共查询到20条相似文献，搜索用时 78 毫秒
1.
An extended structure-function model is developed by including the new effect in the p-model of Meneveau and Sreenivasan which shows that the averaged energy cascade rate changes with scale, a situation which has been found to prevail in nonfullydeveloped turbulence in the inner solar wind. This model is useful for the small-scale fluctuations in the inner heliosphere, where the turbulence is not fully developed and cannot be explained quantitatively by any of the previous intermittency turbulence models. With two model parameters, the intrinsic index of the energy spectrum <alpha>, and the fragmentation fraction P 1, the model can fit, for the first time, all the observed scaling exponents of the structure functions, which are calculated for time lags ranging from 81 s to 0.7 h from the Helios solar wind data. From the cases we studied we cannot establish for P 1 either a clear radial evolution trend, or a solar-wind-speed or stream-structure dependence or a systematic anisotropy for both the flow velocity and magnetic field component fluctuations. Generally, P 1 has values between 0.7 and 0.8. However, in some cases in low-speed wind P 1 has somewhat higher values for the magnetic components, especially for the radial component. In high-speed wind, the inferred intrinsic spectral indices (<alpha>) of the velocity and magnetic field components are about equal, while the experimental spectral indices derived from the observed power spectra differ. The magnetic index is somewhat larger than the index of the velocity spectrum. For magnetic fluctuations in both high- and low-speed winds, the intrinsic exponent <alpha> has values which are near 1.5, while the observed spectral exponent has much higher values. In the solar wind with considerable density fluctuations near the interplanetary current sheet near 1 AU, it is found that P 1 has a comparatively high value of 0.89 for V x . The impact of these results on the understanding of the nature of solar wind fluctuations is discussed, and the limitations in using structure functions to study intermittency are also described. 相似文献
2.
Pradeep V. Mandapaka Witold F. Krajewski Ricardo Mantilla Vijay K. Gupta 《Advances in water resources》2009
This study examines the role of rainfall variability on the spatial scaling structure of peak flows using the Whitewater River basin in Kansas as an illustration. Specifically, we investigate the effect of rainfall on the scatter, the scale break and the power law (peak flows vs. upstream areas) regression exponent. We illustrate why considering individual hydrographs at the outlet of a basin can lead to misleading interpretations of the effects of rainfall variability. We begin with the simple scenario of a basin receiving spatially uniform rainfall of varying intensities and durations and subsequently investigate the role of storm advection velocity, storm variability characterized by variance, spatial correlation and intermittency. Finally, we use a realistic space–time rainfall field obtained from a popular rainfall model that combines the aforementioned features. For each of these scenarios, we employ a recent formulation of flow velocity for a network of channels, assume idealized conditions of runoff generation and flow dynamics and calculate peak flow scaling exponents, which are then compared to the scaling exponent of the width function maxima. Our results show that the peak flow scaling exponent is always larger than the width function scaling exponent. The simulation scenarios are used to identify the smaller scale basins, whose response is dominated by the rainfall variability and the larger scale basins, which are driven by rainfall volume, river network aggregation and flow dynamics. The rainfall variability has a greater impact on peak flows at smaller scales. The effect of rainfall variability is reduced for larger scale basins as the river network aggregates and smoothes out the storm variability. The results obtained from simple scenarios are used to make rigorous interpretations of the peak flow scaling structure that is obtained from rainfall generated with the space–time rainfall model and realistic rainfall fields derived from NEXRAD radar data. 相似文献
3.
The geometrical and scaling properties of the energy flux of the turbulent kinetic energy in the solar wind have been studied. Using present experimental technology in solar wind measurements we cannot directly measure the real volumetric dissipation rate, <varepsilon>(t), but are constrained to represent it by its surrogate the energy flux near the dissipation range at the proton gyro scale. There is evidence for the multifractal nature of the so defined dissipation field <varepsilon>(t), a result derived from the scaling exponents of its statistical moments. The generalized dimension D q has been determined and reveals that the dissipation field has a multifractal structure, which is not compatible with a scale-invariant cascade. The related multifractal spectrum f(<alpha>) has been estimated for the first time for MHD turbulence in the solar wind. Its features resemble those obtained for turbulent fluids and other nonlinear multifractal systems. The generalized dimension D q can for turbulence in high-speed streams be fitted well by the functional dependence of the p-model with a comparatively large parameter p 1=0.87, indicating a strongly intermittent multifractal energy cascade. The experimental value for D p/3 used in the scaling exponent s(p) of the velocity structure function gives an exponent that can describe some of the observations. The scaling exponent <mu> of the autocorrelation function of <varepsilon>(t) has also been directly evaluated, being 0.37. Finally, the mean dissipation rate was determined, which could be used in solar wind heating models. 相似文献
4.
The solar wind velocity and polarity of the B x-component of the interplanetary magnetic field have been analyzed for the first eight months of 2005. The interplanetary magnetic field had a four-sector structure, which persisted during nine Carrington rotations. Three stable clusters of a high-speed solar wind stream and one cluster of a low-speed stream were observed during one solar rotation. These clusters were associated with the interplanetary magnetic field sectors. The predicted solar wind velocity was calculated since July 2005 one month ahead as an average over several preceding Carrington rotations. The polarity of the B x-component of the interplanetary magnetic field was predicted in a similar way based on the concept of the sector structure of the magnetic field and its relation to maxima of the solar wind velocity. The results indicate a satisfactory agreement of the forecast for two rotations ahead in July–August 2005 and pronounced violation of agreement for the next rotation due to a sudden reconfiguration of the solar corona and strong sporadic processes in September 2005. 相似文献
5.
Cancellation exponents and multifractal scaling laws in the solar wind magnetohydrodynamic turbulence 总被引：1，自引：0，他引：1
Some signed measures in turbulence are found to be sign-singular, that is their sign reverses continuously on arbitrary finer scales with a reduction of the cancellation between positive and negative contributions. The strength of the singularity is characterized by a scaling exponent , the cancellation exponent. In the present study by using some turbulent samples of the velocity field obtained from spacecraft measurements in the interplanetary medium, we show that sign-singularity is present everywhere in low-frequency turbulent samples. The cancellation exponent can be related to the characteristic scaling laws of turbulence. Differences in the values of , calculated in both high- and low-speed streams, allow us to outline some physical differences in the samples with different velocities. 相似文献
6.
The stability of the power law scaling of earthquake recurrence time distribution in a given space–time window is investigated,
taking into account the magnitude of completeness and the effective starting time of aftershock sequences in earthquake catalogs
from Southern California and Japan. A new method is introduced for sampling at different distances from a network of target
events. This method allows the recurrence times to be sampled many times on the same area. Two power laws with unknown exponents
are assumed to govern short- and long-recurrence-time ranges. This assumption is developed analytically and shown to imply
simple correlation between these power laws. In practice, the results show that this correlation structure is not satisfied
for short magnitude cutoffs (m
c
= 2.5, 3.5, 4.5), and hence the recurrence time distribution departs from the power law scaling. The scaling parameters obtained
from the stack of the distributions corresponding to different magnitude thresholds are quite different for different regions
of study. It is also found that significantly different scaling parameters adjust the distribution for different magnitude
thresholds. In particular, the power law exponents decrease when the magnitude cutoff increases, resulting in a slower decrease
of the recurrence time distribution, especially for short time ranges. For example, in the case of Japan, the exponent p2 of the power law scaling at large recurrence times follows roughly the relation: , where m
c
is the magnitude cutoff. In case of Southern California, it is shown that Weibull distribution provides a better alternative
fit to the data for moderate and large time scales. 相似文献
7.
M. O. Riazantseva V. P. Budaev L. S. Rakhmanova N. L. Borodkova G. N. Zastenker Yu. I. Yermolaev J. Safrankova Z. Nemecek L. Prech A. Pitna 《Geomagnetism and Aeronomy》2017,57(6):645-654
The properties of turbulent fluctuations of the solar wind plasma near the interplanetary shock observed at September 12, 2014 by the BMSW instrument are considered. The spectra of the density fluctuations in the solar wind and their statistical characteristics up-and downstream of the shock front are analyzed. They are compared with each other and with characteristics corresponding to different turbulence models. It is shown that the spectral and statistical characteristics of the density fluctuations in the solar wind conserve their basic properties after the arrival of an interplanetary shock. Intermittency is observed both before and after the front, but its level increases on average in the second case. In both regions, the scaling of the structure functions of the density fluctuations in the solar wind differ from the scaling of the classical Kolmogorov model and can be described by the log-Poisson turbulence model. Parameterization of the scaling of the structure functions revealed the presence of filamentary structures in the solar wind plasma, which provide the density intermittency in the studied space regions. 相似文献
8.
9.
Z. Vörös 《Annales Geophysicae》2000,18(10):1273-1282
10.
Small-scale (scales of ∼0.5–256 km) electric fields in the polar cap ionosphere are studied on the basis of measurements of
the Dynamics Explorer 2 (DE-2) low-altitude satellite with a polar orbit. Nineteen DE-2 passes through the high-latitude ionosphere
from the morning side to the evening side are considered when the IMF z component was southward. A rather extensive polar cap, which could be identified using the ɛ-t spectrograms of precipitating particles with auroral energies, was formed during the analyzed events. It is shown that the
logarithmic diagrams (LDs), constructed using the discrete wavelet transform of electric fields in the polar cap, are power
law (μ ∼ s
α). Here, μ is the variance of the detail coefficients of the signal discrete wavelet transform, s is the wavelet scale, and index α characterizes the LD slope. The probability density functions P(δE, s) of the electric field fluctuations δE observed on different scales s are non-Gaussian and have intensified wings. When the probability density functions are renormalized, that is constructed
of δE/s
γ, where γ is the scaling exponent, they lie near a single curve, which indicates that the studied fields are statistically
self-similar. In spite of the fact that the amplitude of electric fluctuations in the polar cap is much smaller than in the
auroral zone, the quantitative characteristics of field scaling in the two regions are similar. Two possible causes of the
observed turbulent structure of the electric field in the polar cap are considered: (1) the structure is transferred from
the solar wind, which is known to have turbulent properties, and (2) the structure is generated by convection velocity shears
in the region of open magnetic field lines. The detected dependence of the characteristic distribution of turbulent electric
fields over the polar cap region on IMF B
y
and the correlation of the rms amplitudes of δE fluctuations with IMF B
z
and the solar wind transfer function (B
y
2 + B
z
2)1/2sin(θ/2), where θ is the angle between the geomagnetic field and IMF reconnecting on the dayside magnetopause when IMF B
z
< 0, together with the absence of dependence on the IMF variability are arguments for the second mechanism. 相似文献
11.
Modelling assumptions, boundary and loading conditions have a significant effect on analytical assessment of ductility supply and demand measures for RC bridges, a structural form which had suffered extensively in recent earthquakes. In recognition of the important role played by analysis in advancing seismic design of bridges, this paper is concerned with assessing the effect of model characteristics and earthquake strong-motion selection on analytical action and deformation seismic design parameters. This is of particular significance when viewed in the light of the large capital investment and problems with the satisfaction of dynamic similitude encountered in physical testing of piers and pier-deck assemblies. The models studied range between simple fixed-base cantilever and inclusion of both soil and deck effects, represented by assemblies of springs in translational and rotational degrees of freedom. Moreover, two sets of earthquake records are used in dynamic analysis, each comprising six records covering low, intermediate and high a/v, where a and v are the peak ground acceleration and velocity, respectively. The two sets differ in the scaling procedure employed to bring them to a common level of severity; the first set is obtained by direct acceleration scaling whilst the second utilizes the concept of velocity spectral intensity. The results from static and dynamic analysis, using advanced material characterization and solution procedures, are assessed and discussed. Subject to the limitations of the study, outlined in the paper, the results indicate that the inclusion of deck stiffness and/or soil representation is essential to avail of accurate seismic response parameters. However, the effect of variations in soil stiffness and/or deck torsional rigidity applied in the analysis is rather small, compared to the inclusion/exclusion of the model feature. Moreover, it is also observed that using acceleration scaling leads to much larger scatter in the results than when velocity spectral intensity scaling is used. Finally, the results from two particular earthquakes, Friuli and El Centro, highlight the peril of using a small number of records selected without due consideration to the relationship between their wave form, predominant periods and spectral shapes on the one hand and the response periods of the structure on the other. 相似文献
12.
We studied the applicability of two types of existing three-dimensional (3-D) basin velocity structure models of the Osaka
basin, western Japan for long-period ground motion simulations. We synthesized long-period (3–20 s) ground motions in the
Osaka basin during a M6.5 earthquake that occurred near the hypothetical Tonankai earthquake source area, approximately 200 km
from Osaka. The simulations were performed using a 3-D finite-difference method with nonuniform staggered grids using the
two basin velocity structure models. To study the ground motion characteristics inside the basin, we evaluated the wave field
inside the basin using the transfer functions derived from the synthetics at the basin and a reference rock site outside the
basin. The synthetic waveforms at the basin site were obtained by a convolution of the calculated transfer function and the
observed waveform at the reference rock site.
First, we estimated the appropriate Q values for the sediment layers. Assuming that the Q value depends on the S wave velocity V
S and period T, it was set to Q = (1/3V
S)(T
0/T) where V
S is in m/s and the reference period T
0 is 3.0 s. Second, we compared the synthetics and the observations using waveforms and pseudovelocity response spectra, together
with a comparison of the velocity structures of the two basin models. We also introduced a goodness-of-fit factor to the pseudovelocity
response spectra as an objective index. The synthetics of both the models reproduced the observations reasonably well at most
of the stations in the central part the basin. At some stations, however, especially where the bedrock depth varies sharply,
there were noticeable discrepancies in the simulation results of the models, and the synthetics did not accurately reproduce
the observation. Our results indicate that the superiority of one model over the other cannot be determined and that an improvement
in the basin velocity structure models based on simulation studies is required, especially along the basin edges. We also
conclude that our transfer function method can be used to examine the applicability of the basin velocity structure models
for long-period ground motion simulations. 相似文献
13.
The effects of the characteristics of the interplanetary medium on the radar scattering occurrence, related to the whole array of SuperDARN radars installed in the Northern Hemisphere, have been studied over a two-year period. Statistically significant correlations of the variation of the scattering occurrence are found with the merging electric field and with the negative Bz component of the interplanetary magnetic field, independent of the seasonal period considered. This result demonstrates that the merging rate (and in particular the reconnection process) between the interplanetary magnetic field and the magnetosphere is a relevant factor affecting the occurrence of scattering. For comparison, we note that no statistically significant correlations are obtained when the interplanetary ion density or the solar wind speed are considered, although also these variables affect to a small degree the scattering occurrence variation. The study of the latitudinal and magnetic local time dependence of the observations shows an association between the considered correlation and the location of the auroral oval and the cusp/cleft region. 相似文献
14.
A set of monoial scaling relations to parametrize several measures of strong motion (peak velocity, peak starting-phase acceleration, peak stopping acceleration) is proposed. Dynamic solutions are obtained for a 2-D (P-SV) stress-drop model of faulting, and ground motion from these calculations is used to calibrate the scaling relations. Geometrical spreading, radiation patterns, low frequency near-field radiation, and free surface response are analysed, and introduced as corrections. The calculational finite difference method is sound within its frequency range of validity, which is found to be about 0 to 5–6 Hz for the chosen grid steps. A strong difference is obtained between theP andSV motion scalings, mainly with the source rupture velocity. Also noted are significant differences between the starting and stopping accelerations due to different frequency content and the influence of low frequency near-field radiation. To test the estimated scaling relationships, some synthetic predictions of the kinematic parameters are made, with quite good agreement when compared with dynamic computations (errors within 30%). The results emphasize some features of the near-fieldP-SV radiation and allow a kinematic prediction for a simple and smooth source model, but show the limited reliability of such predictions, arising from the problem's complexity. 相似文献
15.
The need to estimate velocity and discharge indirectly in gravel-bedded rivers is a commonly-encountered problem. Semilogarithmic friction equations are used to estimate mean velocity using a friction factor obtained from depth and grain size information. Although such equations have a semi-theoretical basis, in natural gravel-bed channels, an empirical constant (6.8 or 3.5) has to be introduced to scale-up the characteristic grain size (D50 or D84) to represent the effective roughness length. In this paper, two contrasting approaches are used to suggest that the multiplier of characteristic grain size is attributable to the effect of small-scale form resistance, reflecting the occurrence of microtopographic bedforms in gravel-bedded environments. First, spatial elevation dependence in short, detailed bed profiles from a single gravel-bedded river is investigated using semivariogram and zero-crossing analyses. This leads to objective identification of two discrete scales of bed roughness, associated with grain and microtopographic roughness elements. Second, the autocorrelation structure of the three-dimensional near-bed velocity field is examined to identify regularities associated with eddy shedding and energy losses from larger grains and microtopographic bedforms. Apart from improving the capacity to determine friction factors for velocity and discharge estimation, the findings have implications in general for the initial motion of gravelly bed material. 相似文献
16.
《Journal of Atmospheric and Solar》2007,69(15):1851-1863
Geomagnetic storms are large disturbances in the Earth's magnetosphere caused by enhanced solar wind–magnetosphere energy transfer. One of the main manifestations of a geomagnetic storm is the ring current enhancement. It is responsible for the decrease in the geomagnetic field observed at ground stations. In this work, we study the ring current dynamics during two different levels of magnetic storms. Thirty-three events are selected during the period 1981–2004. Eighteen out of 33 events are very intense (or super-intense) magnetic storms (Dst ⩽−250 nT) and the remaining are intense magnetic storms (−250<Dst ⩽−100 nT). Interplanetary data from spacecraft in the solar wind near Earth's orbit (ACE, IMP-8, ISEE-3) and geomagnetic indices (Dst and Sym-H) are analyzed. Our aim is to evaluate the interplanetary characteristics (interplanetary dawn–dusk electric field, interplanetary magnetic field component BS), the ε parameter, and the total energy input into the magnetosphere (Wε) for these two classes of magnetic storms. Two corrections on the ε energy coupling function are made: the first one is an already known correction in the magnetopause radius to take into account the variation in the solar wind pressure. The second correction on the Akasofu parameter, first proposed in this work, accounts for the reconnection efficiency as a function of the solar wind ram pressure. Geomagnetic data/indices are also employed to study the ring current dynamics and to search for the differences in the storm evolution during these events. Our corrected ε parameter is shown to be more adequate to explain storm energy balance because the energy input and the energy dissipated in the ring current are in better agreement with modern estimates as compared with previous works. For super-intense storms, the correction of the Akasofu ε is on average a scaling factor of 3.7, whilst for intense events, this scaling factor is on average 3.4. The injected energy during the main phase using corrected ε can be considered a criterion to separate intense from very intense storms. Other possibilities of cutoff values based on the energy input are also investigated. A threshold value for the input energy is much more clear when a new classification on Dst=−165 nT is considered. It was found that the energy input during storms with Dst<−165 nT is double of the energy for storms with Dst>−165 nT. 相似文献
17.
E. Costa E. Echer M.V. Alves B.T. Tsurutani F.J.R. Simões F.R. Cardoso G.S. Lakhina 《Journal of Atmospheric and Solar》2011,73(11-12):1405-1409
We present a new method of calculating cross-field diffusion of charged particles due to their interactions with interplanetary magnetic decreases (MDs) in high heliospheric latitudes. We use a geometric model that evaluates perpendicular diffusion to the ambient magnetic field as a function of particle's gyroradius, MD radius, ratio between fields outside and inside the MD, and a random impact parameter. We use Ulysses magnetic field data of 1994 to identify the MDs and get the empirical size and magnetic field decrease distribution functions. We let protons with energies ranging from 100 keV to 2 MeV interact with MDs. The MD characteristics are taken from the observational distribution functions using the Monte Carlo method. Calculations show that the increase in diffusion tends to saturate when particles' gyroradius becomes as large as MD radii, and that particles' gyroradius increases faster than diffusion distance as the energy of the particles is increased. 相似文献
18.
Measurements of turbulent fluctuations of horizontal and vertical components of velocity, salinity and suspended particulate matter are presented. Turbulent Prandtl numbers are found to increase with stratification and to become larger than 1. Consequently, the vertical turbulent mass transport is suppressed by buoyancy forces, before the turbulent kinetic energy (TKE) and vertical turbulent momentum exchange are inhibited. With increasing stratification, the buoyancy fluxes do not cease, instead they become countergradient. We find that buoyantly driven motions play an active role in the transfer of mass. This is in agreement with trends derived from Monin–Obukhov scaling. For positive Richardson flux numbers (Ri f ), the log velocity profile in the near-bed layer requires correction with a drag reduction. For negative Ri f , the log velocity profile should be corrected with a drag increase, with increasing |Ri f |. This highlights the active role played by buoyancy in momentum transfer and the production of TKE. However, the data do not appear to entirely follow Monin–Obukhov scaling. This is consistent with the notion that the turbulence field is not in equilibrium. The large stratification results in the decay of turbulence and countergradient buoyancy fluxes act to restore equilibrium in the energy budget. This implies that there is a finite adjustment timescale of the turbulence field to changes in velocity shear and density stratification. The energy transfers associated with the source and sink function of the buoyancy flux can be modeled with the concept of total turbulent energy. 相似文献
19.
The spatial scaling effect of continuous canopy Leaves Area Index retrieved by remote sensing 总被引：1，自引：0，他引：1
Leave Area Index (LAI) is one of the most basic parameters to describe the geometric structure of plant canopies. It is also important input data
for climatic model and interaction model between Earth surface and atmosphere, and some other things. The spatial scaling
of retrieved LAI has been widely studied in recent years. Based on the new canopy reflectance model, the mechanism of the scaling effect of
continuous canopy Leaf Area Index is studied, and the scaling transform formula among different scales is found. Both the
numerical simulation and the field validation show that the scale transform formula is reliable.
Supported by National Basic Research Program of China (Grant No. 2007CB714402) and National Natural Science Foundation of
China (Grant Nos. 40401036, 40734025 and 40401036) 相似文献
20.
In this study, observed seismic attributes from shot gather 11 of the SAREX experiment are used to derive a preliminary velocity and attenuation model for the northern end of the profile in southern Alberta. Shot gather 11 was selected because of its prominent Pn arrivals and good signal to noise ratio. The 2-D Gaussian beam method was used to perform the modeling of the seismic attributes including travel times, peak envelope amplitudes and pulse instantaneous frequencies for selected phases. The preliminary model was obtained from the seismic attributes from shot gather 11 starting from prior tomographic results. The amplitudes and instantaneous frequencies were used to constrain the velocity and attenuation structure, with the amplitudes being more sensitive to the velocity gradients and the instantaneous frequencies more sensitive to the attenuation structure. The resulting velocity model has a velocity discontinuity between the upper and lower crust, and lower velocity gradients in the upper and lower crust compared to earlier studies. The attenuation model has Q
p
-1 values between 0.011 and 0.004 in the upper crust, 0.0019 in the lower crust and a laterally variable Q
p
-1 in the upper mantle. The Q
p
-1 values are similar to those found in Archean terranes from other studies. Although the results from a single gather are non-unique, the initial model derived here provides a self-consistent starting point for a more complete seismic attribute inversion for the velocity and attenuation structure. 相似文献