of leakage errors using the GLDAS hydrological model and postglacial rebound effects using the glacial isostatic adjustment model IJ05, the variations in the ice sheet mass are obtained. The results reveal that the rate of melting of the Antarctic ice sheet is 80. 0 Gt/a and increasing and contributes 0. 22 mm/a to the global sea-level rise; the mass loss rate is 78. 3 Gt/a in the West Antarctic and 1. 6 Gt/a in the East Antarctic. The average mass loss rate increases from 39. 3 Gt/a for the period 2002-2005 to 104. 2 Gt/a for the period 2006-2010, and its corresponding contribution to the global sea-level rise increases from 0. 11 to 0. 29 mm/a, which indicates accelerated ice mass loss over the Antarctic since 2006. Moreover, the mass accumulation rates for Enderby Land and Wilkes Land along the coast of East Antarctica decrease for the period 2006-2008 but increase evidently after 2009. © 2011 Science China Press and Springer-Verlag Berlin Heidelberg.

Cai H.T.,Wuhan University | Cai H.T.,Key Laboratory of Geospace Environment and Geodesy | Yin F.,Wuhan University | Yin F.,Key Laboratory of Geospace Environment and Geodesy | And 3 more authors.
Annales Geophysicae | Year: 2011

In this paper, we present observational evidence for the trans-polar propagation of large-scale Traveling Ionospheric Disturbances (TIDs) from their nightside source region to the dayside. On 13 February 2001, the 32 m dish of EISCAT Svalbard Radar (ESR) was directing toward the geomagnetic pole at low elevation (30°) during the interval 06:00-12:00 UT (MLT ≈ UT + 3 h), providing an excellent opportunity to monitor the ionosphere F-region over the polar cap. The TIDs were first detected by the ESR over the dayside north polar cap, propagating equatorward, and were subsequently seen by the mainland UHF radar at auroral latitudes around geomagnetic local noon. The propagation properties of the observed ionization waves suggest the presence of a moderately large-scale TIDs, propagating across the northern polar cap from the night-time auroral source during substorm conditions. Our results agree with the theoretical simulations by Balthazor and Moffett (1999) in which poleward-propagating large-scale traveling atmospheric disturbances were found to be self-consistently driven by enhancements in auroral heating. © Author(s) 2011.

Zhang Y.,Wuhan University | Zhang Y.,Key Laboratory of Geospace Environment and Geodesy | Xu Y.,Wuhan University | Xia J.,Wuhan University | And 3 more authors.
Soil Dynamics and Earthquake Engineering | Year: 2014

The analytical dispersion and waveform solutions of Rayleigh surface wave for the Biot fluid-saturated model are obtained at low frequencies for a homogeneous half space. The equivalent solution is also obtained by the equivalent-viscoelastic representation of the fluid-saturated model based on a single viscoelastic element for each wave modulus. The effective characteristics of the validations and limitations for the equivalent-viscoelastic model are analyzed by comparison of the numerical solutions of the fluid-saturated model and the equivalent model for the surface wave propagations. Our calculations show that the free boundary effects on the frequency dependent dispersion and time dependent dynamical waveforms of the surface wave in the Biot model are well fitted in a relative narrow low frequency band by the Zener elements in case of the frequency is much lower than the critical frequency f c of the porous material. The effective characteristics for air filled cases with a higher f c show a better result. Furthermore, if the critical frequency f c is low, always with high permeability κ under near surface condition, at low frequencies (e.g. the seismic frequency band <200Hz) the surface fluid drainage conditions influence Rayleigh-wave propagations obviously. The frequency range must hence be carefully checked for the viscoelastic representations. When the validated frequency range is defined, the viscoelastic elements can solve the transient surface wave propagation in porous media effectively. The convolution integral in wave modeling can be replaced by memory variables, which makes the field quantities calculated at every time step need not be stored. The effective representation saves the consumptions of computer time and storages, and supplies a more convenient approach to apply the surface wave considering poroelasticity. © 2013 Elsevier Ltd.

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