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991.
Near-Earth asteroid (99942) Apophis currently resides among the top positions on the list of objects with small, yet non-zero impact probability with the Earth. For that reason an unusual observational and theoretical effort has been dedicated to precisely characterize its future orbit. Here we discuss orbital perturbation of Apophis due to incident and reflected solar radiation pressure (SRP). We both revisit recent analytical estimate of the SRP effects for this body and also formulate a numerical approach allowing us to compute the SRP orbital perturbation under general assumptions. Contrary to some previous results, we show that SRP has a much smaller effect on the Apophis trajectory than does the thermal re-radiation force which produces the Yarkovsky effect. When the Yarkovsky effect becomes constrained enough in the future, our approach may be used to improve the orbit determination for this asteroid. 相似文献
992.
?uk et al. (?uk, M. Gladman, B.J., Stewart, S.T. [2010]. Icarus 207 590-594) concluded that the the lunar cataclysm (late heavy bombardment) was recorded in lunar Imbrian era craters, and that their size distribution is different from that of main belt asteroids (which may have been the dominant pre-Imbrian impactors). This result would likely preclude the asteroid belt as the direct source of lunar cataclysm impactors. Malhotra and Strom (Malhotra, R., Strom, R.G. [2011]. Icarus) maintain that the lunar impactor population in the Imbrian era was the same as in Nectarian and pre-Nectarian periods, and this population had a size distribution identical to that of main belt asteroids. In support of this claim, they present an Imbrian size distribution made from two data sets published by Wilhelms et al. (Wilhelms, D.E., Oberbeck, V.R., Aggarwal, H.R. [1978]. Proc. Lunar Sci. Conf. 9, 3735-3762). However, these two data sets cannot be simply combined as they represent areas of different ages and therefore crater densities. Malhotra and Strom (Malhotra, R., Strom, R.G. [2011]. Icarus) differ with the main conclusion of Wilhelms et al. (Wilhelms, D.E., Oberbeck, V.R., Aggarwal, H.R. [1978]. Proc. Lunar Sci. Conf. 9, 3735-3762) that the Nectarian and Imbrian crater size distributions were different. We conclude that the available data indicate that the lunar Imbrian-era impactors had a different size distribution from the older ones, with the Imbrian impactor distribution being significantly richer in small impactors than that of older lunar impactors or current main-belt asteroids. 相似文献
993.
994.
Handbook of Hydrogeology is a professional reference book guiding the work and re-search of hydrogeology and providing enlightenment to hydrogeologists and those working in re-lated fields. This paper introduces the revision and enlargement of Handbook of Hydro-geology (2nd edition) on the basis of its first edition. It gives a brief account of the background, principles, and necessity of revising the handbook. The content, features and application of the handbook are also addressed. 相似文献
995.
Naifang?Bei Fuqing?ZhangEmail author John?W.?Nielsen-Gammon 《Pure and Applied Geophysics》2012,169(3):539-554
This study examines the effectiveness of targeted meteorological observations for improving ozone prediction in Houston and
the surrounding area based on perfect-model simulation experiments. Supplementary observations are targeted for the location
that has the highest impact factor (maximum Kalman gain) estimated from an ensemble and is expected to minimize ozone forecast
uncertainty at the verification time. It is found that the observational impact factor field varies with time and is sensitive
to ensemble resolutions and physics parameterizations. The efficiency of observation targeting is further examined through
assimilating observations in areas with different impact factors using an ensemble Kalman filter. It is found that the ensemble
sensitivity analysis is capable of locating supplementary observations that may reduce meteorological and ozone forecast error,
but not as effectively as expected. 相似文献
996.
997.
While it is obvious that large-scale gravity studies should account for the sphericity of the Earth, each case should be examined.
If a geometry model is very large for the 3D-gravity calculation, it cannot be correctly defined in Cartesian coordinates.
Because of the Earth’s curvature it is necessary to use spherical coordinates, the importance of which is shown in this paper.
The calculation of the gravity for a cylinder reveals, 1 m above the center of the cylinder, a relative difference of 13%
between the models with Cartesian and spherical coordinates. 相似文献
998.
A catalog for northeast India and the adjoining region for the period 1897–2009 with 4,497 earthquakes events is compiled
for homogenization to moment magnitude M
w,GCMT in the magnitude range 3–8.7. Relations for conversion of m
b and M
s magnitudes to M
w,GCMT are derived using three different methods, namely, linear standard regression, inverted standard regression (ISR) and orthogonal
standard regression (OSR), for different magnitude ranges based on events data for the catalog period 1976–2006. The OSR relations
for M
s to M
w,GCMT conversion derived in this paper have significantly lower errors in regression parameters compared to the relations reported
in other studies. Since the number of events with magnitude ≥7 for this region is scanty, we, therefore, considered whole
India region to obtain the regression relationships between M
w,GCMT and M
s,ISC. A relationship between M
w,GCMT and M
w,NEIC is also obtained based on 17 events for the range 5.2 ≤ magnitude ≤ 6.6. A unified homogeneous catalog prepared using the
conversion relations derived in this paper can serve as a reference catalog for seismic hazard assessment studies in northeast
India and the adjoining region. 相似文献
999.
Bohuslav R??ek Jaroslava Plomerová Vladislav Babu?ka 《Studia Geophysica et Geodaetica》2012,56(1):107-140
Joint inversion of teleseismic P-waveforms and local group velocities of surface waves retrieved from ambient seismic noise
has been performed to model velocity structure of the crust and uppermost mantle of the Bohemian Massif. We analysed P-waveforms
of 381 teleseismic earthquakes recorded at 54 broadband seismic stations located on the territory of the Czech Republic and
in its close surroundings. Group velocities of Rayleigh and Love surface waves were obtained by cross-correlating long-term
recordings of seismic noise. The basis for waveform inversion is the well-known methodology of P-to-S receiver functions constructed
from converted phases. Due to instabilities in direct inversion of receiver functions caused by the necessity of applying
deconvolution, we propose an alternative formulation to fit observed and calculated radial components of P waveforms. The
joint inversion is transformed into a search for the minimum of the cost function defined as a weighted sum of waveform and
group velocity misfits. With the use of the robust stochastic optimizer (Differential Evolution Algorithm), neither derivatives
nor a starting model are needed. The task was solved for 1D layered isotropic models of the crust and the uppermost mantle.
We have performed a sequence of inversions with models containing one, two, three and four layers above a half-space. By using
statistical criteria (F-test) we were able to select the simplest velocity models satisfying data and representing local geological
structures. Complex crustal models are typical for stations located close to boundaries of major tectonic units. The relatively
low average P to S wave-velocity ratio is in agreement with the generally accepted view that the BM crust is predominantly
felsic. 相似文献
1000.
The standard ray theory (RT) for inhomogeneous anisotropic media does not work properly or even fails when applied to S-wave
propagation in inhomogeneous weakly anisotropic media or in the vicinity of shear-wave singularities. In both cases, the two
shear waves propagate with similar phase velocities. The coupling ray theory was proposed to avoid this problem. In it, amplitudes
of the two S waves are computed by solving two coupled, frequency-dependent differential equations along a common S-wave ray.
In this paper, we test the recently developed approximation of coupling ray theory (CRT) based on the common S-wave rays obtained
by first-order ray tracing (FORT). As a reference, we use the Fourier pseudospectral method (FM), which does not suffer from
the limitations of the ray method and yields very accurate results. We study the behaviour of shear waves in weakly anisotropic
media as well as in the vicinity of intersection, kiss or conical singularities. By comparing CRT and RT results with results
of the FM, we demonstrate the clear superiority of CRT over RT in the mentioned regions as well as the dangers of using RT
there. 相似文献