A comprehensive model current system for high-latitude magnetic activity - I. The steady state system

Summary. Magnetic variations at high latitudes represent the combination of magnetic perturbations from several different current systems. The relative effects of each of the contributory current systems vary from event to event, resulting in changes in magnetic perturbation patterns in a given local time sector which are a function of several variables, e.g. level of magnetospheric activity, spectrum of precipitating particles responsible for ionospheric conductivity, configuration of interplanetary magnetic field, etc. In this paper we extend the work of Hughes & Rostoker in which the steady state perturbation pattern in each local time sector was established from ground-based meridian line magnetometer data. We shall define the various current systems which contribute to the high-latitude magnetic perturbation pattern and we shall construct the model composite current system which best explains the high-latitude observations in the winter months. We shall discuss changes in this pattern as a function of season and as a function of the configuration of the interplanetary magnetic field. Finally, we shall relate the geometrical configuration of the ionosphere—magnetosphere current flow to the convection of plasma in the magnetosphere.     

Development of a polar magnetic substorm: a two-dimensional magnetometer array study

A polar magnetic substorm on 1974 September 11 was recorded by a two-dimensional array of 25 three-component magnetometers, so located that the westward ionospheric current passed over the array. The mean perturbation fields over five-minute intervals are presented at six representative epochs of the substorm, the first just before its onset and the sixth 21/2 hr later in the coda of the event. At four of the resulting 'time frames', the perturbation fields have been fitted, on a least-squares criterion, by calculated fields of three-dimensional current loops having downward field-aligned current at the east end of the ionospheric westward current, upward field-aligned current at the west end and closure in the magnetotail. The current density was constant across the width of each of these model currents. In three of the four time frames it proved necessary to introduce a bend to the northwest in the ionospheric current; this bend occurred within 27° (geomagnetic) west of geomagnetic midnight. An association with the Harang discontinuity is possible. The field-aligned current nearer to the array (in one time-frame the downward, east-end current, in two others the upward current at the west end) proved essential to secure any reasonable fit to the data. The ionospheric segment of the current loop moved at least 20° of longitude eastward, relative to the surface of the earth, between 07.48 and 08.58 UT. In this 70-min period the ionospheric segment moved at least 38° eastward relative to magnetic midnight.     

A study of two polar magnetic substorms with a two-dimensional magnetometer array

Summary. The paper reports studies of the three-dimensional magnetospheric—ionospheric current systems which produced polar magnetic substorms on 1974 September 7 and September 18. The data were magnetic perturbation fields observed with a two-dimensional array of 23 three-component magnetometers located in western Canada beneath the auroral oval. In an earlier study of a substorm of September 11 (Bannister & Gough) the fields fitted calculated field for a Boström Type 1 current loop with field-aligned currents at east and west ends of the ionospheric segment, and with uniform current density across the width. The substorms here reported could not be modelled with uniform current density. An inverse method due to Oldenburg was therefore used to estimate current density distributions, and satisfactory fits of calculated to observed field resulted. Each substorm was modelled at six representative epochs. In general the principal ionospheric current seem by the array was westward. At four epochs of the September 7 substorm and throughout the September 18 substorm, significant eastward ionospheric current (or its equivalent in terms of the fields produced) was observed north of the westward electrojet. Northwestward bends in the ionospheric current segments were found at four epochs on September 7 and at three epochs on September 18. As in the September 11 substorm (Paper 1), these bends were either west of or close to magnetic midnight. In some cases the bends may follow the auroral oval, but in others they are sharper and may be associated with the Harang discontinuity. East of geomagnetic the ionospheric currents tend to run in a constant geomagnetic midnight latitude range. The developments of the three substorms, of September 7, 11 (Paper 1) and 18, are compared. They showed a variety of shifts in longitude, though all moved eastward relative to magnetic midnight.     

The interaction of surge and tide in the North Sea and River Thames

Summary. Although the tendency for surge peaks in the Thames to occur on rising tide has been recognized for some time, no satisfactory physical explanation has been presented. The phenomenon almost certainly results from non-linear interaction between tide and surge and it is the mechanism of this interaction which is examined in the present study.
A statistical analysis of surges recorded at 10 ports located along the east coast of Britain demonstrated the development of interaction as surges propagate southwards. This analysis showed that surges tend to develop a peak on the rising tide in the Thames irrespective of the phase relationship between tide and surge in the northern North Sea.
A one-dimensional model of the River Thames was used to examine how surge-tide interaction varied for surges of differing types. In order to identify the mechanics of interaction, a new modelling technique was developed involving two models, one of tidal propagation and one of surge propagation, operated simultaneously with cross-linkages in the form of perturbation terms providing the effects of interaction. By this means it was shown that quadratic friction is the dominant interaction mechanism in the Thames.     

Geophysical model of the dynamical flattening of the Earth in agreement with the precession constant

The dynamical flattening of the Earth, as observed by geodetic techniques, is different by about 1 per cent from the value associated with the PREM density profile with hydrostatic equilibrium. In this paper, we compute a new dynamical flattening H induced by PREM mean density with hydrostatic equilibrium, to which we add lateral heterogeneities associated with (1) seismic velocity variations observed by tomography and (2) internal boundary topographies. First, we compute mantle circulation associated with the density anomalies derived from a tomography model. The flow-induced boundary deformations are then converted into additional mass anomalies which are added to the tomography model for computing the associated perturbation to the Earth's inertia tensor. Finally, we show that it is possible to obtain a dynamical flattening from the total inertia tensor (i.e. the sum of the PREM inertia tensor and of the perturbation) in agreement with that observed.     

Non-linear processes in the geodynamo: palaeomagnetic evidence

Summary. Palaeomagnetic data relating to secular variation, westward drift, non-dipole field scatter, field intensities and polarity transitions are considered together in an attempt to improve our understanding of the geodynamo. An attempt is then made to interpret these data in the light of magnetohydrodynamic theory. It is suggested that polarity transitions are not characterized by significant periods of convection stasis but that dynamic processes appear to be acting throughout. Phenomena such as variable westward drift rates are explained in terms of non-linear effects relating to the magnetic field intensity. Several lines of evidence suggest that the Lorentz force is a significant body force within the Earth's core and cannot be treated as a perturbation. However, it is also suggested that the Lorentz force may become insignificant during polarity transitions, particularly during that part of a transition when the intensity is reduced and the magnetic field is predominantly non-dipolar.     

基于西通道的国家西向流通网络特征与组织模式

“一带一路”倡议促使国家进入全面开放、陆海统筹、门户引领的发展新阶段。相对以往侧重海洋方向上的城市流通网络研究,内陆方向在内容和方法上都有待拓展。构建“流通成本—通达能力与生产外向型程度—市场竞争与政策保护—流通结构与组织模式”分析框架,利用空间分析技术,基于西通道对国家西向流通网络结构特征与货流组织模式进行研究。研究表明：高等级流通节点在最小时间与运费成本路径上,表现出整体上的近似性;流通节点货流生产能力差异显著;构建西向多层级轴—辐物流网络,市场竞争条件下沿海地区尤其是长三角、珠三角节点具有显著的货流规模优势,而地方补贴政策下内陆节点可辐射沿海地区并展开货源竞争;根据节点、线路、流量等方面特征,可提炼出国家西向流通的“三角洲结构”。在筛选出西向流通核心节点群体基础上,提出西向货流组织的四种模式：以顶点城市为主导的统一编组集结模式、以内陆省会城市为主导的货源集结模式、以发达外贸型城市为主导的货源产区发车模式、以重要港口城市为主导的货源集散型模式。西向流通经历了独创、扩散、分散阶段,并将走向整合阶段,西向流通的主导权将最终回归位于优势空间的核心节点城市。     

南极地区地磁扰日变化S_D场特征

利用南极地磁资料研究了地磁扰日变化ＳＤ场特征。１）ＳＤ场变化不如Ｓｑ场变化规则,无明显的昼夜差异;太阳活动高年（１９９１年）比下降年（１９９４年）ＳＤ场变化更强烈。２）极隙区中山站ＳＤ场变幅比亚极光区长城站和北半球低纬区北京台的ＳＤ场变幅大１０倍以上。３）中山站Ｓ０Ｄ（Ｈ）变化呈“正弦波”形态,与长城站和北京台Ｓ０Ｄ（Ｈ）变化形态相反。４）中山站Ｓ０Ｄ等效电流矢量在早晨－下午（０２～１５ｈ）矢量方向为反时针西向,下午－夜间（１５～０２ｈ）矢量方向为顺时针东向,这是由极区东向、西向电射流和极区电流涡旋所引起;长城站与北京台Ｓ０Ｄ等效电流矢量很小,这是由极区东向和西向电射流的影响所致。     

Modelling of solar quiet magnetic field variations near a conductivity anomaly

Summary. A simplified model of the solar quiet-time ionospheric current system is used to calculate the induced currents in a model earth. The conductivity is assumed to be constant below a depth of about 400 km and zero above that depth. The current induced in the north—south conductivity anomaly under the Rocky Mountains is then estimated from the time-varying potential difference between points at 30 and 45° latitude at the surface of the conducting sphere. The purpose of these calculations is to investigate whether variations in the latitude of the northern hemisphere current system vortex will substantially alter the relationship between the observed magnetic field components at the Earth's surface and the local magnetic field gradient caused by the conductivity anomaly. We find that a 10° shift in the latitude of the ionospheric current focus causes a change of 6 per cent or less in the transfer function from the field components to the gradient in the total field. Thus such latitude shifts cannot explain much of the magnetic field gradient variation at periods near 24 hr that has been observed near Boulder, Colorado.     

Equation for the field lines of an axisymmetric magnetic multipole

Summary. An exact equation is derived for the magnetic field lines of the general axisymmetric magnetic multipole of arbitrary degree ( n ). This new result has important applications in studies of the possible nature of solarterrestrial physics during geomagnetic polarity reversals. In the limiting case of a magnetic dipole ( n=1 ), the equation for the magnetic field lines of the general axisymmetric magnetic multipole simplifies correctly to the well-known dipolar form, which is used extensively in geomagnetism, magnetospheric physics and cosmic-ray physics as a first-order approximation to the actual configuration of the geomagnetic field.
It is also shown theoretically that suites of similar magnetic field lines of the general axisymmetric multipole attain their maximum radial distances from the origin on a set of circular conical surfaces, with coincident vertices at the centre of the Earth; this set includes the equatorial plane if the degree ( n ) of the multipole is odd. The magnetic field is horizontal everywhere on all these surfaces.
Palaeomagnetic studies have suggested that during some polarity reversals the magnetic field in the inner magnetosphere can be represented approximately either by a single, non-dipolar, low-degree (2 < n < 4), axisymmetric magnetic multipole or by a linear combination of such multipoles. In this situation, the equation for the field lines of an axisymmetric magnetic multipole of low degree (2 < n < 4) would be as fundamental to a proper understanding of magnetospheric, ionospheric and cosmic-ray physics during polarity reversals as is the equation for dipolar field lines in the case of the contemporary geomagnetic field.     

A study on the multi-component substrom current

A new technique of eigen mode analysis, Method of Natural Orthogonal Components (MNOC) is used to analyze the ionospheric equivalent current systems obtained on the basis of magnetic data at six meridian magnetometer chains in the northern hemisphere during March 17 19, 1978. The results show that the whole current pattern for any given instant consists of a few eigen modes with different intensities. The first eigen mode exhibits a two cell current construction, characterizing the large scale magnetospheric convection and directly driven process, while the second eigen mode shows a concentrated westward electrojet at midnight sector, characterizing the substorm current wedge and the loading unloading process. The first mode consistently exists whenever during quiet periods or at substorms, and its intensity increases from the beginning of the growth phase of substorms, then quickly intensifies in the expansion phase, followed by a gradual decrease in the recovery phase. On the other hand, the intensity of the second mode remains to be near zero during both quiet time and the growth phase of substorms. Its rapid enhancement occurs in the expansion phase. These characteristics in the current patterns and the intensity variations coincide with the defined physical processes of the directly driven and loading unloading components.     

Modelling the Venice Lagoon

In this paper, an overview is given on the modelling activities at the National Research Council in Venice (ISDGM-CNR) concerning the Venice Lagoon. A short history of former modelling efforts is given. A finite element hydrodynamic model is presented that has been created especially for the Venice Lagoon. This model has been applied to various situations. Residual currents have been computed for one year. A specific storm surge event has been simulated with spatially varying real winds. A transport diffusion model has been applied to model the impact of a treatment plant on the central lagoon. And a sub-basin has been modeled with a high resolution grid.     

Identifying factors of influence in the spatial distribution of domestic fires

Domestic fires at the city level, being causes for casualties and causing significant material damages, are stored as a point pattern in a GIS. In this paper we apply a statistical point pattern analysis to derive major causes from related layers of information. We fit a G-function to analyse neighbourhood relations and a Strauss process for inferring causal relations. Using open-source software we find significant differences in patterns and explaining factors between the different parts of the day, in particular for different building types and income groups. We conclude that a quantitative spatial model can be fitted and that this provides a useful opportunity for fire brigades to improve planning their efforts.     

Propagation and source of Pc5 frequency range pulsation at cusp latitude

Two induction magnetometers have been installed at Chinese Zhongshan Station and Australia Davis Station, Antarctica respectively. We adopt the cross spectral analysis technique to analyze the data of the two induction magnetometers, in June, September, December 1996 and March 1997, and to investigate Pc5 frequency range pulsation (150 600 s) occurrence and propagation in cusp latitude. The results are summarized as follows: At Zhongshan Davis Station, the magnetic pulsations in Pc5 frequency band can occurs over a wide time, but more frequently at pre local magnetic noon and pre local magnetic midnight. The Pc5 pulsations have no significant seasonal variation in the amplitude, occurrence and propagation. The amplitude has a small peak at pre local magnetic noon and large value sometimes at pre local magnetic midnight. In daytime, the Pc5 pulsations propagate westward in morning and eastward in afternoon, and reversal at local magnetic noon. In nighttime, the Pc5 pulsations propagate westward before 20:00 MLT and eastward after 20:00 MLT. Near dusk time, the Pc5 pulsations propagate irregularly. These characteristics indicate that the Pc5 pulsations have different source at different local magnetic time.     

Tectonic heritage in drainage pattern and dynamics: the case of the French South Alpine Foreland Basin (ca. 45–20 Ma)

The spatial and temporal organization of depositional environments in drainage networks of foreland basins reflect the tectonic and erosional dynamics associated with the development of mountain belts. We provide field evidences for the initiation and evolution of a complex drainage system in the French South Alpine Foreland Basin related to Western Alps exhumation. Sedimentological and structural analyses of the Eocene–Early Miocene succession were investigated in the (1) Argens/Peyresq, (2) Barrême/Blieux/Taulanne and (3) Montmaur/St‐Disdier sectors. Combined with the existing structural data set, we propose a new model that integrates the regional tectonic activity, the palaeovalley orientation and their dynamics through time. The Eocene–Miocene deposits clearly show the existence of N–S‐oriented palaeovalleys. The systematic presence of early NE–SW‐ to N–S‐oriented strike‐slip and extensional faults in the palaeovalleys suggests that these tectonic structures were responsible for the formation of the initial N–S‐oriented basin‐floor topographies. The vertical offset of the strike‐slip faults induced sufficient accommodation space for the Cenozoic sedimentation since the Middle Eocene. It implies the creation of N–S‐oriented palaeovalleys during the northward Pyrenean‐Provençal phase, pre‐dating westward Alpine compression. Later, the Oligocene Alpine tectonic phase induced drainage expansion toward the orogenic wedge and the erosion of the exhumed internal massifs by transverse streams. The establishment of new connections between the old topographic lows formed a longitudinal drainage pattern that remains the locus of deposition in a regional sedimentary routing system. In this model, former strike‐slip faults correspond to weakness zones overprinted by the westward Alpine shortening that allowed the formation of the modern piggyback basin structure of the foreland and the long‐time preservation of the palaeovalley geometry.     

近53年内蒙古寒潮时空变化特征及其影响因素

利用内蒙古及其周边121个气象台站1960-2013年逐日最低气温数据,辅以分段线性回归模型、趋势分析及相关分析等方法,本文探讨了近53年内蒙古寒潮频次的时空变化特征及其影响因素。研究发现 (1) 近53年内蒙古单站寒潮频次总体呈下降趋势,降速为-0.5次/10a (-2.4~1.2次/10a),其中1991年之前降速为-1.1次/10a (-3.3~2.5次/10a),而1991年之后呈增加趋势,增速为0.45次/10a (-4.4~4.2次/10a);春季寒潮变化趋势与年变化趋势一致,且在各季节中变化最为显著;寒潮频次年内变化呈“双峰”结构特征,且以11月最多;(2) 空间上,内蒙古单站寒潮频次具有显著的空间差异特征,高发区集中在内蒙古的北部和中部地区,且北部高于中部。年代尺度对比来看,20世纪60-90年代寒潮高频区域范围在减少、低频区域范围在增加;而21世纪初期高频区域范围有所增加,增加区主要为内蒙古东部的图里河、小二沟,以及中部的西乌珠穆沁旗等地;(3) 就年尺度而言,寒潮主要受AO、NAO、CA、APVII和CQ控制,而各季节驱动因素有所差别,冬季寒潮与AO、NAO、SHI、CA、TPI、APVII、CW和IZ均达到显著相关关系,说明冬季寒潮受多种因素共同控制;秋季寒潮主要受CA和IM影响;而春季寒潮与CA和APVII关系显著。     

Spectral-finite-element approach to two-dimensional electromagnetic induction in a spherical earth

We present a spectral-finite-element approach to the 2-D forward problem for electromagnetic induction in a spherical earth. It represents an alternative to a variety of numerical methods for 2-D global electromagnetic modelling introduced recently (e.g. the perturbation expansion approach, the finite difference scheme). It may be used to estimate the effect of a possible axisymmetric structure of electrical conductivity of the mantle on surface observations, or it may serve as a tool for testing methods and codes for 3-D global electromagnetic modelling. The ultimate goal of these electromagnetic studies is to learn about the Earth's 3-D electrical structure.
Since the spectral-finite-element approach comes from the variational formulation, we formulate the 2-D electromagnetic induction problem in a variational sense. The boundary data used in this formulation consist of the horizontal components of the total magnetic intensity measured on the Earth's surface. In this the variational approach differs from other methods, which usually use spherical harmonic coefficients of external magnetic sources as input data. We verify the assumptions of the Lax-Milgram theorem and show that the variational solution exists and is unique. The spectral-finite-element approach then means that the problem is parametrized by spherical harmonics in the angular direction, whereas finite elements span the radial direction. The solution is searched for by the Galerkin method, which leads to the solving of a system of linear algebraic equations. The method and code have been tested for Everett & Schultz's (1995) model of two eccentrically nested spheres, and good agreement has been obtained.     

Estimating spatial efficiency using cyber search,GIS, and spatial optimization: a case study of fire service deployment in Los Angeles County

ABSTRACT

The efficiency of public investments and services has been of interest to geographic researchers for several decades. While in the private sector inefficiency often leads to higher prices, loss of competitiveness, and loss of business, in the public sector inefficiency in service provision does not necessarily lead to immediate changes. In many cases, it is not an entirely easy task to analyze a particular service as appropriate data may be difficult to obtain and hidden in detailed budgets. In this paper, we develop an integrative approach that uses cyber search, Geographic Information System (GIS), and spatial optimization to estimate the spatial efficiency of fire protection services in Los Angeles (LA) County. We develop a cyber-search process to identify current deployment patterns of fire stations across the major urban region of LA County. We compare the results of our search to existing databases. Using spatial optimization, we estimate the level of deployment that is needed to meet desired coverage levels based upon the location of an ideal fire station pattern, and then compare this ideal level of deployment to the existing system as a means of estimating spatial efficiency. GIS is adopted throughout the paper to simulate the demand locations, to conduct location-based spatial analysis, to visualize fire station data, and to map model simulation results. Finally, we show that the existing system in LA County has considerable room for improvement. The methodology presented in this paper is both novel and groundbreaking, and the automated assessments are readily transferable to other counties and jurisdictions.     

极区电离层与磁层-电离层耦合

本文利用欧洲空间局设在北极区的非相干散射雷达系统EISCAT的观测资料讨论磁层-电离层耦合。磁静日和磁暴期间的观测样本比较表明,磁层基本过程对极区电离层有重要影响。这不仅说明极区电离层可看作这些过程的“全景屏幕”,也表明在南、北极区(特别是磁共轭点)进行同时观测的重要意义。文中还对进一步的南极研究课题提出了一些建议。     

Validity of the great circular average approximation for inversion of normal mode measurements

Summary. Synthetic seismograms based upon first-order perturbation theory are analysed to test the validity of assumptions which form the basis of current velocity inversion procedures. It is found that the lowest order geometrical optics approximation, namely that measured normal mode eigen-frequencies reflect the average structure underlying the source–receiver great circle path, becomes less valid near nodes in the source radiation pattern and near the surface wave foci at the source and its antipode. These failures are a consequence of singlet interference within an isolated normal mode multiplet. The technique of determing frequency by fitting a single resonance peak to a multiplet yields results which agree well with the first-order theory for slow and fast paths where excitation is dominated by one pair of singlets but on intermediate paths where singlet interference is more of a problem, agreement is not as good. Inversion of small data sets is particularly sensitive to frequency fluctuations near radiation nodes, while larger sets are influenced more by antipodal deviations from geometrical optics. The latter leads to inversions which fail to recover the short wavelength structure of the starting model. Basing inversions directly upon first-order theory shows promise of improving recovery of short wavelengths.