The analytic properties of the flapping current sheets in the earth magnetotail

The current sheet in Earth’s magnetotail often flaps, and the flapping waves could be induced propagating towards the dawn and dusk flanks, which could make the current sheet dynamic. To explore the dynamic characteristics of current sheet associated with the flapping motion holistically and provide reasonable physical interpretations, detailed direct calculation and analysis have been applied to one approximate analytic model of magnetic field in the flapping current sheet. The main results from the model demonstrate: (1) the magnetic fluctuation amplitude is attenuated from the center of current sheet to the lobe regions; The larger wave amplitude would induce the larger magnetic amplitude; (2) the curvature of magnetic field lines (MFLs), with maximum at the center of current sheet, is only dependent on the displacement Z along the south-north direction from the center of current sheet, regardless of the tilt of current sheet; (3) the half-thickness of neutral sheet, h, the minimum curvature radius of MFLs, R_cmin, and the tilt angle of current sheet, δ, satisfies h=R_cmin cos δ; (4) the gradient of magnetic strength forms a double-peak profile, and the peak value would be more intense if the local current sheet is more tilted; (5) current density j and its j_y, j_z components reach the extremum at the center of CS. j and j_z would be more intense if the local current sheet is more tilted, but it is not the case for j_y; and (6) the field-aligned component of current density mainly appears in the neutral sheet, and the sign of it would change alternatively as the flapping waves passing by. To check the validity of the model, one simulation on the virtual measurements has been made, and the results are in well consistence with actual observations of Cluster.     

Low-frequency current variability observed at the shelfbreak in the northeastern Gulf of Mexico: November 2004–May 2005

Fourteen acoustic Doppler current profilers (ADCPs) were deployed on the shelf and slope for 1 year just west of the DeSoto Canyon in the Northeastern Gulf of Mexico by the Naval Research Laboratory (NRL) as part of its Slope to Shelf Energetics and Exchange Dynamics (SEED) project. The winter and spring observations are discussed here in regards to the low-frequency current variability and its relation to wind and eddy forcing. Empirical orthogonal function (EOF) analyses showed that two modes described most of the current variability. Wind-forced variability of the along-shelf flow was the main contributor in Mode 1 while eddies contributed much of the variability in Mode 2. Wind-stress controlled currents on the shelf and slope at time scales of about a week. On longer time scales, variations in the currents on both the outer shelf and slope appear to be related to seasonal variations in the time-cumulated wind stress curl. Winds were dominant in driving the along-shelf transports, particularly along the slope. However, the effective wind stress component was found to be aligned with the west Florida shelf direction rather than the local shelf direction. Eddy intrusions, which were more numerous in winter and spring than in summer and fall, and winds were found to contribute significantly to cross-shelf exchange processes.     

Stable isotope ratios in Cape gannets around the southern coasts of Africa reveal penetration of biogeographic patterns in oceanic signatures

The southern coasts of Africa are influenced by two major oceanic currents, leading to biogeographic patterns in inshore and offshore species assemblages, and in the stable isotope signatures of suspended particulate matter and filter-feeding mussels. We used the stable isotope ratios of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) from the blood and feathers of adult and chick Cape gannets (Morus capensis) to investigate whether the geographic differences observed at the lower levels in the marine communities are deep penetrating effects that reach top predators. Additionally, we evaluated whether trophic segregation occurs between adult and reared chick gannets, and whether a shift to wintering habitat occurs in adults. The study was conducted during the 2006 breeding season on Bird Island in the Agulhas system, and on Malgas and Ichaboe Islands, in the south and north Benguela respectively. Our results showed significant differences in the isotope ratios of members of different colonies, but no intra-colony differences between tissues or age groups. These results indicate that there is neither age-related nor temporal segregation in the diet of members of the same colony. Feather isotopic values suggest that adults remain all year round in the same habitats, and do not undertake long migration after reproduction. Since all gannets tend to target similar prey, we attributed among-colony differences in isotope signatures mostly to the oceanic conditions experienced by the main prey of birds rather than substantial differences in diet composition. Overall, isotopic signatures segregate the two current systems, with depleted carbon values in the Agulhas and enriched nitrogen values in the upwelled waters of the Benguela. Within the Benguela birds from Ichaboe in the north had higher δ¹⁵N values than those from Malgas in the south, which we attributed to differences in the functioning of the upwelling cells in the vicinity of the two colonies. Finally, slight variation in the proportion of main prey and discards from fisheries may contribute to the variation in the stable-isotope signatures between colonies in the Benguela.     

较大尺度下潮流物模试验理论可行性的数值研究

以渤海及其局部海域为例,通过数值模拟量化地转缺失对水动力过程的影响,评价不同空间尺度下实施潮流物理模型试验的理论可行性。结果表明,对整个渤海、辽东湾这么大尺度的海域实施物模试验理论上是不可行的;对90km尺度的渤海湾,地转缺失使模拟结果存在较大偏差;对莱州湾61km尺度的海域可进行物模试验。研究确定60km为实施潮流物模试验一般的理论允许尺度。     

无光照条件下CMOS图像传感器噪声分析

在无光照条件下,对采用CMOS图像传感器的尼康D90相机在不同的曝光时间下进行拍摄实验,分析CMOS图像传感器在不同曝光时间下的暗电流噪声以及系统噪声在传感器像面上的空间分布。得出如下结论：系统噪声并没有随曝光时间增加而增大的趋势,因此,系统的暗电流噪声比较小;而整体噪声在不同的曝光时间下,都有在空间分布上有像面外围较大而像面中间较小的情况,根据该分布给出计算不同位置像素系统噪声大小的公式。     

2011年宁夏川区一次高温天气分析

文章利用Micaps资料及数值预报产品,对2011年6月15日宁夏川区高温天气进行了分析,结果表明:此次高温天气主要影响系统是河西一带产生暖性高压东移形成河套高压。从低层到高层盛行暖平流是高温出现的直接因素,气流的上升下沉加快了暖流扩散。     

发射电流波形对瞬变电磁响应的影响

本文研究了不同发射电流波形对瞬变电磁(TEM)响应的影响,利用自由空间中回线作为目标体,推导出方波、梯形波、半正弦波、三角波激励的响应.研究表明:对于低电导率目标体,同种波形激励的响应在有发射电流期间(on-time)与无发射电流期间(off-time)的幅度相近,四种波形中方波激励响应最强.由于off-time采集可以获得更高的信噪比,因而探测低电导率目标体时一般采用方波激励,采集off-time信号.对于高电导率目标体,off-time响应幅度随电导率增加而迅速下降,而半正弦与三角波激励的on-time响应幅度基本不随电导率变化,因而航空TEM可以采用半正弦与三角波进行on-time采集,并通过适当地提高激励电流的基频,以提高信噪比与空间分辨率.     

直流电阻率法2.5维正演的外推瀑布式多重网格法

引入外推瀑布式多重网格法(EXCMG)求解2.5维直流电阻率有限元计算形成的大型稀疏线性方程组, 结合基于地址矩阵的压缩存贮方式以及最优化离散波数, 使得2.5维电阻率正演程序的计算速度大大提高而内存需求大大减小. 研究结果表明:EXCMG法的收敛速度与网格尺寸无关,计算速度明显优于不完全Cholesky共轭梯度(ICCG)法. 并且, 随着问题规模的增大, EXCMG法的效率优势更加明显. 对1600×1600网格的2.5维电阻率法模拟问题, 正演程序仅耗时28 s, 视电阻率平均相对误差控制在0.22%以内, 为进一步研究快速反演奠定了基础.     

磁暴环电流衰减率对磁层能量状态的影响

赤道环电流是引起磁暴扰动的主要电流体系,环电流衰减速率极大地影响着磁层能量收支估计和磁暴预报.本文提出评价环电流衰减率的两条新指标;（1）E指标（磁暴事件总能量收支平衡指标）,即磁暴全过程的积分能量收支平衡;（2）L指标（长期总能量收支平衡指标）,即几年、十几年或更长时段内积分能量收支平衡.我们用1998~2003年44个磁暴事件以及第23太阳周（1998~2008年）11年的连续资料,分别检验了几类衰减率模型对上述两条指标符合的情况.结果表明,PA1978和XD2010两类模型对E指标符合得最好,即无论磁暴强弱,它们均显示出事件总能量收支平衡的基本特征;同时,这两类模型与L指标符合得也最好,即它们的长期积分能量基本平衡,而且磁暴期间的能量消耗表现出明显增强的重要特征.     

基于多种方法联合反演的河床岩性结构特征研究

河床结构探测是研究河水-地下水转化过程的关键,复杂河床结构探测过程中,单一物探方法通常精度不高。本文采用多普勒声学流速仪、地质雷达法、高密度电阻率法相结合,在淮北平原典型工作区进行河床形态和沉积物分布特征探测,构建了完整的联合反演河床岩性结构特征的技术体系,取得了较好的识别效果。结果表明:新汴河节制闸上游呈"蝶形",河床结构均匀,底泥厚度变化范围小;下游河床形态受分水坝影响起伏较大,河床形态不平整,底泥厚度为0.8~1 m;涡河处河道形态呈半椭圆形,且底泥两岸薄中间厚。韩村剖面受抽砂干扰严重,河床出现中间高两侧低的情况,底泥分布不均匀;两河口断面下存在弱透水层(粉质黏土),下方为承压含水层,其潜水含水层分布不均匀。实践证明,多种技术联合反演在河床形态探测上能够相互印证,提高成果解译的可靠性和准确性,为河水-地下水转化研究提供支撑。