Dynamos with a flat α-effect distribution

Abstract

In order to obtain a better insight into the excitation conditions of magnetic fields in flat objects, such as galaxies, we have calculated critical dynamo numbers of different magnetic field modes for spherical dynamos with a flat α-effect distribution. A simple but realistic approximation formula for the rotation curve is employed. In most cases investigated a stationary quadrupole-type solution is preferred. This is a consequence of the flat distribution of the α-effect. Non-axisymmetric fields are in all cases harder to excite than axisymmetric ones. This seems to be the case particularly for flat objects in combination with a realistic rotation curve for galaxies. The question of whether non-axisymmetric (bisymmetric) fields, which are observed in some galaxies, can be explained as dynamos generated by an axisymmetric αω-effect is therefore still open.     

Small Scale Kinematic Dynamos: Beyond the α-Effect

In geodynamo simulations which simulate the generation of an axial dipolar magnetic field, the generation mechanism appears to be adequately described as an α²-dynamo with an anisotropic α-effect. The anisotropy in the α-effect favors an equatorial dipole field, however, which calls into question the interpretation in terms of an α²-dynamo. It is shown in this article with kinematic dynamo calculations and exemplary velocity fields with an anisotropic α-effect that both types of dipolar magnetic field can be generated. Two examples of working dynamos in a sphere with flows with zero α-effect are also provided.     

Dynamic Variations of the Gravity Field before and after the M5. 1 Wen’an Earthquake in Hebei Province

In order to explore the abnormal variations before and after the Wen'an M5. 1 earthquake on July 4,2006,the gravity data observed by the Wenba Gravity Observation Network before and after the earthquake was analyzed. The relationships between gravity change and vertical displacement and shallow groundwater level were discussed,and elevation correction of the gravity was preliminarily performed. The results show that there were abnormal gravity changes before the M5. 1 earthquake,which appeared as gravity rising in the whole observation network,especially in the south part. A high gradient of gravity variation appeared around the epicenter before the M5. 1 earthquake,especially during the short period between October 2005 to April 2006. The boundary of the positive and negative gravity variations gradually deflected and began to recover from October 2007.     

Dynamic Variations of the Gravity Field before and after the M5. 1 Wen’an Earthquake in Hebei Province

In order to explore the abnormal variations before and after the Wen'an M5. 1 earthquake on July 4, 2006, the gravity data observed by the Wenba Gravity Observation Network before and after the earthquake was analyzed. The relationships between gravity change and vertical displacement and shallow groundwater level were discussed, and elevation correction of the gravity was preliminarily performed. The results show that there were abnormal gravity changes before the M5. 1 earthquake, which appeared as gravity rising in the whole observation network, especially in the south part. A high gradient of gravity variation appeared around the epicenter before the M5. 1 earthquake, especially during the short period between October 2005 to April 2006. The boundary of the positive and negative gravity variations gradually deflected and began to recover from October 2007.     

Spherical dynamos with anisotropic α-effect

Abstract

The mean field induction equation of Steenbeck, Krause and Rädler (1966) is solved for anisotropic α _ik -tensors of varying anisotropy. The attention is restricted to α²-dynamos in spheres with steady axisymmetric magnetic fields. The ratio of the electrical conductivity outside and inside the sphere is varied, but in all cases it is found that a steady dynamo does not exist when the anisotropy of the α _ik -tensor exceeds a critical value. Such a critical value does not exist in the exceptional case of the Fermi boundary condition. The results emphasize the important effect of boundaries on the existence of solutions of the dynamo problem.     

Dynamic behavior and applicability of lead alloy absorber

Introduction As the result of economic development, metropolitans spring out in China. In metropolitans,the cities contact with each other through life line systems, the density of population is larger thanother areas, and social treasure in the unit area is much higher than other cities (GAO, 2003). Atthe same time, the risk of potential earthquake disaster in metropolitans increases greatly. One ofcharacters of metropolitan earthquake damage is that each city in the metropolitan will …     

Correlated Crust and Mantle Strain Field in China

INTRODUCTION Moststudiesonactiveblockshavebeenfocusedonidentificationofblockboundariesandtheiractivity;inotherwords,mostoftheworkwaslimitedtothehorizontalmovementoftheblocks.Inreality,theblocksarenotonlysurroundedbyactivefaultsofhorizontalmotion,butalsoco…     

The Datong earthquake and its anomalous tilt Field

ＴｈｅＤａｔｏｎｇｅａｒｔｈｑｕａｋｅａｎｄｉｔｓａｎｏｍａｌｏｕｓｔｉｌｔｆｉｅｌｄＡｎ－ＦｕＮＩＵ（牛安福）；Ｙｉ－ＬｉｎＷＵ（吴翼麟）；Ｄｅ－ＦｕＣＨＥＮ（陈德福）ａｎｄＸｉａｏ－ＪｕｎＬＩ（李晓军）（ＩｎｓｔｉｔｕｔｅｏｆＳｅｉｓｍｏｌｏｇｙ...     

Analytical Description of the Near Planetary Bow Shock Based on Gas-Dynamic and Magneto-Gas–Dynamic Modeling for the Magnetic Field Parallel and Perpendicular to the Plasma Flow

Geomagnetism and Aeronomy - An analytical semiempirical model of the bow shock based on theoretical MGD calculations, accurate analytical solutions, and experimental data continues to be developed....     

Activity Tendency and Dynamic Characteristics of Shanxi Fault Zone

INTRODUCTIONTheShanxifaultzoneisoneofthemajorseismiczonesinNorthChinaandmanymoderateandsmallearthquakeshaveoccurredthere (WangXiuwen ,1995 ) .TheM6 1Datongearthquakewhichoccurredatthenorthernendofthefaultin 1989isoneofthem (YangGuohuaetal.,1994 ) .Inordertomakeearthquakepredictionsandtostudytherelationshipbetweentheseismicityandfaultactivityofthefault,theSeismologicalBureauofShanxiProvinceestablishedaGPSmonitoringnetworkalongthefaultfromthesouthtothenorthin 1995 .Now ,thereisalarge…     

Global Cloud Cover and the Earth’s Mean Surface Temperature

The well-known 11-year cycle in low cloud cover amount for Solar Cycle Number 22 and the trend with time for Solar Cycle Number 23 are interpreted as being due to similar changes, but of opposite phase, in the mean global surface temperature of the Earth. An analysis of cloud amounts in two higher altitude bands shows that they, and the surface temperature, are roughly in phase with each other. The suggested mechanism to explain this result is that a warming of the Earth’s surface causes low clouds to rise and to be reclassified in the next upper category. The energetics of the process are shown to be satisfactory for this to be the correct explanation.     

Preliminary Research on Relation Between Field and Source in Earthquake Preparation

The "field" and "source" are two important branches in seismology and earthquake research.In this paper,we use a computer model to study the relation between the field and the source in earthquake activity.In our modeling,a network of nonlinear elements is used to simulate seismic activity of seismic zones in a seismotectonic block.Constant strain rate is enforced on the boundary of the model,cyclic seismic activity,quasi-periodic variation of stress field intensity,and strain energy are observed with high and low fluctuations.There is a main seismicity area in seismic cycles,and the main seismicity area shows the spatial migration during different seismic cycles.If the precursory area is related to high element stress,it is found that the development of precursors in our model is quite complicated.No certain relation between the precursors and earthquakes has been discovered.Anomalies show different characteristics in the seismic quiet period and active period.All of the seismic zones in the system hav     

Calculation of Calibration Functions and Explosive Aftershock Magnitudes in the Near Field

The current calibration function used in calculating the magnitude of natural earthquakes within 5km is a constant; a fact that causes several serious difficulties for the calculation of the magnitude of small and shallow-focus earthquakes. According to the attenuation law of explosions and the propagation theory of elastic waves, the calibration function is calculated for near field quakes from 0kin to 5kin. Magnitudes of two aftershock sequences are calculated. The magnitudes of most explosion earthquakes are small, ranging mainly from magnitude -0.5 to 1.0. The M-t chart of the explosive aftershocks is completely different from that of strong earthquake aftershocks. It not only shows positive columnar lines indieatJng large magnitudes but also short negative columnar lines indicating small magnitudes.     

Calculation of magnetic field turbulent diffusivities and α-effect

Abstract

First the exact numerical solutions of DIA system of equations describing the transportation of magnetic field in an infinite medium are presented. It is assumed that the turbulence is stationary, homogeneous, isotropic and incompressible. The spectra of turbulence of δ-type and Kolmogorov's type were used. The steady state values of magnetic field diffusivity D_T and the α-effect coefficient α _T were calculated for various values of space-scale and lifetimes of these spectra and the spectra of helicity. Also investigated is the dependence of D_T and α _T on the degree of helicity. The corrections to the α _T -coefficient due to the contribution of four-order velocity correlators are given. The results are compared with those due to the self-consistent technique.     

Regional Stress Field and Seismic Dynamics Along the Border Zone Between Fujian, Guangdong and Jiangxi Provinces

The regional stress field and seismic dynamics along the border zone between Fujian, Guangdong and Jiangxi Provinces are studied based on the seismo-geological data, GPS measurement, and seismicity. The results show that: (1) the principal compressional stress of the stress field is oriented in NW-SE direction and the principal extensional stress is in NE-SW direction; (2) the WNW-ward compression and collision of the Philippine Sea Plate to the eastern coast of Taiwan Island are the most direct and most important dynamic source for preparation and occurrence of strong earthquakes in the Taiwan area and along the border zone between Fujian, Guangdong and Jiangxi Provinces.     

Dynamic stiffness matrix of a poroelastic multi-layered site and its Green＇s functions

Few studies of wave propagation in layered saturated soils have been reported in the literature. In this paper, a general solution of the equation of wave motion in saturated soils, based on one kind of practical Biot‘s equation,was deduced by introducing wave potentials. Then exact dynamic-stiffness matrices for a poroelastic soil layer and halfspace were derived, which extended Wolf‘s theory for an elastic layered site to the case of poroelasticity, thus resolving a fundamental problem in the field of wave propagation and soil-structure interaction in a poroelastic layered soil site. By using the integral transform method, Green‘s functions of horizontal and vertical uniformly distributed loads in a poroelastic layered soil site were given. Finally, the theory was verified by numerical examples and dynamic responses by comparing three different soil sites. This study has the following advantages: all parameters in the dynamic-stiffness matrices have explicitly physical meanings and the thickness of the sub-layers does not affect the precision of the calculation which is very convenient for engineering applications. The present theory can degenerate into Wolf‘s theory and yields numerical results approaching those for an ideal elastic layered site when porosity tends to zero.     

Recent Crustal Stress Field and Dislocation Along Seismic Faults in the East China Region

INTRODUCTIONThere exist many methods for studying recent crustal stress field.One of the most commonmethodsis to use focal mechanismsolution data of earthquakes to deduce the recent crustal stressfield.The method is simple and feasible and the data obtained is reliable.Yan Jiaquan,et al.(1979)usedthe method to study the recent tectonic stress field in China and its adjacent areas andgave a rough configuration and regional characteristics of the stress field.Li Qinzu(1980),WeiGuangxing,e…