Features Of Arc Research Articles

Andaman-Sumatra island arc: 1. Spatiotemporal manifestations and focal mechanisms of the earthquakes

The spatiotemporal manifestations of seismicity in the Andaman-Sumatra island arc are studied using the instrumental data for 1900–2010. The data on the largest tsunamigenic earthquakes of the 18th–19th centuries were also taken into account. The epicenters of the earthquakes are established to cluster in some areas; their possible relation to the structural features of the island arc is considered. A distinctive feature of seismicity in the region of the Andaman Sea is the presence of compact swarms of numerous earthquakes occurring during short intervals of time. The distribution of the earthquakes by the depth of their hypocenters in different segments of the island arc is investigated. The focal mechanisms of the earthquakes are analyzed using the centroid-moment-tensor (CMT) determinations over the period of 1980–2004, and the characteristic features of their parameters in different segments of the Andaman-Sumatra island arc are formulated. The focal parameters of the earthquakes determined by CMT and the moment-tensor-solution (MTS) are compared; the possible uncertainty in the estimates of the focal mechanisms is assessed. The pattern of the spatiotemporal manifestations of the Andaman-Sumatra earthquakes and their focal mechanisms are compared to the data on the Kuril-Kamchatka and the Aleutian island arcs previously studied by the authors. The results of analyzing the long-term seismicity and focal mechanisms in the Andaman-Sumatra island arc provide a necessary basis for the further thorough investigation of the geological conditions and source parameters of the major Sumatra earthquakes of 2000–2010.

New Static “AirArc” EMTP Model of Long Arc in Free Air

In this paper, long arcs in free air are investigated. A long arc was induced in the high-power test laboratory at FGH-Mannheim, Germany. From the recorded arc data, the features of the arc were derived and analyzed, including elongation effects associated with these arcs. Based on this analysis of a real arc, a new “AirArc” static arc model, including the length variations, was derived and incorporated into the Alternate Transients Program-Electromagnetic Transients Program software package in which it was simulated. The arc is modeled as a current-dependent voltage source with a characteristic-distorted rectangular voltage that elongates nonlinearly over time. Fault arcs are a significant source of harmonics that distort other voltages and currents in the network. The time-domain and spectral-domain features of the simulated arc are compared with those of the real arc. The assessment of transient processes during arcing faults was carried out using the discrete wavelet transform.

Progress in the design of a Sub-Harmonic Arc Detector for ITER

Arc detection systems are developed for ICRH on ITER to prevent arcs from damaging the ICRF components. One of the detectors, the Sub-Harmonic Arc Detector (SHAD) is based on the detection of the frequencies emitted in the MHz range by arcs [1], and is consequently resilient to fast load changes. A prototype has been in operation on ASDEX-Upgrade for ten years and has demonstrated its capability to detect arcs. However, to reach the safety level required by ITER, the design has to be modified to eliminate the spurious detections triggered by plasma emission in the MHz range and to enhance the sensitivity to low-voltage arcs. These improvements, which must preserve the simplicity of the system to guarantee its reliability, are based on the experience of RF arcs in resonant structures acquired on ASDEX-Upgrade and on dedicated test-benches. We present the main features of arcs in time and frequency domains and their impact on the behaviour of the SHAD system. Several technological solutions are based on these data and have to support the harsh environment of a burning plasma experiment. The shielding, the location of the detector, the reliability can be affected by the nuclear radiations. These elements are integrated in the design to make of the SHAD a primary safety system for ICRF heating.

Chronology, Geochemistry and Tectonic Settings of the Hadamiao Granodiorite on the Northern Margin of the North China Platform

Abstract:The Hadamiao granodiorite, located on the northern margin of the North China platform and acting as the country rock of gold deposits in the Hadamiao region, was formed in the same age and similar tectonic settings with the Hadamiao gold deposit and the large‐scale Bilihe gold deposit in the same area. By using the LA‐ICP‐MS method, the U‐Pb age obtained is 267±1.3 Ma, which represents the crystallized age of the granodiorite, and that of the xenolithic zircon is 442.8±5 Ma. Base on the main elements, it exhibits the features of calc‐alkaline to high‐potassium calc‐alkaline series, low silicon, and quasi‐aluminous I‐type granites, and with high magnesium (Mg#=0.45–0.57) and high sodium contents (Na2O/K2O=0.98–2.29). The SREE values (81.6–110.15 ppm) are relatively low, the fractionations between LREE and HREE are obvious, showing a right‐inclined dispersion in the REE distribution diagram. Compared with the primitive mantle, the rock is relatively rich in LREE (La and Ce), LILE (K, Sr, and Th), and intensively depleted in HFSE (Ti, P, Nb and Ta). The ratios of Sr/Y and (La/Yb)N and the contents of Rb, Nb and Y are relatively low, the Sr values are high (436.35–567.26 ppm), and the Yb contents of most samples are low (1.25–1.8), which indicate the features of typical continental margin arc and adakitic rocks. According to the values of εNd(t) (–2.4 to +0.2) and ISr (0.7028–0.7083), and variations of the La/Sm ratios, the Hadamiao granodiorite was formed from mixing of the thickened molten lower crust and the mantle wedge substances. The rock was related to the southward subduction and accretion of the Paleo‐Asia Ocean in the Late Paleozoic, being Late Paleozoic magma of the continental margin arc formed on the basement of the Early Paleozoic accretion complexes, and showing a trend of turning into adakitic rocks, which indicates their great metallogenic (Au) potential.

Late Cambrian/Ordovician magmatic arc type volcanism in the Southern Gemericum basement, Western Carpathians, Slovakia: U–Pb (SHRIMP) data from zircons

The low-grade crystalline basement of the Southern Gemeric Unit (Inner Western Carpathians) consists mainly of siliciclastic deep-water turbidite sequence associated with a huge mass of redeposited rhyolite/dacite volcanics and their volcaniclastics. In situ U–Pb SHRIMP zircon dating applied to metavolcanic rocks contributed to specification of the Southern Gemeric crystalline basement stratigraphy (the Gelnica Group). Zircon populations from ten samples of three lithostratigraphic units of the Gelnica Group rock complex have been analysed. The zircons presented significant magmatic zoning. Part of the magmatic zircons contained inherited older cores. The Vlachovo Formation gives the 494 ± 1.6 Ma zircon average age. The Bystrý potok Formation gives 465.8 ± 1.5 Ma and the Drnava Formation 463.9 ± 1.7 Ma average age. The geochemical data are compatible with an orogenic geodynamic setting. Dominant rhyolite–dacite volcanites belong to the peraluminous, calc-alkaline high-K magmatic series. The age data let us presume two phase longlasting magmatic activity, starting in the Late Cambrian and continuing to the Middle/Late Ordovician. Thus, the studied Late Cambrian volcanic sequence of the Gelnica Group reflects the active continental margin setting. The second Middle Ordovician volcanic phase documents an assumed rifting stage, accompanied by a strong volcanic activity with inherited features of the Late Cambrian magmatic arc. The inherited cores of magmatic zircons have Proterozoic (600–760, 1,000–1,400, 2,000–2,700 Ma) and Archean (3,099–3,164 Ma) ages.

North Qinling Paleozoic granite associations and their variation in space and time: Implications for orogenic processes in the orogens of central China

Integrated zircon U-Pb dating and whole rock geochemical analyses have been carried out for two typical S- and I-type granitoids in the north Qinling. Zircon dating by SIMS of the Piaochi S-type granitoids yields an emplacement age of 495±6 Ma. The granitoids show whole-rock ɛ Nd(t)=−8.2–−.8.8, zircon ɛ Hf (t)=−6–−39. The Huichizi I-type granitoids have emplacement ages of 421±27 Ma and 434±7 Ma established by LA-ICP-MS and SIMS methods, respectively. Their whole-rock ɛ Nd(t)=−0.9–0.9 and zircon ɛ Hf(t)=−11–8.4. Combined with statistical analyses of 28 zircon ages of granitoid plutons collected from the literature, Paleozoic magmatism in the north Qinling can be divided into three stages. The first-stage magmatism (∼505−470 Ma) mainly occurred in the east part of the north Qinling and has features of an I-type arc, associated with which are S-type granitoids such as Piaochi pluton. The early granitoids (∼505−490 Ma) have close spatio-temporal relations to ultra-high-pressure (UHP) rocks, and thus are interpreted as an oceanic subduction system along a continental margin. The second-stage magmatism (∼450−422 Ma) occured through the whole north Qinling, and is characterized by I-type granitoids represented by the Huichizi pluton. The magma is interpreted as partial melting of lower crust mixed by mantle-derived magma in a collisional setting with the uplift of terranes. The third-stage magmatism (∼415−400 Ma) is dominated by I-type granitoids and only took place in the middle part of the north Qinling, and is regarded as a late-stage collision. The spatial and temporal variations of the Qinling Paleozoic magmatism reveal protracted subduction/collision. The subduction was initiated from the east part of the north Qinling, earlier than that in the Qilian-northern Qaidam, Kunlun, and northern Dabie regions. This demonstrates variations in time of subduction, accretion and collision of separate blocks or terranes in the orogenic systems in central China.

Special features of arc mechanised welding of aluminium with controlled pulse electrode wire feed

Special features of arc mechanised welding of aluminium with controlled pulse electrode wire feed

저압배선계통에서 직렬아크의 검출에 관한 연구

This paper dealt with the detection algorithm of series arcing, which is a cause of electric fires in low-voltage wiring systems. To find the distinguished electrical features of series arc, we simulated series arcing by the arc generator specified in UL1699. An electric heater, an inverter-controlled vacuum cleaner, and a phase-controlled incandescent lamp were used as loads to generate series arcing. A high-pass filter (HPF) with the low cut-off frequency of 3 kHz at -3 dB was fabricated and applied to separate the series arc signal from the AC voltage source. The experiment showed that the high frequency signal generates randomly during series arcing, and the phase-controlled incandescent lamp produces high frequency pulses even in normal state. In this case, the magnitude, the width, and the randomness of high frequency signal should be analyzed to estimate series arcing precisely.

Nonlinear oscillations due to nonlinear Arc characteristics

The negative U-I-characteristic of a high-pressure arc causes unstable current oscillations in selected LC-networks. In the present paper three different types of oscillations are analyzed. The relevant nonlinear differential equations are solved by numerical and analytical methods as well as by graphical integration procedures. Two types of stable oscillations occur. They can be represented either by stable sinusoidal or almost triangular waveforms or by a Van der Pol oscillation. The third kind of oscillation is unstable and grows exponentially only for small amplitudes whilst it increases considerably faster for larger amplitudes. Our results agree with the experimentally observed features of arcs in electrical power networks.

Features of stable diffuse arcs observed by means of auroral tomography

Abstract. In this paper we study the spatial distribution of optical volume emission rates and peculiarities of the luminosity intensity within weak diffuse auroral arcs recovered by means of auroral tomography. The tomographic images are obtained from sets of scanning multi-channel photometer data obtained in February 1999 on the Kola Peninsula in Russia at three sites of a chain extending 226 km along the geomagnetic meridian. The 427.8- and 557.7-nm emissions of a 15-s time resolution observed within one hour during low geomagnetic activity are analyzed. We found that the intensity profile of an individual arc along the geomagnetic meridian has an inverted-V-shape. The luminosity maximum altitude decrease by 4–14 km at about 140 km distance in the south-north direction can be observed during two or more diffuse arcs. The parameters of the precipitating electron flux are obtained from an integral equation, which determines the best relationship between the 427.8-nm intensity height profile and an arbitrary particle energy spectrum. A dimensionless function of the energy dissipation is used as the core in the integral equation. The estimated average energy of electron flux, which generated the isolated diffuse arc, is 1–2 keV higher in the central part of the arc in comparison to values at its borders.

VPPA welds of Al-2024 alloys: Analysis and modelling of local microstructure and strength

The microstructural features of variable polarity plasma arc welded Al–Cu–Mg 2024-T351 with 2319 filler have been studied by TEM, SEM and DSC. Fusion zone, partial melting zone, re-solutionising zone, overageing (for S-phase), peak ageing (for S-phase) and under ageing zones (for S-phase) have been identified. The Ω-phase has been observed between re-solutionising zone and peak ageing zone. The hardness profile contains two peaks. The microstructure development, and resulting hardness and yield strength profiles are modelled using a model which combines primary precipitation, resolution, partial/full melting and resolidification (Scheil type) and re-precipitation, in a two precipitate–two mechanism approach. Hardness profiles and microstructures are accurately predicted. The hardness peak in the re-solutionising zone is due to re-solutionising and subsequent Cu–Mg co-cluster formation; and the second hardness peak is caused by S-phase strengthening.

Reply to Comment on “Steady-state 226Ra/ 230Th disequilibrium in mantle minerals: implications for melt transport rates in island arcs” by R. George, M. Reagan, S. Turner, J. Gill, B. Bourdon

We agree with George et al. that high 226Ra/ 230Th and correlated Ba/Th, 238U/ 230Th, and other elemental and isotopic ratios are the result of fluid addition to the wedge. The Feineman and DePaolo [Earth Planet. Sci. Lett. 215 (2003) 339–355] manuscript addresses more specifically the pathway followed by the fluid en route to the surface. In particular, we address whether the 226Ra/ 230Th data directly yield the total time available for the fluid to migrate from the point of origin in the slab to the melting region in the wedge, and ultimately to the surface with the melt. All of the processes involved in generating arc volcanism would of necessity have to take place very rapidly within the constraints proposed by George et al. The alternative is that the upward fluid movement is hindered by the fluid flow regime and solid mantle flow, allowing more time for reaction with the mantle wedge. There is evidence in the U-series data for more than one time scale, and our model helps to explain how this could come about. The extent to which grainscale 226Ra/ 230Th disequilibrium affects the inferred melt transport time is yet another issue, and this could be achieved either with the small-degree melt mechanism we discussed or by additional effects associated with melt and fluid migration at later stages in the melt generation process. We agree that the final melt transport event is likely to happen quickly. One of the most puzzling features of volcanic arcs is that the volcanic front emerges considerably behind the presumed location of primary water release from the subducting slab (depth to slab is ∼120 km beneath the volcanic front, as opposed to the predicted ∼80 km). This offset between the expected site of fluid release and the location of the volcanic front can be attributed to coupled transport of the fluid—in the form of hydrous minerals, such as amphibole and/or phlogopite in the wedge [J. Geophys. Res. 97 (1992) 2037–2070] or redirection of the melt due to flow and stresses in the mantle matrix as the magma slowly percolates upward [J. Geol. 95 (1987) 285–307]. In either case, the fluid and/or melt is required to ascend slowly, at least for some part of its journey, to explain the location of the volcanic front. Furthermore, 238U/ 230Th disequilibria in arc lavas seem to suggest a relatively long interval for fluid addition (∼30–60 ka) [Science 292 (2001) 1363–1366; Earth Planet. Sci. Lett. 179 (2000) 581–593; Geochim. Cosmochim. Acta 61 (1997) 4855–4884], which is difficult to reconcile with the 226Ra/ 230Th disequilibrium observed in the lavas. For these reasons, we sought to find a means whereby apparently short-lived isotopic disequilibria could be maintained during complex fluid and melt migration in the mantle wedge. Below, we address a few of the main issues presented by George et al. [Earth. Planet. Sci. Lett. this issue] in their comment.

Long arc in free air elongation effects modeling and simulation

AbstractIn the paper the elongation effects on the long arc in still air are investigated and modelled. Arc is initiated under laboratory conditions in the high power test laboratory FGH‐Mannheim (Germany). Based on the analysis of laboratory arc voltage and current wave forms, the new dynamic arc model, taking into account the arc length variation, is derived. An example of arc computer simulation using the new model is given. The main features of the simulated arc are presented. The main features of both the real and the simulated arc are given in the time and the spectral domains.

Long arc in free air: laboratory testing, modelling, simulation and model-parameters estimation

The results of an investigation into the long arc in free air is presented. The long arc in free air is initiated under laboratory conditions in the FGH-Mannheim high-power test laboratory (Germany). Based on the arc voltage and the arc current data records obtained in the laboratory, the main features of the arc are derived. In the second stage of the research, a new arc model is developed. The arc is modelled as a current dependent voltage source with the characteristic distorted rectangular waveform. Examples of computer simulations of the arc using new models are given. The main features of the simulated arc are also described. Features of the real and modelled arc are given in the time and the spectral domains. The interaction between the arc and the network is analysed. Through computer simulation, a typical example of an arcing fault on an overhead transmission line is given. Finally, the least squares error estimation method for arc model parameter estimation is presented. The unknown model parameters are successfully estimated from the computer-simulated and laboratory-obtained data.

New dynamic model, laboratory testing and features of long arc in free air

In this paper the long arc in free air is investigated. First, arc is initiated under laboratory conditions in the high power test laboratory FGH-Mannheim (Germany). Based on the laboratory obtained data records, the main features of arc are derived. By this arc, voltage and current are processed. An important feature of arc, the variation of arc length, is analyzed. Based on the analysis of arc voltage and current wave forms, the new dynamic arc model, taking into account the arc length variation, is derived. Arc is modeled as a current dependent voltage source with the characteristic distorted rectangular waveform and arc length dependent amplitude. Arc is considered as a source of higher harmonics distorting by this other signals in the network. An example of arc computer simulation using the new model is given. The main features of simulated arc are presented, too. Features of both the real and the simulated arc are given in the time and the spectral domains.

Possible Role of ion Demagnetization in the Plasma Sheet in Auroral arc and Substorm Generation

Ion demagnetization in the plasma sheet causes the formation of field-aligned current that can trigger a magnetosphere-ionosphere coupling feedback instability, which may play an important role in substorm and auroral arc generation. Since field-aligned currents close ionospheric currents, their magnitude is controlled by ionospheric conductivity. The cause of instability is the impact of increasing upward field-aligned currents on ionospheric conductivity, which in turn stimulates an increase in the field-aligned currents. When the magnitude of these currents becomes sufficiently large for the acceleration of precipitating electrons, a feedback mechanism becomes possible. Upward field-aligned currents increase the ionospheric conductivity that stimulates an explosion-like increase in field-aligned currents. It is believed that this instability may be related to substorm generation. Demagnetization of hot ions in the plasma sheet leads to the motion of magnetospheric electrons through a spatial gradient of ion population. Field-aligned currents, because of their effect on particle acceleration and the magnitude of ionospheric conductivity, can also lead to another type of instability associated with the breaking of the earthward convection flow into convection streams. The growth rate of this instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to the formation of auroral arcs with widths of the order of 10 km. This instability is able to explain many features of auroral arcs, including their conjugacy in opposite hemispheres. However, it cannot explain very narrow (less than 1 km) arcs.

Possible role of magnetosphere-ionosphere coupling in auroral arc generation

Abstract. Three models for the magnetosphere-ionosphere coupling feedback instability are considered. The first model is based on demagnetization of hot ions in the plasma sheet. The instability takes place in the global magnetosphere-ionosphere system when magnetospheric electrons drift through a spatial gradient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetospheric electrons move earthward through a radial gradient of hot ions. This leads to the formation of field-aligned currents. The effect of upward field-aligned current on particle precipitation and the magnitude of ionospheric conductivity leads to the instability of this earthward convection and to its division into convection streams oriented at some angle with respect to the initial convection direction. The growth rate of the instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such arcs, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow arcs. For such arcs another type of instability must be considered. In the other two models the instability arises because of the generation of Alfven waves from growing arc-like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfven wave. The other model takes into consideration the reflection of Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000 km. The growth of structures in both models takes place when the ionization function associated with upward field-aligned current is shifted from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different from the E×B drift. Although both models may work, the growth rate for the model, based on the modulation of the precipitating accelerated electrons, is significantly larger than that of the model based on the Alfven wave reflection. This mechanism is suitable for generation of auroral arcs with widths of about 1 km and less. The growth rate of the instability can be as large as 1 s-1, and this mechanism enables us to justify the development of auroral arcs only in one ionosphere. It is hardly suitable for excitation of wide and conjugate auroral arcs, but it may be responsible for the formation of small-scale structures inside a wide arc.Key words: Ionosphere (auroral ionosphere) - Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions)

Possible role of magnetosphere-ionosphere coupling in auroral arc generation

Three models for the magnetosphere-ionosphere coupling feedback instability are considered. The first model is based on demagnetization of hot ions in the plasma sheet. The instability takes place in the global magnetosphere-ionosphere system when magnetospheric electrons drift through a spatial gradient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetospheric electrons move earthward through a radial gradient of hot ions. This leads to the formation of field-aligned currents. The effect of upward field-aligned current on particle precipitation and the magnitude of ionospheric conductivity leads to the instability of this earthward convection and to its division into convection streams oriented at some angle with respect to the initial convection direction. The growth rate of the instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such arcs, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow arcs. For such arcs another type of instability must be considered. In the other two models the instability arises because of the generation of Alfven waves from growing arc-like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfven wave. The other model takes into consideration the reflection of Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000 km. The growth of structures in both models takes place when the ionization function associated with upward field-aligned current is shifted from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different from the E×B drift. Although both models may work, the growth rate for the model, based on the modulation of the precipitating accelerated electrons, is significantly larger than that of the model based on the Alfven wave reflection. This mechanism is suitable for generation of auroral arcs with widths of about 1 km and less. The growth rate of the instability can be as large as 1 s-1, and this mechanism enables us to justify the development of auroral arcs only in one ionosphere. It is hardly suitable for excitation of wide and conjugate auroral arcs, but it may be responsible for the formation of small-scale structures inside a wide arc.Key words: Ionosphere (auroral ionosphere) - Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions)

Problems for Evaluation of the Scenario of the Permian -Triassic Boundary Biotic Crisis and of Its Causes

Determination of the causes of the Permian -Triassic boundary (PTB) biotic crisis is hindered primarily by the diachronous nature of the used PTB, poor stratigraphic control of compared Upper Permian and Lower Triassic faunas, espec ially in cont inental biotopes, poor knowledge of the lower and middle Scythian faunas from m,UlY environments, and by interpolation of the unknown (lower and middle) Scythian diversity from the known Upper Pennian and Middle Triassic diversity data in many major fossil gro ups. Most of these problems can be reso lved by using the firs t appearance datum (FAD) of Nil1deodlls pan'Ii,I' as either an isochronous PTB, or as an isochronous marker [eve I very dose 10 lhe base of lhe Triassi c; careful studies of fossil-rich, comp[ele conlinental boundary sections (e.g., Dalongkou in Sinkiang), and uti li sation of uninterpoJated diversity data, which are based on known Scythian data, for the reconstruction of the extinction and recovery patterns in all fossil groups. The most important features of the PTB biotic crisis arc: (I) Among the marine biota, only the plankton and the wann-water benthos, nektobenthos and nekton are strongly affected by the PTB biotic crisis. (2) The recovery of the warm-water nekton and nektobenthos was very fast (after one conodont zone). The recovery of the warmwater benthos, some of the plankton (radiolarians) and the terrestrial plant product ivity was strongly delayed for several million years, and occurred on ly in the upper Ol enekian (upper Scythian) and in the Middle Triassic. (3) The number of the Lazarus taxa that re-appeared in the upper Olenekian and above all in the Middle Triassic, is very high (about 50%) and in some fossi l groups 90-100% at generic level. The reconstruction of the scenario for the PTB biotic crisis requires not only the conside'rlItion of the uninterpolaled extinction and recovery patterns of all foss il groups across all envi ronments, but must also account for the main feat ures of geological evolution from the Middle Permian to the Lower Triassic. The most important causal factors in the PTB biotic crisis are the ex tinction event at the Guadalupian-Lopingian boundary that restricted the diverse Upper Permian wann-water benthos 10 the Tet hyan shelves, the long-[asling, widespread Siberian Trap volcanism (Dzhulfian - lower Scythian) which was Ihe grealest volcanic event during the Phanerozoic, and (he very strong explosive felsic 10 intermediate volcanism around the PTB, close to the margin between eastern Tethys and Panlhalassa. These volcanic activities resulted in those climatic changes that were dircclly and indireclly (as cause of the oceanic sll peranoxia) responsi ble for the PTB biotic crisis, such as periodic cooling of Ihe climate by volcanic dust and sulphate aerosols (mai nly caused by the Siberian Trap volcanism), acid rain, a 3-6 month vo[canic winter at low lat iIlldes and Ihe strongly reduced inpul of sunl ight during the uppennost Dorashamian (both caused by lhe very strong explosive volcanism at the Tethys/Panthalassa margin), followed by global warming in the lower Scythian, and uppennost Dorashamian to lower Scylhian superanoxia.

Dynamical effects of ionospheric conductivity on the formation of polar cap arcs

By using a magnetosphere‐ionosphere (M‐I) coupling model of polar cap arcs [Zhu et al., 1993], a systematic model study of the effects of ionospheric background conductivity on the formation of polar cap arcs has been conducted. The variations of the ionospheric background conductivity in the model study cover typical ionospheric conditions, including solar minimum, solar maximum, winter, and summer. The simulation results clearly indicate that the ionospheric background conductivity can dynamically affect the mesoscale features of polar cap arcs through a nonlinear M‐I coupling process associated with the arcs.