Evidence of guide field magnetic reconnection in flapping current sheets

Based on current sheet flapping motion on 27 August 2018 in the dusk flank magnetotail,as recorded by instruments aboard Magnetospheric Multiscale(MMS)spacecraft,we present the first study of guide field reconnection observed in the flux rope embedded in kink-like flapping current sheets near the dusk-side flank of the magnetotail.Unlike more common magnetotail reconnections,which are symmetric,these asymmetric small-scale(λ_(i)~650 km)reconnections were found in the highly twisted current sheet when the direction normal to the sheet changes from the Z direction into the Y direction.The unique feature of this unusual reconnection is that the reconnection jets are along the Z direction-different from outflow in the X direction,which is the more usual situation.This vertical reconnection jet is parallel or antiparallel to the up-and-down motion of the tail’s current sheet.The normalized reconnection rate R is estimated to be~0.1.Our results indicate that such asymmetric reconnections can significantly enlarge current sheet flapping,with large oscillation amplitudes.This letter presents direct evidence of guide field reconnection in a highly twisted current sheet,characterized by enlarged current sheet flapping as a consequence of the reconnection outflow.

Lower-Hybrid Drift Instability in Modified Harris Current Sheet

Lower-hybrid drift instability （LHDI） in a Harris current sheet including a uniform background distribution is investigated in linear local kinetic theory. It is found that the introduction of a uniform background distribution reduces the growth rate and real frequency of LHDI at all wavelengths. Some physical explanations about the effects of the background distribution are provided.

Cluster observations of large-scale southward movement and dawnward-duskward flapping of Earth's magnetotail current sheet

In August 2001,Cluster satellites observed that the mid-tail current sheet(CS) moved southward continuously for almost seven hours.Meanwhile,Cluster crossed back and forth the CS repeatedly.This means that the large-scale southward movement of the CS was accompanied by a small-scale CS flapping during this period.Using the minimum-variation-analysis(MVA) method and the multi-spacecraft data,we calculated the normal vector,current density and the magnetic curvature of the CS,the results showed that the CS alternated between flattened CS and tilted CS for several times.Strong dawn-dusk oscillations were found for the tilted CS,which caused the repeated crossings of the center of CS by the satellites.This feature is obviously different from the previous observations of the vertical flapping of the CS induced by the kink instability.Two types of flapping were observed:One of them is accompanied with bursty bulk flows(BBFs) and the other is not.This suggests that in this event there was no direct relationship between the CS flapping and BBFs.

New method for determining central axial orientation of flux rope embedded within current sheet using multipoint measurements

A new method for determining the central axial orientation of a two-dimensional coherent magnetic flux rope(MFR)via multipoint analysis of the magnetic-field structure is developed. The method is devised under the following geometrical assumptions:(1) on its cross section, the structure is left-right symmetric;(2) the projected structure velocity is vertical to the line of symmetry. The two conditions can be naturally satisfied for cylindrical MFRs and are expected to be satisfied for MFRs that are flattened within current sheets. The model test demonstrates that, for determining the axial orientation of such structures, the new method is more efficient and reliable than traditional techniques such as minimum-variance analysis of the magnetic field,Grad-Shafranov(GS) reconstruction, and the more recent method based on the cylindrically symmetric assumption. A total of five flux transfer events observed by Cluster are studied using the proposed approach, and the application results indicate that the observed structures, regardless of their actual physical properties, fit the assumed geometrical model well. For these events, the inferred axial orientations are all in excellent agreement with those obtained using the multi-GS reconstruction technique.

3D MHD simulation of the double-gradient instability of the magnetotail current sheet

Flapping motion of the current sheet(CS) is an important physical process in the Earth's magnetotail. The magnetic doublegradient model, which includes both the instability and wave modes, offers a reasonable explanation for the exciting and propagation of the flapping wave. In this paper, we apply an advanced numerical magnetohydrodynamic(MHD) scheme(conservation element and solution element(CESE)-MHD) to simulate the magnetic double-gradient instability in an idealized current sheet that mimics the magnetotail configuration. We initialize the simulations with a numerically relaxed magnetotail equilibrium, in which the normal component of the magnetic field has a tailward gradient. It is confirmed in our simulation that the instability develops in the current layer. The growth rate of the instability yielded from the simulation is very close to the prediction of theory, with a relative deviation of only ten percent. The results demonstrate that the CESE-MHD scheme is very powerful in numerical study of the double-gradient mechanism of the CS flapping mode, and can be used for further investigations of the flapping motion in more realistic CS configurations.

Statistical research on the motion properties of the magnetotail current sheet:Cluster observations

The origin of the flapping motion of the earth’s magnetotail current sheet is one of the most important problems in the magnetotail dynamics.Using Cluster data,we make a statistical research on the motion properties of the magnetotail current sheet of 2001 and 2003.We calculate the velocities of the magnetotail current sheet using new methods and obtain the distribution of the magnetotail current sheet velocities in the X-Y plane in GSE coordinate system.Our results show that although most of the current sheets were propagating toward the tail flanks and those of the exceptions lay in dusk side,which is consistent with previous studies,the proportions of the current sheet which were propagating toward midnight (where |YGSE|=0) were higher than those in previous studies.Motions of the current sheet in the middle area (|YGSE|【8 Re) of the magnetotail are investigated.Relatively high value of the Z component of the velocity further confirms that the middle area of the magnetotail might be a source region for the motion of the current sheets which were propagating towards the tail flanks.According to our case studies,the way the current sheets propagated toward midnight area differs significantly from that toward dusk and dawn side,from which we infer that there might be two different kinds of current sheet motions originated from different sources.The statistical results of this paper may give some clues for further studies on the origin of the flapping motion of the magnetotail current sheet.

THEMIS observation of a magnetotail current sheet flapping wave

A flapping wave was observed by THEMIS-B(P1)and THEMIS-C(P2)probes on the dawn side of the magnetotail,while the solar wind was generally stable.The magnetic activity was quite weak,suggesting that this flapping wave was generated by an internal instability,which normally occurs during magnetic quiet times.Our analysis shows that the flapping wave was propagating downward with a tail-aligned scale of at least 3.7 R E and did not show much change in shape during its propagation from P1 to P2.Correlation analysis employed to estimate the time lag between the corresponding half waveforms of P1 and P2 shows that the propagating velocities along the current sheet normal directions were close to each other in the beginning,but increased linearly later on.The average wavelength of the flapping wave is approximately 4 R E.Theoretical analysis suggests that the ballooning type wave model may not be the mechanism for the observed flapping wave,but that the magnetic double-gradient instability model is a more plausible candidate.

Numerical study of magnetic reconnection process near interplanetary current sheet

The third order accurate upwind compact difference scheme has been applied to the numerical study of the magnetic reconnection process possibly occurring near the interplanetary current sheet, under the framework of the two-dimensional compressible magnetohydrodynamics (MHD). Our results here show that the driven reconnection near the current sheet can occur within 10-30 min for the interplanetary high magnetic Reynolds number, RM =2 000-10 000, the stable magnetic reconnection structure can be formed in hour-order of magnitude, and there are some basic properties such as the multiple X-line reconnections, vortical velocity structures, filament current systems, splitting and collapse of the high-density plasma bulk. These results are helpful in understanding and identifying the magnetic reconnection phenomena near the interplanetary current sheets.

Statistical survey on the magnetic field in magnetotail current sheets: Cluster observations

The distribution properties of the magnetic field in magnetotail current sheets have been explored statistically with the magnetic measurement data of the Cluster mission from June to November of the years 2001–2005.It is found that,on average,the strength of the magnetic field and its Bz component in the current sheet are weaker in the region close to midnight but stronger near the dawnside and duskside flanks,which implies that,in general,a thinner current sheet occurs near midnight and thicker ones near both flanks.The occurrence of tail current sheet flapping is higher on both flanks than in the midnight region,although it is most frequent in the dawn flank.Current sheets with a negative Bz component or a strong By component have a higher probability of occurring at magnetic local times of 21:00–01:00,indicating that magnetic activity,e.g.magnetic reconnection and current disruption occur more frequently there.Statistically,the probability distributions of the By component and the tilt angle of magnetic field lines in the current sheet are approximately normal distributions,and the occurrence probability of the flattened current sheet is about one third that of the normal current sheet.The magnetic field and Bz component in the current sheet mainly vary from 1 nT to 10 nT.The By component in the tail central current sheet is on average twice the IMF By at 1 AU.

Electron surfing acceleration in a current sheet of flares

A model of electron acceleration in a current sheet of flares is studied by the analytical approximation solution and the test particle simulation. The electron can be trapped in a potential of propagating electrostatic wave. The trapped electron moving with the phase velocity v p of wave may be effectively accelerated by ev p/c × B z force along the outflow direction in the current sheet, if a criterion condition K > 0 for electron surfing acceleration is satisfied. The electron will be accelerated continuously until the electron de-trap from the wave potential at the turning point S.

A numerical study for boundary layer current and sheet flow transport induced by a skewed asymmetric wave

An analytical model with essential parameters given by a two-phase numerical model is utilized to study the net boundary layer current and sediment transport under skewed asymmetric oscillatory sheet flows. The analytical model is the first instantaneous type model that can consider phase-lag and asymmetric boundary layer development. The two-phase model supplies the essential phase-lead, instantaneous erosion depth and boundary layer development for the analytical model to enhance the understanding of velocity skewness and acceleration skewness in sediment flux and transport rate. The sediment transport difference between onshore and offshore stages caused by velocity skewness or acceleration skewness is shown to illustrate the determination of net sediment transport by the analytical model. In previous studies about sediment transport in skewed asymmetric sheet flows, the generation of net sediment transport is mainly concluded to the phase-lag effect.However, the phase-lag effect is shown important but not enough for the net sediment transport, while the skewed asymmetric boundary layer development generated net boundary layer current and mobile bed effect are key important in the transport process.

Parametric effect of large impulse current excited coil on electromagnetic force in coil-sheet system

Based on the method of controlling welding stress with trailing, the electromagnetic force in coil-sheet system was simulated with finite element software ANSYS. The effect of parameters of coil on the electromagnetic force density fy was analyzed. The results show that the maximum electromagnetic force density fy, max in sheet appears in the position near the inner radius of single-turn coil. The position is independent of section shape of coil. fy, max for flat coil is larger than that for long coil and the coil with wedge shape section, while section areas of all coils are equal to each other. The effect of turn number of multiple-turn coil on fy is dependent on the loop resistance in circuit. The kind of coil with more turns and larger inductance is commended while there is larger loop resistance in circuit. fy increases in a certain magnitude while a magnetic core is located in coil. However, the magnitude of fy is limited by saturating magnetic flux of the core.

My Limiting Behavior of MHD Flow with Hall Current, Due to a Porous Stretching Sheet

An electrically conducting fluid is driven by a stretching sheet, in the presence of a magnetic field that is strong enough to produce significant Hall current. The sheet is porous, allowing mass transfer through suction or injection. The limiting behavior of the flow is studied, as the magnetic field strength grows indefinitely. The flow variables are properly scaled, and uniformly valid asymptotic expansions of the velocity components are obtained through parameter straining. The leading order approximations show sinusoidal behavior that is decaying exponentially, as we move away from the surface. The two-term expansions of the surface shear stress components, as well as the far field inflow speed, compare well with the corresponding finite difference solutions;even at moderate magnetic fields.

Unsteady magnetic reconnection in non-periodic multiple current sheets

The process of magnetic reconnection in non_periodic three_layer current sheets is studied numerically by using two_dimensional magnetohydrodynamic simulation. The results show that unlike periodic current sheets, it is complex unsteady magnetic reconnection. It may be important for solar flare and corona heating.

Ti_(2)AlNb合金瓦楞板电流辅助辊压成形研究

目前采用辊弯-热校形复合工艺制备Ti_(2)AlNb合金瓦楞板存在工艺周期长、能耗高、构件易氧化及易回弹等缺陷的问题,提出了Ti_(2)AlNb合金瓦楞板电流辅助辊压成形工艺,通过有限元数值模拟分析了Ti_(2)AlNb合金瓦楞板成形及电流密度场演化规律。结果表明,瓦楞板变形主要分布在辊弯成形位置,且辊压前角的应变略高于后角。辊弯成形位置的电流密度与辊轮接触状态相关,在整个成形周期内,其电流密度先增加后减小。电流辅助辊压成形利用电迁移效应可显著抑制瓦楞板的回弹缺陷,瓦楞板的成形角度由125.53°降低为120.07°。

深空探测中磁强计在轨标定研究进展

磁场测量是深空探测的重要任务之一,高精度的磁场测量对研究行星宜居性、空间等离子体环境等具有重要意义.磁通门磁强计是空间探测任务中最广泛使用的仪器之一.然而其磁补偿会随时间缓慢变化,因此,在卫星飞行期间需要定期对磁强计的磁补偿实施在轨标定.本文综述了基于空间物理现象对星载磁强计实施在轨标定的方法.根据计算磁补偿的方式,这些方法可分为两类.第一类方法通过公式计算磁补偿,如基于阿尔芬波动特征的Belcher方法、Hedgecock方法和Davis-Smith方法,以及一维和三维磁镜模方法;这类方法中,Davis-Smith方法被认为是计算磁补偿的最优方法.第二类方法通过概率获取磁补偿的最优解,该类方法被称为新一代在轨标定技术,目前已发展出六种算法;该技术可使用阿尔芬波动、磁镜结构和电流片中的一种或多种现象来实施在轨标定,另外,该技术也可以使用足够数据量(如4小时)的任意行星际磁场数据来计算磁补偿.与Davis-Smith方法相比,新一代在轨标定技术具有很好的兼容性(可同时使用不同的物理现象)和拓展性.除了太阳风,该技术还能在火星磁鞘中实施在轨标定.目前,该技术已为“天问一号”环绕器的磁场数据提供标定服务.

基于力场原理的有轨电车地面供电模块的故障分析与对策

针对基于力场原理的有轨电车地面供电模块频繁出现的正极触指烧损的故障,通过对触指工作状态的测试,掌握了其工作瞬间的电气特性。结合触指负载能力的分析,指出容性电流引发的电弧和受电靴电流不均衡是导致触指烧损的直接原因。通过针对性的措施,实现了故障的消除。

一种可不停电安装的开合式电流互感器设计

针对电流互感器现场拆装更换的停电问题,文中设计了一种新型开合式电流互感器,实现了不停电在线安装。该设计将高导磁性能的硅钢铁芯作为互感器的芯体材料,基本结构则采用了圆形铁芯中空方式。同时利用辅助绕组,基于二次电势的方法对其测量误差进行了补偿。实验测试结果表明,通过补偿可以使开合式电流互感器的比差及角差最大差值仅为0.034%和10.64′,满足0.2级的使用要求,且补偿后的电流互感器稳定性良好,受温度影响较小,能够满足实际工程使用需求。

金属波纹管的阵列涡流检测

制作了多层不锈钢薄板和金属波纹管人工缺陷试样并进行阵列涡流检测试验,研究了阵列涡流检测的检测能力及技术特点。试验结果表明,阵列涡流检测技术能有效检测多层不锈钢薄板和金属波纹管的表面及内部线性缺陷;缺陷信号幅值和相位与缺陷埋藏深度有关,可通过幅值和相位来综合判断缺陷的埋藏深度。

2-D Theoretical Model for Current–Voltage Characteristics in AlGaN/GaN HEMT’s

A threshold-voltage-based 2-D theoretical model for the Current–Voltage characteristics of the AlGaN/GaN high electron mobility transistors (HEMT’s) is developed. The present work proposes an improved charge-control model by employing the Robin boundary condition when introduced the solution of the 2-D Poisson’s equation in the density of charge depleted in the AlGaN layer. The dependence of 2-DEG sheet carrier concentration on the aluminum composition and AlGaN layer thickness has been investigated in detail. Current–voltage characteristics developed from the 2-DEG model in order to take into account the impact of gate lengths. The relation between the kink effect and existing deep centers has also been confirmed by using an electrical approach, which can allow to adjust some of electron transport parameters in order to optimize the output current.