Field emission characteristics of gold dust grains

The space plasma environment usually contains charged dust grains. The grain charge is an important parameter determining its migration through the space, coagulation, formation of dust clouds and so on. The knowledge of its charge is thus one of the basic information we want to know. There are several emission processes leading to both positive and negative charges, among others photoemission, all kinds of secondary emissions, field emissions, etc. The present study is focused on a laboratory simulation of emissions caused by impacts by energetic ions. Our experiment is based on the Paul trap which gives us an opportunity to catch a single dust grain for several days inside the vacuum vessel and exposed it by electron/ion beams. This experimental approach allows us to separate an individual charging process. We have chosen spherically shaped gold grains and discussed the processes leading to the limitation of the grain charge. We suggested that the implantation of charging ions leads to deformations of the grain surface. The deformations enhance the local electric field strength that becomes sufficient for the field ionization.     

Numerical Simulation for the 16 August 1999 EUV Brightenings

The 16 August 1999 EUV brightenings are numerically simulated by a third-order upwind compact scheme, basing on the TRACE observation. The present simulation can give a possible explanation to its formation and evolution. The numerical results show that the initial reconnection jets at around X-point are responsible for the occurrence of EUV brightening. The strong and superposed ejections caused by the first and second coalescence of magnetic islands are possibly related to the lifted material which initially appeared as absorption features and Later EUV-emitting structures respectively. The bi-directional reconnection jets may correspond to the lifted material that either continued to move upward along the apparently open field lines or fell down to the surface.      

赤道地区和中纬度地区扩展F的特点和比较

通过数值模拟研究了赤道地区和中纬度地区扩展F的触发机制间的联系,发现相同的触发机制在不同纬度的条件下所演化出的结构形态不相同,因此观测上会有所不同;研究了西向电场在赤道地区的作用,发现在适合的电离层参数组合下会产生观测到的电离层F区顶部扰动出现率高于底部扰动出现率的状况;在数值模拟的基础上提出了一个中纬度地区电波散射的模型,用以解释观测到的中纬度地区电波散射的出现率高于线性理论预期的现象.      

引导场对无碰撞电流片低频电磁不稳定性的影响

通过数值求解无碰撞电流片中可压缩磁流体力学模型下得到的一般形式的色散关系,讨论了无碰撞电流片中引导场对低频电磁波不稳定性的影响.结果表明,平衡态磁场中的引导场,对于三维扰动传播的波不稳定性有很强的影响.(1)在电流片中间平面上(z=0),无引导场时,没有不稳定性发生,但若存在引导场,不稳定性便发生,并随着引导场的增强,不稳定性明显增强,不稳定的波模可能是低混杂模.(2)在中间平面附近(z=0.2),电流片是不稳定的.随着引导场的增强,不稳定性增长率明显地增强,不稳定的波模从平行和反平行两个方向传播变为反平行方向一个方向传播,并且是斜传播的,具有低频哨声模或低混杂模的特征.(3)在电流片边缘附近(z=0.8),引导场对不稳定的波模和增长率没有明显影响,不稳定的波模都是准平行的哨声波.      

极端低温下电液伺服阀温漂特性分析

针对电液伺服阀在极端低温下温漂大的问题，以＋40℃时电液伺服阀初始零偏为基准，采用线性回归方法分析了某型射流管伺服阀不同温度的零偏试验数据，得到了极端低温下的伺服阀温漂与＋40℃时初始零偏的数学关系。分析与试验结果表明：极端低温时射流管伺服阀的温漂与＋40℃时的初始零偏存在非常显著的线性关系。温漂与电液伺服阀制造与装配工艺过程密切相关，与结构及其装配不对称有关。电液伺服阀结构上的微观不对称现象，在极端低温下显现出来，尤其是呈现出较大的温漂。降低温漂的主要措施是提高电液伺服阀结构与装配的对称性，降低初始零偏。     

Post-midnight equatorial irregularity distributions and vertical drift velocity variations during solstices

Longitudinal distributions of post-midnight equatorial ionospheric irregularity occurrences observed by ROCSAT-1 (1st satellite of the Republic of China) during moderate to high solar activity years in two solstices are studied with respect to the vertical drift velocity and density variations. The post-midnight irregularity distributions are found to be similar to the well-documented pre-midnight ones, but are different from some published distributions taken during solar minimum years. Even though the post-midnight ionosphere is sinking in general, longitudes of frequent positive vertical drift and high density seems to coincide with the longitudes of high irregularity occurrences. Large scatters found in the vertical drift velocity and density around the dip equator in different ROCSAT-1 orbits indicate the existence of large and frequent variations in the vertical drift velocity and density that seem to be able to provide sufficient perturbations for the Rayleigh-Taylor (RT) instability to cause the irregularity occurrences. The need of seeding agents such as gravity waves from atmospheric convective clouds to initiate the Rayleigh-Taylor instability may not be necessary.     

空间等离子体与垂直无碰撞激波相互作用的数值实验

应用混合模拟方法数值研究了高Ａｌｆｖｅｎ－Ｍａｃｈ数垂直无碰撞激波与等离子体间的相互作用。结果表明，激波上游的磁场非常稳定，粒子分布近似为Ｍａｘｗｅｌｌ分布。激波下游磁场存在不规则的湍动，质子分布有一个高能尾，且有些质子被激波反射。跟踪少量高速质子的计算结果表明，在ｔ＝４０Ωｉ＾－１时约有４０％的质子被激波反射，而７％质子可被加速，最大速度值可达到２０ＶＡ激波区的电场分布对质子的减速或加速起了主要     

三维非线性撕裂模耦合和快速磁场重联

基于三维不可压缩电阻性ＭＨＤ方程，在长柱形位型下，数值研究了电阻撕裂模不稳定性所引起的磁场重联过程。研究结果表明，撕裂模的非线性相互作用和耦合，将产生许多模式的强裂解稳和导致快速的磁场重联。这一过程的进一步发展，导致具有螺旋形结构，不同模式的磁岛磁力线重叠，从而引起较大区域内的磁场各态经历和磁力线随机走向，形成撕裂模端动。     

基于交叉验证的陀螺仪温度漂移建模方法

根据k次交叉验证思想,分别在测量方差已知与未知的情况下引入了一种模型选择准则,该准则充分考虑了模型的拟合能力与预测能力,在获得极大似然估计的同时,避免了参数估计中的过拟合问题。利用这一准则,对陀螺仪误差系数温度漂移建模进行了分析,获得了温度漂移的最优多项式模型。并与传统的AIC准则、MDL准则进行对比,通过Monte Carlo仿真说明了该选择准则在样本较小的情况下具有更好的性质。     

光纤陀螺随机漂移ARMA模型研究

从理论上证明了山多个ARMA过程组成的平稳随机过程，可以南一个等效ARMA模型描述，并推导废模型的数学表达式。提出了一种新的平稳随机过程建模的有效方法，并应用于光纤陀螺随机漂移建模。采用功率谱密度分析光纤陀螺随机漂移组成，从而确定随机漂移的ARMA模型的形式和阶数。通过对实际光纤陀螺数据分析和建模，利用所得随机漂移模型对陀螺输出数据补偿，检验模型的适用件。结果表明废建模方法正确町行，采用该方法建立的光纤随机漂移模型具有很好的适用性，可以有效抑制光纤陀螺随机漂移。