闪电双向先导理论及观测:极性不对称、不稳定及间歇性

闪电双向先导原理的提出及观测验证是闪电物理研究近几十年最重要的进展之一,而正、负先导及流光的极性不对称性及传输的持续、间歇性,是理解闪电各种过程物理机制的关键.本文对闪电双向先导的概念及进展进行了总结和讨论,重点强调了正、负先导流光传输机制特别是门限电场的不对称性,阐述了正先导传输的连续性及其在闪电始发、负先导的空间先导形成、不稳定先导通道中的反冲先导建立等过程中独特的启动作用.     

On the nature of bipolar flashes that share the same channel to ground

The paper examines the relationship between sequential events in bipolar flashes of two groups sharing the same channel to ground: one initiated by an upward positive leader, the other by a natural flash. Two quite different mechanisms are involved in the occurrence of the bipolar flashes of each group: The bipolar events that start as a natural flash are results of recoil leaders sharing parts of the initial branching structure; the bipolar events that start as upward positive leaders are the result of the interaction between these leaders' initial branching structures and the branches of the intracloud flashes they triggered.     

一次人工触发闪电上行正先导的传输特征

利用山东一次人工触发闪电的高速摄像和30 m, 60 m和480 m的同步电场测量, 清晰显示了人工触发闪电上行正先导的传输特征.上行正先导头部光强相对较强, 二维发展速度变化波动较大并呈现明显不规则性,表明上行正先导发展具有明显的梯级特征. 在先导开始阶段从340 m到705 m高度之间上行正先导平均发展速度为9.8×10⁴ m/s,起始速度是 3.8×10⁴ m/s,局部速度总体上随高度呈现增加趋势.电场变化在近距离产生有规律的梯级状 变化,记录到的28个梯级相邻梯级间隔变化从14 μs 到 39 μs,几何平均值为25.1 μs. 估计的先导梯级长度分布在0.9 m到3.7 m,几何平均值为1.7 m.先导电场变化由慢的正向梯级状变化和 脉冲变化组成,结合光学和电场变化测量结果,得出正先导头部通道发生弯曲可使其电场变化的梯级特征减弱 或消失;正负先导梯级形成机制可能类似,均由其先导头部前端的双向流光发展而来.     

Physical processes during development of upward leaders from tall structures

The objective of our study was to identify and interpret the various processes during development of upward positive leaders from tall structures. We provide a physical interpretation for the varying luminosity of the leader channel during its initial ascent and the pulsing luminosity during much later stages of the established leader. Our analysis confirms that pulsed luminosity is a result of recoil leaders intercepting the current-carrying channel connected to ground. This interception produces an M-component. Applying the bidirectional leader concept and a simplified electrostatic approximation, we introduce physical models of the dart leader-return stroke sequence and the M-component.     

Diffusion measurements at long observation times in the presence of spatially variable internal magnetic field gradients

Diffusion measurements in heterogeneous media may contain a significant source of error, the influence of the coupling between the applied and internal magnetic field gradients on the attenuation of the NMR signal. The application of bipolar magnetic field gradients has been introduced to suppress this error. The basic assumption for the successful removal of the coupling is that the diffusing molecules are experiencing a constant internal gradient during the experiment. We will provide theoretical and experimental evidence that the application of bipolar magnetic field gradients may fail to suppress the effect from all the cross terms between internal and applied gradients effectively at long observation times. It will be shown experimentally that a successful suppression of the cross terms is strongly dependent on the observation time, and on the tau value in the bipolar pulsed field gradient stimulated echo experiment. Copyright 2000 Academic Press.     

Investigation of the possibility of γ-background discrimination in a neutron spectrometer with a proportional counter of recoil protons

The possibility of expanding the range of neutron spectrum measurements to lower energies in fields of mixed n-γ radiation via separation of pulses from recoil protons and γ-induced electrons, using the proportional counter developed for neutron spectrometry at the All-Russia Research Institute of Experimental Physics, has been investigated. With the use of a digital oscilloscope, a data set on the shape of counter pulses under irradiation in a field of mixed n-γ radiation (²⁵²Cf) and in a field of γ rays (SOSGI-M set) has been obtained. Mathematical processing of the obtained data set showed a possibility of distinguishing pulses from recoil protons and electrons; hence, for the ²⁵²Cf source, the lower boundary of the neutron spectrometer operating range can be reduced to ≈80 keV.     

Subband mutiresolution by multi-object phase-only joint transform correlator

Subband multiresolution technique presents decomposition and reconstruction properties for 2D images using multichannel spatial filtering. Each subband image represents multiresolution of the input image. In this paper, optic subband multiresolution architecture is proposed based on multi-object phase-only joint transform correlator (JTC). Multichannel filtering is realized using multi-object input joint image that includes bipolar data. The bipolar data (containing positive, zero and negative values) is encoded by phase in the input and Fourier planes of the phase-only JTC. For image decomposition and reconstruction, two optoelectronic subband multiresolution architectures have been developed. Each architecture has been tested using a simulation program.     

Surface magnetic anisotropy in glass-coated amorphous microwires as determined from ferromagnetic resonance measurements

The ferromagnetic resonance frequency of different Co base glass-coated amorphous magnetic microwires about 3.5 μm in diameter with negative, vanishing and positive magnetostriction has been investigated from power absorption measurements in the microwave frequency range. The experimental technique employed here involves the replacement of the dielectric of a coaxial transmission line by the sample to be measured. From the evolution of the resonance frequency with DC applied magnetic field, the surface magnetic anisotropy field of the microwires has been quantitatively obtained and, as expected, found to depend on the sign and strength of the magnetostriction. Similar values for the surface anisotropy are obtained in comparison with bulk anisotropy as determined from quasi-static hysteresis loops measurements.     

The use of positive muons in metal physics

The present status of work in metal physics by the new method of “muon spin rotation” is reviewed. This spectroscopy is based on the spin interactions of positive or negative muons and resembles NMR as far as the interpretation of interactions in metals is concerned. The positive muon behaves in several respects as a light isotope of hydrogen in metals. Local properties like site symmetry, local magnetic field, dynamic effects from surrounding spins as well as effects from the diffusion of the particle itself can be measured with high sensitivity.A brief review of the technical aspects is given. The problems of diffusion of light positive particles in metals are discussed, with regard to specific mechanisms at low temperatures, trapping of muons by impurities, etc. The local electronic structure around this kind of impurity in normal metals as well as ferromagnets has been subject to a large nnumber of studies. Other applications include the interaction of muons with other kinds of defects, the study of metal hydrides and measurements on the dynamics of spin glasses.     

Automatic detection of bioacoustics impulses based on kurtosis under weak signal to noise ratio

Passive acoustic monitoring (PAM) of marine mammal vocalizations has been efficiently used in a wide set of applications ranging from marine wildlife surveys to risk mitigation of military sonar emissions. The primary use of PAM is for detecting bioemissions, a good proportion of which are impulse sounds or clicks. A click detection algorithm based on kurtosis estimation is proposed as a general automatic click detector. The detector works under the assumption that click trains are embedded in stochastic but Gaussian noise. Under this assumption, kurtosis is used as a statistical test for detection. The algorithm explores acoustic sequences with the optimal frequency bandwidth for focusing on impulse sounds. The detector is successfully applied to field observations, and operates under weak signal to noise ratios and in presence of stochastic background noise. The algorithm adapts to varying click center frequency. Kurtosis appears as a promising approach to detect click trains, alone or in combination with other clicks detector, and to isolate individual clicks.     

Cross sections for the n+d breakup reaction

The n+d breakup cross section has been measured between 8 and 22 MeV by integrating the energy distributions of breakup protons and recoil deuterons observed in a deuterated scintillator. The results obtained deviate from the trend suggested by recent measurements of the cross section for proton-induced breakup and favour predictions based on a local N-N potential having a hard core in the singlet state.     

Separating positive and negative magnetoresistance in organic semiconductor devices

We study the transition between positive and negative organic magnetoresistance (OMAR) in tris-(8 hydroxyquinoline) aluminium (Alq_{3}), in order to identify the elementary mechanisms governing this phenomenon. We show how the sign of OMAR changes as function of the applied voltage and temperature. The transition from negative to positive magnetoresistance (MR) is found to be accompanied by an increase in slope of log(I) versus log(V). ac admittance measurements show this transition coincides with the onset of minority charge (hole) injection in the device. All these observations are consistent with two simultaneous contributions with opposite sign of MR, which may be assigned to holes and electrons having different magnetic field responses.     

Remote sensing of surface reflective properties: Role of regularization and a priori knowledge

Linear kernel-driven bidirectional reflectance distribution function (BRDF) models have been used for mapping albedo with single field-of-view satellite measurements such as Moderate Resolution Imaging Spectroradiometer (MODIS). Due to limited samplings and poor angular configurations available from these satellite remotely sensed data, BRDF models inversion is often plagued by numerical instability. In order to overcome the ill-posedness of the BRDF model inversion and robustly estimate terrestrial surface albedo, a regularization technique is employed for the cases where the number of observations is insufficient, or the angular distribution is poor. Emphasis is also placed on the combination of a priori knowledge with the regularized inversion. Numerical performances and case study results with ground measurements and MODIS observations suggest that the method is sound and robust for ill-posed BRDF inverse problems. The method presented in this study is promising for land surface reflective parameters retrieval even for regions where only sparse observations are available.     

Transverse effects in collective atomic recoil lasing

We investigate transverse effects in collective atomic recoil lasing (CARL), where a cold atomic sample is lightened by a far detuned laser beam resonant with the internal atomic transition. The gradient force of the scattered radiation field produces a collective self-focusing on the atoms, which could be observed in a Bose-Einstein condensate stored in a bidirectional optical ring cavity or in the superradiant CARL-BEC regime.     

Effect of air flow on corona discharge in wire-to-plate electrostatic precipitator

This paper analyses corona discharge in ambient air with laboratory-scaled wire-to-plate electrostatic precipitator (WPESP). The electric field is behind the electro hydrodynamic (EHD) flow in air. Its measurements provide complementary results for the corona discharge study because the classical theory based on the current and voltage data is unsatisfactory. Taking into account the dynamic air flow velocity is perpendicular to the active wires, measurement method of the positive and negative DC corona current density and electric field, has been introduced. It has been shown also that the dynamic air flow velocity modifies the current density and the electric field distributions on the planes surfaces of the WPESP.     

Site-specific recoil diffraction of backscattered electrons in crystals

A novel diffraction effect in high-energy electron backscattering is demonstrated: the formation of element-specific diffraction patterns via nuclear recoil. For sapphire (Al(2)O(3)), the difference in recoil energy allows us to determine if an electron scattered from aluminum or from oxygen. The angular electron distribution obtained in such measurements is a strong function of the recoiling lattice site. These element-specific recoil diffraction features are explained using the dynamical theory of electron diffraction. Our observations open up new possibilities for local, element-resolved crystallographic analysis using quasielastically backscattered electrons in scanning electron microscopy.     

基于Alq3的有机发光二极管的磁电导效应

制备了结构为 ITO/CuPc/NPB/Alq₃/LiF/Al 的有机发光二极管,并在300,260,220和180 K 四个温度测量了器件在恒压偏置下注入电流的磁场效应(磁电导效应).在注入电流从双极电流过渡到单极电流的过程中,随电流减小,器件的磁电导呈现先上升后下降的变化趋势.当温度降低,磁电导的值下降.但在任何测量条件下,器件的磁电导始终为正,没有出现如文献报道的磁电导从正到负的变化.实验结果表明,有机发光二极管中正负磁电导现象的产生,并非仅取决于注入电流是单极电流还是双极电流,它还与有机材料、器件结构等密切相关.利用受磁场调控的“电子-空穴对”机理与“双极化子”模型,分别解释了器件双极电流和单极电流的正磁电导效应. 关键词： 有机发光二极管 磁电导 双极化子     

Interaction of a supersonic beam of toluene with a resonant RF electric field

Deflection of a cold supersonic toluene beam seeded in He has been observed when these molecules interact with both a static and a resonant oscillating electric field. The toluene beam splits into two beams each one peaking at a deflection angle of 1 degree towards the positive and negative direction of the Stark field when the employed resonant frequency between the two Stark levels of the toluene molecule is 1411 kHz. This deflection angle is about four orders of magnitude higher than the value one would expect from the toluene dipole moment and the employed RF field gradient. Different hypothesis are suggested to explain the observed strong beam splitting including the possibility of transverse beam interferences induced by both the resonant RF field and the transverse uniform electric field. A theoretical model is presented based on molecular beam interferences induced by the resonant RF field which seems to account satisfactorily for the present observations.     

Influence of the Sign of Solvatochromism of Merocyanines on their Photoelectric Properties in Polymer Films

It has been established that the intensities of the longwave, electron-absorption bands of merocyanine dyes differing in the sign of solvatochromism redistribute under the action of an external electric field. This is explained by the fact that the probability of vibronic transitions changes under the action of an electric field. It has been revealed that the photoconductivity of poly-N-epoxypropylcarbazole films doped with merocyanines increases with decrease in negative solvatochromism and increase in positive solvatochromism. It is suggested that the more bipolar structure of the excited state of the above-mentioned films in the first case as compared to the structure of these films in the second case is favorable for electron transfer from the poly-N-epoxypropylcarbazole molecules to the dye molecule and the formation of a carbazole cation-radical providing hole conduction in the polymer.