Dynamics of equilibrium upset and electromagnetic energy transformation in the geomagnetotail: A theory and simulation using particles. 2. Numerical simulation using particles

The process of rapid geomagnetotail reconfiguration after the equilibrium upset (substorm activation) has been studied using the numerical simulation. The decisive reconfiguration feature consists in the spontaneous formation of nonlinear kinetic small-scale structures in the position of the initial current sheet (CS). Substantially different reconfiguration channels are possible depending on the value of the magnetic field component normal to CS: the dominating process can be the formation of a collisionless slow magnetosonic shock or a forced kinetic current sheet (FKCS). The first option is presented in this part of the work. During the late stages, the process is independent of the type of an initial disturbance; the disturbance intensity is spontaneously formed at a certain final level. Far from CS, the disturbance has the form of a rarefaction wave propagating from CS in background plasma. The reconfiguration results in a large-scale transformation of electromagnetic energy into the energy of plasma flows and heat.     

Dynamics of equilibrium upset and electromagnetic energy transformation in the geomagnetotail: A theory and simulation using particles. 1. Evolution of configurations in an MHD approximation

The process of equilibrium disruption in the system with a current sheet (CS) under the conditions of small magnetic field component normal to CS, which is induced by an external disturbance, has been theoretically studied within the scope of MHD. In the geomagnetotail, this disturbance can be caused by a tearing instability developing in the more distant tail section, or by a ballooning instability in the tail nearest section, or by a rapid reconfiguration at the magnetopause during the disturbance passage in the solar wind. Locally, in a limited CS section, a longitudinal momentum balance is rapidly (on the Alfvén time scale) upset when a fast MHD disturbance, the form of which depends on the presence of CS, passes along the tail. The nonequilibrium temperature, which subsequently evolves through splitting of CS into several current structures, originates on a substantially larger (due to the smallness of the normal field component) time scale. Such a reconfiguration SPONTANEOUSLY develops after the initial equilibrium upset under the action of an external (weak) disturbance. During an analysis within the scope of MHD, this reconfiguration can be described as the well-known process with two pairs of nonlinear waves propagating in both directions from the central sheet plane at constant velocities: these are fast rarefaction waves and the following slow “switching-off” shocks. However, the kinetic theory reveals substantially different relaxation channels. These channels are studied in the second and third work sections, where the kinetic numerical simulation of the problem is presented and the results of this simulation are analyzed.     

Amplification and energy transformation of the magnetized Rossby waves in the ionosphere with an inhomogeneous zonal wind. I. A theory

The generation and further dynamics of the planetary magnetized Rossby waves and inertial waves in a dissipative ionosphere in the presence of a smooth inhomogeneous zonal wind (shear flow) have been studied. The magnetized Rossby waves are caused by the interaction with the spatially inhomogeneous geomagnetic field and represent the ionospheric manifestations of usual tropospheric Rossby waves. The effective linear mechanism of amplification and mutual transformation of the Rossby and inertial waves has been revealed. For shear flows, the operators of linear problems are not self-adjoint, and the corresponding eigenfunctions are non-orthogonal; therefore, a canonical modal approach is of little use in studying such motions. It becomes necessary to apply the so-called nonmodal mathematical analysis, which has actively been developed for the last years. The nonmodal approach makes it possible to reveal that the transformation of wave-like disturbances in shear flows is caused by the nonorthogonality of eigenfunctions in the problem of linear dynamics. Thus, there appear a new degree of the system freedom and a new way of disturbance evolution in the medium.     

Dynamics of energy exchange in model samples subjected to elastic and electromagnetic impacts

The effects of elastic and electromagnetic (EM) fields are studied as an additional factor of energy exchange in the process of the deformation of a heterogeneous medium. The threshold value of initiating energy, Ktr(kp), relative to the current relaxation process is quantitatively estimated. It is shown that the estimated energy impacts below the threshold can initiate relaxation of local structural stresses and, thus, reduce the risk of a macrofracture. In a seismically active region, a similar scenario of initiation of dynamic development is considered in the local zones of potential sources of earthquakes. The possibility to determine the location, the time, and the intensity of the initiating EM impact is considered. From the experiments, the coefficient of electromechanical conversion is calculated.     

Numerical simulation on the evolution of cloud particles in 3-D convective cloud

A 3-D convective cloud model with compressible non-hydrostatic dynamics and the spectral bin microphysics of a 2-D slab-symmetric model has been used to simulate an observed supercell storm occurring on 29 June, 2000 near Bird City, Kansas, USA. The main objective of this paper is to study the evolution of particles in this convective storm with bin spectral microphysics scheme. Graupels form and grow through two mechanisms, deposition and riming, with the riming process dominant on top of the inflow and in the upper portion of main updraft. Over the outflow and during the developing and mature stages of the storm, graupel particles mainly grow through deposition with dominant unimodal spectra. Most fall out after growing up. Reducing initial relative humidity disturbance (increasing initial potential temperature disturbance) has negative impact on the formation and growth of graupels over the inflow (outflow). This study shows that large graupel and hail could be suppressed by altering the deposition and coalescence process over the inflow and main updraft. At different locations of the convective cells and with different initial humidity and potential temperature disturbance, the graupel formation and growth mechanisms are different, so as to the feasible hail suppression locations and methods. Supported by National Natural Science Foundation of China (Grant Nos. 40537034, 40805057), and Foundation of Key Laboratory of Meteorological Disaster of Ministry of Education in Nanjing University of Information Science & Technology (Grant No. KLME060202)     

Relationship between the equilibrium morphology of river islands and flow-sediment dynamics based on the theory of minimum energy dissipation

River islands are vital geomorphic units in alluvial rivers, and the variation of their morphology and position plays a significant role in regulating flow-sediment transport and channel stability. Based on the theories of minimum energy dissipation theory of fluid movement and river morphodynamics, this study uses the river islands in anabranching channels to analyze the relationship between the shape coefficient of river island and the flow-sediment dynamics under stable equilibrium conditions...     

Spontaneous tangential discontinuities and the optical analogy for static magnetic fields. VI. Topology of current sheets

Abstract

The electric surface current in a tangential discontinuity in a force-free magnetic field is conserved. The direction of the current is halfway between the direction of the continuous fields on either side of the surface of discontinuity. Hence the current sheets, i.e. the surface of tangential discontinuity, have a topology that is distinct from the lines of force of the field. The precise nature of the topology of the current sheet depends upon the form of the winding patterns in the field. Hence, invariant winding patterns and random winding patterns are treated separately. Current sheets may have edges, at the junction of two or more topological separatrices. The current lines may, in special cases, be closed on themselves. The lines of force that lie on either side of a current sheet somewhere pass off the sheet across a junction onto another sheet. In most cases the current sheets extending along a field make an irregular honeycomb.

The honeycomb pattern varies along the field if the winding pattern of the field varies. The surface current density in a tangential discontinuity declines inversely, or faster, with distance from its region of origin. The edges of weaker tangential discontinuities (originating in more distant regions) are bounded by the stronger tangential discontinuities (of nearby origin).

An examination of the force-free field equations in a small neighborhood of the line of intersection of two tangential discontinuities shows that the lines of force twist around to cross the line of intersection at right angles. If the angle between the tangential discontinuities exceeds π/2, there is also the possibilitity that the lines twist around so as to come tangent to the line of intersection as they cross it.     

Core formation and chemical equilibrium in the Earth—I. Physical considerations

Current models of planetary formation suggest a hierarchy in the size of planetesimals from which planets were formed, causing formation of a hot magma ocean through which metal-silicate separation (core formation) may have occurred. We analyze chemical equilibrium during metal-silicate separation and show that the size of iron as well as the thermodynamic conditions of equilibrium plays a key role in determining the chemistry of the mantle (silicates) and core (iron) after core formation. A fluid dynamical analysis shows that the hydrodynamically stable size of iron droplets is less than 10⁻² m for which both chemical and thermal equilibrium should have been established during the separation from the surrounding silicate magma. However, iron may have been separated from silicates as larger bodies when accumulation of iron on rheological boundaries and resultant large scale gravitational instability occurred or when the core of colliding planetesimals directly plunged into the pre-existing core. In these cases, iron to form the core will be chemically in dis-equilibrium with surrounding silicates during separation. The relative role of equilibrium and dis-equilibrium separation has been examined taking into account of the effects of rheological structure of a growing earth that contains a completely molten near surface layer followed by a partially molten deep magma ocean and finally a solid innermost proto-nucleus. We show that the separation of iron through a completely molten magma ocean likely occurred with iron droplets assuming a hydrodynamically stable size ( 10⁻² m) at chemical equilibrium, but the sinking iron droplets are likely to have been accumulated on top of the partially molten layer to form a layer (or a lake) of molten iron which sank to deeper portions as a larger droplet. The degree of chemical equilibrium during this process is determined by the size of droplets which is in turn controlled by the size and frequency of accreting planetesimals and the rheological properties of silicate matrix. For a plausible range of parameters, most of the iron that formed the core is likely to have been separated as large droplets or bodies and chemical equilibrium with silicate occurred only at relatively low temperatures and pressures in a shallow magma ocean or in their parental bodies. However, a small portion of iron that separated as small droplets was in chemical equilibrium with silicate at high temperatures and pressures in a deep magma ocean during the later stage of core formation. Therefore the chemistry of the core is mostly controlled by the chemical equilibrium with silicates at relatively low temperatures and pressures, whereas the chemistry of the mantle controlled by the interaction with iron during core formation is likely to have been determined mostly by the chemical equilibrium with a small amount of iron at high temperatures and pressures.     

A new method for the determination of the specific kinetic energy (SKE) released to pyroclastic particles at magmatic fragmentation: theory and first experimental results

Brittle magmatic fragmentation plays a crucial role in explosive eruptions. It represents the starting point of hazardous explosive events that can affect large areas surrounding erupting volcanoes. Knowing the initial energy released during this fragmentation process is fundamental for the understanding of the subsequent dynamics of the eruptive gas-particle mixture and consequently for the forecasting of the erupting column’s behavior. The specific kinetic energy (SKE) of the particles quantifies the initial velocity shortly after the fragmentation and is therefore a necessary variable to model the gas-particle conduit flow and eruptive column regime. In this paper, we present a new method for its determination based on fragmentation experiments and identification of the timings of energy release. The results obtained on compositions representative for basaltic and phonolitic melts show a direct dependence on magma material properties: poorly vesiculated basaltic melts from Stromboli show the highest SKE values ranging from 7.3 to 11.8 kJ/kg, while experiments with highly vesiculated samples from Stromboli and Vesuvius result in lower SKE values (3.1 to 3.8 kJ/kg). The described methodology presents a useful tool for quantitative estimation of the kinetic energy release of magmatic fragmentation processes, which can contribute to the improvement of hazard assessment.     

Dynamics of the mid-ocean ridge plate boundary: Recent observations and theory

The global mid-ocean ridge system is one of the most active plate boundaries on the earth and understanding the dynamic processes at this plate boundary is one of the most important problems in geodynamics. In this paper I present recent results of several aspects of mid-ocean ridge studies concerning the dynamics of oceanic lithosphere at these diverging plate boundaries. I show that the observed rift valley to no-rift valley transition (globally due to the increase of spreading rate or locally due to the crustal thickness variations and/or thermal anomalies) can be explained by the strong temperature dependence of the power law rheology of the oceanic lithosphere, and most importantly, by the difference in the rheological behavior of the oceanic crust from the underlying mantle. The effect of this weaker lower crust on ridge dynamics is mainly influenced by spreading rate and crustal thickness variations. The accumulated strain pattern from a recently developed lens model, based on recent seismic observations, was proposed as an appealing mechanism for the observed gabbro layering sequence in the Oman Ophiolite. It is now known that the mid-ocean ridges at all spreading rates are offset into individual spreading segments by both transform and nontransform discontinuities. The tectonics of ridge segmentation are also spreading-rate dependent: the slow-spreading Mid-Atlantic Ridge is characterized by distinct bulls-eye shaped gravity lows, suggesting large along-axis variations in melt production and crustal thickness, whereas the fast-spreading East-Pacific Rise is associated with much smaller along-axis variations. These spreading-rate dependent changes have been attributed to a fundamental differences in ridge segmentation mechanisms and mantle upwelling at mid-ocean ridges: the mantle upwelling may be intrinsically plume-like (3-D) beneath a slow-spreading ridge but more sheet-like (2-D) beneath a fast-spreading ridge.     

Laboratory simulation of the salt weathering of schist: II. Fragmentation of fine schist particles

Recent developments in long term landform evolution modelling have created a new demand for quantitative salt weathering data, and in particular data describing the size distribution of the weathered rock fragments. To enable future development of rock breakdown models for use in landscape evolution and soil production models, laboratory work was undertaken to extend existing schist/salt weathering fragmentation studies to include an examination of the breakdown of sub‐millimetre quartz chlorite schist particles in a seasonally wet tropical climate. Laser particle sizing was used to assess the impact of different experimental procedures on the resulting particle size distribution. The results reveal that salt weathering under a range of realistic simulated tropical wet season conditions produces a significant degree of schist particle breakdown. The fragmentation of the schist is characterized by splitting of the larger fragments into mid‐sized product with finer material produced, possibly from the breakdown of mid‐sized fragments when weathering is more advanced. Salinity, the salt addition method and temperature were all found to affect weathering rates. Subtle differences in mineralogy also produce variations in weathering patterns and rates. It is also shown that an increase in drying temperature leads to accelerated weathering rates, however, the geometry of the fracture process is not significantly affected. Copyright © 2006 John Wiley & Sons, Ltd.     

基于自适应有限元的复电阻率法2.5维复杂构造电磁场模拟

复电阻率法在矿产、油气勘探调查中发挥着重要作用.为了认识复杂构造的复电阻率法电磁场的变化规律,本文基于自适应有限元方法,采用非结构化网格,引入Cole-Cole模型,实现了电偶源2.5D复电阻率法电磁场正演,可以模拟复杂地形和地电结构,正演结果更符合野外实际地质情况.通过将本文的计算结果与半空间模型解析解、层状介质和起伏模型结果进行对比,验证了本文算法的正确性.最后,基于复杂地电模型,通过正演模拟,系统分析了地形、激电参数、复杂构造对复电阻率法电磁场的影响特征.

     

基于单能窄束伽马射线算法计算高分辨率屏蔽自然伽马探测器的三维测井响应

本文应用单能窄束伽马射线理论建立高分辨率屏蔽自然伽马探测器三维测井响应的数值模拟算法.首先利用单能窄束原理,用稳态扩散方程描述伽马光子密度的空间分布,根据扩散方程基本解、放射源空间分布、探测器位置和屏蔽情况,将屏蔽探测器上的总伽马通量表示成放射性地层中的有效探测区域上的体积分和晶体表面上有效接收面的面积分形式.并根据伽马射线传播路径和探测器屏蔽情况,给出有效探测区域的解析表达式,通过数值积分法计算任意复杂情况下探测器上的自然伽马通量,获得伽马测井响应的3D数值模拟算法.并通过数值模拟结果与模型井数据的对比验证了该算法的有效性.最后通过3D数值模拟算法系统研究考察晶体形状（圆柱形晶体、方形晶体）、晶体长度、仪器在井轴中的位置（居中屏蔽探测器、贴井壁屏蔽探测器）、以及测速等不同情况下自然伽马测井响应,设计出新型高分辨率自然伽马测井仪器.     

ATTEM系统中电流关断期间瞬变电磁场响应求解的研究

在瞬变电磁法中,由于发射电流关断时间不为零、接收线圈的谐振频率有限,早期瞬变电磁信号发生畸变,只能舍弃,因此存在着探测盲区. 针对这一问题,研究了瞬变电磁方法中发射电流关断期间总磁场的形成过程,论证了一次场、二次场和总瞬变场的关系,分析了接收线圈的频率特性和关断时间对瞬变电磁场的影响,提出从总磁场中剔除一次磁场影响的方法,从而获得电流关断期间和电流关断后的早期瞬变电磁场. 采用吉林大学自主研制的瞬变电磁测量系统(ATTEM)在长春市伊通河活断层进行勘探,进一步验证了算法的有效性,缩短了瞬变电磁法的勘探盲区,实现了近地表4 m以下的勘探,可以清晰地分辨近地表的低阻异常,提高了浅层探测精度和分辨率.     

Diffusive equilibrium in thin films provides evidence of suppression of hyporheic exchange and large‐scale nitrate transformation in a groundwater‐fed river

The hyporheic zone of riverbed sediments has the potential to attenuate nitrate from upwelling, polluted groundwater. However, the coarse‐scale (5–10 cm) measurement of nitrogen biogeochemistry in the hyporheic zone can often mask fine‐scale (<1 cm) biogeochemical patterns, especially in near‐surface sediments, leading to incomplete or inaccurate representation of the capacity of the hyporheic zone to transform upwelling NO₃^?. In this study, we utilised diffusive equilibrium in thin‐films samplers to capture high resolution (cm‐scale) vertical concentration profiles of NO₃^?, SO₄^2?, Fe and Mn in the upper 15 cm of armoured and permeable riverbed sediments. The goal was to test whether nitrate attenuation was occurring in a sub‐reach characterised by strong vertical (upwelling) water fluxes. The vertical concentration profiles obtained from diffusive equilibrium in thin‐films samplers indicate considerable cm‐scale variability in NO₃^? (4.4 ± 2.9 mg N/L), SO₄^2? (9.9 ± 3.1 mg/l) and dissolved Fe (1.6 ± 2.1 mg/l) and Mn (0.2 ± 0.2 mg/l). However, the overall trend suggests the absence of substantial net chemical transformations and surface‐subsurface water mixing in the shallow sediments of our sub‐reach under baseflow conditions. The significance of this is that upwelling NO₃^?‐rich groundwater does not appear to be attenuated in the riverbed sediments at <15 cm depth as might occur where hyporheic exchange flows deliver organic matter to the sediments for metabolic processes. It would appear that the chemical patterns observed in the shallow sediments of our sub‐reach are not controlled exclusively by redox processes and/or hyporheic exchange flows. Deeper‐seated groundwater fluxes and hydro‐stratigraphy may be additional important drivers of chemical patterns in the shallow sediments of our study sub‐reach. © 2015 The Authors. Hydrological Processes Published by John Wiley & Sons Ltd.