54,826 research outputs found
Are Chinese loess deposits essentially continuous?
No full textWe conducted a paleomagnetic examination of the last glacial loess of three representative profiles along an east-west transect in the central Loess Plateau in order to assess the continuity of Chinese loess. The results show that the Xifeng and Luochuan profiles record the Laschamp geomagnetic excursion but with different morphologies. Together with the published results from Weinan, southern plateau, our results suggest that sedimentation of the last glacial loess in the central-southern plateau was continuous at the time scale equivalent to the duration of the Laschamp excursion (~2 kyr), but probably episodic at finer time scales (<2 kyr). No geomagnetic excursion was found at the Yichuan profile near the Yellow River valley, where loess accumulation may be strongly affected by local environmental changes and thus may have been discontinuous. Both site location and time scale therefore need to be considered when considering continuity of Chinese loess.<br/
Probability Density Function Estimation in Multilevel Monte Carlo FDTD Method
No full textWe apply the multilevel Monte Carlo (MLMC) method with the finite-difference time-domain method (FDTD) to estimate the probability density function (PDF) and the cumulative distribution function (CDF) of any quantity of interest for uncertainty quantification in electromagnetic problems. It is shown that, compared with the standard Monte Carlo FDTD (MC-FDTD), the MLMC-FDTD method can provide accurate estimations with high computational efficiency. In addition, as opposed to the polynomial chaos FDTD (PC-FDTD) method that suffers the curse of dimensionality or failure, the MLMC-FDTD method is more reliable
Multilevel Monte Carlo FDTD estimation of the cumulative distribution function with kernel smoothing technique
No full textThis paper addresses the multilevel Monte Carlo finite-difference time-domain (MLMC-FDTD) method for the estimation of the cumulative distribution function (CDF) of an electromagnetic quantity when material parameters in the problem are modeled as random variables. In order to alleviate the effects of the discontinuity of the indicator function, the kernel density estimation (KDE) technique is used. The technique can consider correlations between random parameters. It is shown that MLMC-FDTD has a faster convergence rate
compared with Monte Carlo FDTD (MC-FDTD) and that the estimations of the CDF become smoother with the help of KDE. In addition, MLMC-FDTD preserves the advantages of MC-FDTD, such as robustness and simplicity, and proves to be a powerful approach, superior to other methods
Enhanced Multilevel Monte Carlo Method Applied to FDTD for Probability Distribution Estimation
No full textMultilevel Monte Carlo (MLMC) method, enhanced by a
smoothing technique based on Kernel Density Estimation, is coupled with
the Finite-Difference Time-Domain (FDTD) algorithm in order to estimate
the probability distribution of any quantity of interest, for uncertainty
quantification in electromagnetic problems. It is shown that such enhanced
MLMC-FDTD is faster than conventional Monte Carlo FDTD while
inheriting its advantages of robustness, simplicity and generality, unlike
other uncertainty analysis methods, such as the perturbation and the
moment methods that cannot be used to straightforwardly estimate
probability distribution, or the polynomial chaos method that suffers from
the curse of dimensionality problem or even fails
Multilevel Monte Carlo FDTD method for uncertainty quantification
No full textThe recent multilevel Monte Carlo (MLMC) method
is here proposed for uncertainty quantification in electromagnetic
problems solved by the finite-difference time-domain (FDTD)
method, when material parameters are modeled as random
variables. It improves the estimations of the mean and variance of
the quantities of interest computed on a FDTD spatial grid by
sampling at coarser levels of discretization. The proposed
approach can amply reduce the computational cost of the standard
Monte Carlo FDTD (MC-FDTD), at the price of a small reduction
of its accuracy. It is advantageous with respect to Polynomial
Chaos FDTD (PC-FDTD), when the latter fails or becomes
prohibitive for computational requirements. It also appears to
widely outperform Stochastic FDTD (S-FDTD) in terms of
accuracy
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
Magnetostratigraphic age of the Xiantai Paleolithic site in the Nihewan Basin and implications for early human colonization of Northeast Asia
No full textTiming of the early habitation and stone technologies in the Nihewan Basin, North China has provided insights into our understanding of early human adaptability to high northern latitudes in East Asia. Here we contribute to this topic with detailed magnetostratigraphic investigation, coupled with mineral magnetic measurements and palynological analyses on a lacustrine sequence in this basin, which contains the Xiantai Paleolithic site. Magnetite and hematite were identified as the main carriers for the characteristic remanent magnetizations. Magnetostratigraphic results show that the Xiantai lacustrine sequence recorded the Brunhes chron, the Jaramillo and the Olduvai subchrons, and successive reverse polarity portions of the intervening Matuyama chron. Stratigraphic correlation in terms of lithology, magnetic susceptibility and magnetic polarity sequences between the Xiantai and Xiaochangliang sections indicates that the Xiantai artefact layer is readily contemporary with the Xiaochangliang artefact layer, which has been previously estimated to be about 1.36 Ma [R.X. Zhu, K.A. Hoffman, R. Potts, C.L. Deng, Y.X. Pan, B. Guo, C.D. Shi, Z.T. Guo, B.Y. Yuan, Y.M. Hou, W.W. Huang, Earliest presence of humans in northeast Asia, Nature 413 (2001) 413–417.]. Early humans of the Xiantai Paleolithic site lived in a steppe paleoenvironment indicated from fossil pollens. Furthermore, the combined evidence of our magnetostratigraphy and previously published magnetochronology and paleoclimate data documents that early humans of North China were able to adjust to an increasing variability of paleoclimates and paleoenvironments over the Early Pleistocene
Excitation of toroidal Alfvén eigenmode by barely circulating energetic electrons in low density plasmas
No full textToroidal Alfvén eigenmodes (TAEs) associated with runaway electrons are observed in low density EAST Ohmic discharges (Zhu et al 2022 Phys. Plasmas 29 062504), which motivate the present work to explore the possible destabilization mechanism using simplified hybrid MHD-kinetic simulations. We show that the barely circulating energetic electrons can satisfy the resonance condition with the TAE, mainly due to vanishing transit frequency near the phase space circulating/trapped separatrix. In addition, the nonlinear saturation study suggests that the phase space resonance structure plays an important role in the eventual saturation amplitude, where a low toroidal mode number is favored in this scenario
Candidate section for the base of Cambrian Stage 10 at Wa'ergang, Hunan, China.
No full textA well exposed section through the lower Shenjiawan Formation (uppermost Cambrian-lowermost Ordovician) has been proposed as a candidate stratotype for the GSSP of provisional Stage 10 (Cambrian: Furongian Series). The section contains remains of the agnostoid trilobite Lotagnostus americanus, which is the guide fossil that marks the provisional base of the stage. This section is notable because it is one of few known that also yields the conodont Eoconodontus notchpeakensis.
Trilobite biostratigraphy has been documented through the entire Shenjiawan Formation in the Wa’ergang section. The lower 32 m of the Shenjiawan Formation has been especially well studied, leading to the recognition of three agnostoid zones and identifying the first appearance of L. americanus at 29.65 m above the base of the formation. Most of the Shenjiawan Formation has been intensively sampled for conodonts, leading to recognition of four conodont zones. E. notchpeakensis has been identified at eight sample horizons, with the lowest occurrence being 145 m above the base of the formation, or roughly halfway through the projected thickness of Stage 10. Dense sampling for carbon isotopes (δ18C) reveals a curve showing a negative excursion having a peak of -1.0‰ close to the FAD of L. americanus
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