Variation Of Velocity Gradient Research Articles

Hierarchical hub-filament structures and gas inflows on galaxy-cloud scales

Abstract We investigated the kinematics and dynamics of gas structures on galaxy-cloud scales in two spiral galaxies NGC5236 (M83) and NGC4321 (M100) using CO (2–1) line. We utilized the FILFINDER algorithm on integrated intensity maps for the identification of filaments in two galaxies. Clear fluctuations in velocity and density were observed along these filaments, enabling the fitting of velocity gradients around intensity peaks. The variations in velocity gradient across different scales suggest a gradual and consistent increase in velocity gradient from large to small scales, indicative of gravitational collapse, something also revealed by the correlation between velocity dispersion and column density of gas structures. Gas structures at different scales in the galaxy may be organized into hierarchical systems through gravitational coupling. All the features of gas kinematics on galaxy-cloud scale are very similar to that on cloud-clump and clump-core scales studied in previous works. Thus, the interstellar medium from galaxy to dense core scales presents multi-scale/hierarchical hub-filament structures. Like dense core as the hub in clump, clump as the hub in molecular cloud, now we verify that cloud or cloud complex can be the hub in spiral galaxies. Although the scaling relations and the measured velocity gradients support the gravitational collapse of gas structures on galaxy-cloud scales, the collapse is much slower than a pure free-fall gravitational collapse.

Effect of large-scale inhomogeneity of solar wind velocity on distribution of directions of interplanetary magnetic field vector

Using data with hourly resolution obtained in near-Earth heliosphere in 1965–2014, we have calculated statistical characteristics of the angles describing the direction of the interplanetary magnetic field (IMF): root-mean-square deviations of azimuthal and elevation angles, asymmetries of their distributions, and coefficient of correlation of the angles. It has been shown that the above characteristics varied in the course of solar cycle, and some of them changed their signs when solar polar magnetic field reversed. The results obtained from the experimental data analysis were compared with a model describing transport of large-scale disturbances of IMF lines by the inhomogeneous solar wind. The comparison has shown that the variations in the angular distribution of IMF in the course of solar cycle probably occur due to the appearance of the large-scale latitudinal gradient of solar wind velocity during solar minima. In addition, the angular distribution of IMF has been found to be substantially affected by the longitudinal velocity gradient in trailing parts of high-speed streams and short-term local-scale variations in velocity gradients.

Влияние крупномасштабной неоднородности скорости солнечного ветра на распределение направлений вектора межпланетного магнитного поля

Using data with hourly resolution obtained in near-Earth heliosphere in 1965–2014, we have calculated statistical characteristics of the angles describing the direction of the interplanetary magnetic field (IMF): root-mean-square deviations of azimuthal and elevation angles, asymmetries of their distributions, and coefficient of correlation of the angles. It has been shown that the above characteristics varied in the course of solar cycle, and some of them changed their signs when solar polar magnetic field reversed. The results obtained from the experimental data analysis were compared with a model describing transport of large-scale disturbances of IMF lines by the inhomogeneous solar wind. The comparison has shown that the variations in the angular distribution of IMF in the course of solar cycle probably occur due to the appearance of the large-scale latitudinal gradient of solar wind velocity during solar minima. In addition, the angular distribution of IMF has been found to be substantially affected by the longitudinal velocity gradient in trailing parts of high-speed streams and short-term local-scale variations in velocity gradients.

Phase wrapping of epicyclic perturbations in the Wobbly Galaxy

We use test-particle integrations to show that epicyclic motions excited by a pericentre passage of a dwarf galaxy could account for bulk vertical velocity streaming motions recently observed in the Galactic stellar disc near the Sun. We use fixed potential test-particle integrations to isolate the role of phase wrapping of epicyclic perturbations from bending and breathing waves or modes, which require self-gravity to oscillate. Perturbations from a fairly massive Sagittarius dwarf galaxy, $M_d \sim 2.5 \times 10^{10} M_\odot$, are required to account for the size of the observed streaming motions from its orbital pericentre approximately a Gyr ago. A previous passage of the dwarf through the Galactic disc approximately 2.2 Gyr ago (with a then more massive dwarf galaxy) is less effective. If phase wrapping of epicyclic perturbations is responsible for stellar streaming motions in the Galactic disc, then there should be variations in velocity gradients on scales of a few kpc in the vicinity of the Sun.

Multiple Diving Waves and Steep Velocity Gradients in the Western Taiwan Coastal Plain: An Investigation Based on the TAIGER Experiment

Seismic data collected during explosion experiments performed as part of the TAiwan Integrated GEodynamics Research (TAIGER) project provide an excellent opportunity to obtain high‐resolution images of the structure of the crust and upper mantle beneath Taiwan. The most significant feature observed at near‐source stations located on the western coastal plain in Taiwan is high‐energy later arrivals. These high‐amplitude multiples almost completely mask the lower‐amplitude signals (seismic refraction and wide‐angle reflection) from the deep crust. The later arrivals are identified as free‐surface‐reflected multiples. The nature and generation of these high‐energy, multiple diving waves are demonstrated using synthetic examples. Their generation requires the presence of a steep velocity gradient in the shallow crust. A detailed analysis of the observation data provided information on the velocity gradients in this region. An accurate layer‐velocity model, including the boundary orientation and its depth, and velocity gradient, was constructed based on a 1D waveform simulation and 2D seismic raytracing modeling for travel times. The present results indicate that the thick sediment in the survey area dips shallowly to the east, has a surface P ‐wave velocity of ![Graphic][1] , and an average velocity gradient of about 0.72/s from the surface to 3.0‐km depth. The thick sediment of the 2D model shows lateral variations in velocity gradient, increasing from west to east. This velocity model may provide useful information for future data processing to reduce multiple diving waves with the aim of enhancing the deep‐surface refraction/reflection signal. The velocity gradient calculated for the thick sediment of the western coastal plain may require a revision of the regional seismic velocity model developed for southwestern Taiwan, to improve the accuracy of regional hypocenter determinations, and to predict the strong ground motions produced by large earthquakes beneath this region. [1]: /embed/inline-graphic-1.gif

Upper crustal seismic structure along the Southeast Indian Ridge: Evolution from 0 to 550 ka and variation with axial morphology

The seismic structure of uppermost crust evolves after crustal formation with precipitation of alteration minerals during ridge‐flank hydrothermal circulation. However, key parameters of crustal evolution including depth extent and rates of change in crustal properties, and factors contributing to this evolution remain poorly understood. Here, long‐offset multichannel seismic data are used to study the evolution of seismic layer 2A and uppermost 2B from 0 to 550 ka at three segments of the intermediate spreading rate Southeast Indian Ridge. The segments differ in on‐axis morphology and structure with crustal magma bodies imaged at axial high and rifted high segments P1 and P2, but not at axial valley segment S1 and marked differences in thermal conditions within the upper crust are inferred. One‐dimensional travel time modeling of common midpoint supergathers is used to determine the thickness and velocity of layer 2A and velocity of uppermost 2B. At all three segments, layer 2A velocities are higher in 550 ka crust than on‐axis (by 7–14%) with the largest increases at segments P1 and P2. Velocities increase more rapidly (by 125 ka) at P1 with spatial variations in velocity gradients linked to location of the underlying crustal magma body. We attribute these differences in crustal evolution to higher rates of fluid flow and temperatures of reaction at these ridge segments where crustal magma bodies are present. Layer 2A thickens off‐axis at segments P1 and P2 but not at S1; both off‐axis volcanic thickening and downward propagation of a cracking front linked to the vigor of axial hydrothermal activity could contribute to these differences. In zero‐age crust, layer 2B velocities are significantly lower at segments P1 and P2 than S1 (5.0, 5.4, and 5.8 km/s respectively), whereas similar velocities are measured off‐axis at all segments (5.7–5.9 km/s). Lower on‐axis 2B velocities at segments P1 and P2 can be partly attributed to thinner layer 2A, with lower overburden pressures leading to higher porosities in shallowest 2B. However, other factors must also contribute. Likely candidates include subaxial deformation due to magmatic processes and enhanced cracking with axial hydrothermal activity at these segments with crustal magma bodies.

Velocity measurement of flow in the microchannel with barriers using Micro-PIV

An important application field of microfluidics is in microchemistry. Reactions of premixed reactants call for efficient designs of micromixers. This paper presents the investigations of a relatively little known micromixer embedded with barriers. The micro-resolution particle image velocimetry is used to measure the flow in the micromixer. The obtained results are in good agreement with numerical simulation. The results show that the barriers embedded in the microchannel lead to a large variation of velocity gradient, and make the fluid stretch and fold in the micromixer, which results in a substantial enhancement of the mixing efficiency.

Inversion of 3-D crustal P-wave velocity structure in Ningxia and its neighborhood by using direct, reflected and refracted waves

In this paper, simultaneous inversions for hypocentres and three-dimensional (3-D) crustral P-wave velocity structure of Ningxia and its neighborhood are performed, by using the data of direct P wave, reflected Pm and refracted Pn phases from the Moho discontinuity. The number of arrival-time applied to the inversion is 11 816 total, which are recorded by the Seismic Station Network of Ningxia from 1 107 local natural earthquakes happened since 1970. The LSQR algorithm with damping that has been developed in recent years to solve the large-scale sparse matrix is adopted. At different steps of iterative computing process, weighting is made on the actions of each earthquake at first and on each ray further, to aim at the inhomogeneous features of spatial distribution of earthquake and ray. Numerical analogue and computational results of actual multi-group data, and different iterative control processes, show that under a certain residual standard, there is obvious trade-off on the assignment of travel-time residuals between medium parameters and earthquakes parameters (especially the origin time and depth of earthquake), so the obtained solution is related to the selected initial values of medium parameters to some extent; but there are the basically same variation features on different results in the almost same area. That is the solution of surface layer has a close relation to the strata and landform. In Yinchuan basin there is obvious low velocity feature, and it coincides with the results of a man-made earthquake section crossing the region; tomographic patterns show that there are many low velocity regions in middle-lower crust, and it seems to have some connections between the positions of historical strong earthquakes and the low velocity region or the anomalous variations of velocity gradient.

Lateral variations in P velocity and attenuation in the D″ layer, from diffracted P waves

Abstract Diffracted P waves recorded at two stations along the same great circle are analysed to specify the lateral variations in velocity and velocity gradient in the D″ layer at the base of the mantle. Data come from long-period and broadband GEOSCOPE and (GDSN) stations. The variations in both apparent ray parameter and amplitude decay are studied as a function of the dominant period of the P wave. Regional variations in the ray parameter indicate a ±2.5% variation for the P velocity in D′, with high velocities beneath the North Pacific, Central America and northern Asia, and low velocities beneath the South Pacific, South Atlantic and northern Indonesia. The absence of correlation between these velocity variations and the lateral variations in P attenuation favour a chemical origin rather than a thermal origin for these heterogeneities. On the other hand, lateral variations in the velocity gradient are suggested from the frequency dependence of both ray parameter and P-amplitude decay.