Baryon loading and the Weibel instability in gamma-ray bursts

The dynamics of two counter-streaming electron–positron–ion unmagnetized plasma shells with zero net charge is analysed in the context of magnetic field generation in gamma-ray burst internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios.     

Numerical prediction of MODUs drift during Hurricane Katrina

Severe hurricanes, such as Katrina, broke the mooring lines of a number of mobile offshore drilling units (MODU) deployed in the Gulf of Mexico and some of those MODUs went adrift. A drifting MODU may damage other critical elements of the offshore oil and gas infrastructure by colliding with floating or fixed production systems and transportation hubs, or by rupturing pipelines owing to their dragging anchors over the seabed. To avoid or mitigate the damage caused by a drifting MODU, it is desirable to understand the mechanics of the drift of a MODU under the impact of severe wind, wave and current and have the capability of predicting the trajectory of the drift. To explore the feasibility and accuracy of predicting the trajectory of a drifting MODU based on hindcast met-ocean conditions and limited knowledge of the condition of the drifting MODU, this study employed a simplified equation describing only the horizontal (surge, sway and yaw) motions of a MODU under the impact of steady wind, current and wave forces. The simplified hydrodynamic model neglects the first- and second-order oscillatory wave forces, unsteady wind forces (owing to wind gustiness), wave drift damping, and the effects of the body oscillation on the steady wind and current forces. It was assumed that the net effects of the oscillatory forces on the steady motion are insignificant. To verify the accuracy and feasibility of our simplified approach, the predicted drifting trajectories of two MODUs were compared with the corresponding measurements recorded by the global positioning system (GPS).     

渭河流域东部全新世黄土-古土壤剖面光释光测年及其记录的土壤侵蚀事件

在渭河流域东部老官台(LGT)全新世黄土-古土壤剖面中,系统采集了15个黄土释光测年样品,应用单片再生剂量法(SAR)对这些样品的细颗粒混合矿物进行了光释光(OSL)测年,测定结果表明:红外之后蓝光释光([post-IR]OSL)的年龄偏大,红外释光(IRSL)的年龄偏小或与蓝光释光(BLSL)的年龄基本一致,应用IRSL和BLSL信号测定的OSL年龄整体偏小可能与测定样品中长石矿物的异常衰退有关,而[post-IR]OSL信号由于很少发生异常衰退问题,使得其年龄可靠性更高。基于测定的[post-IR]OSL年龄,结合该剖面的磁化率和粒度指标,进一步探讨了渭河流域东部自全新世以来发生的两期土壤侵蚀事件,第1期土壤侵蚀事件发生在约12.40~9.34kaBP,即全新世大暖期来临之前的气候转暖期间;第2期土壤侵蚀事件发生在约4.60~3.76kaBP,即全新世大暖期即将结束、气候开始出现恶化、黄河中游地区夏文化发展的时期,这为深入探索夏代发生的大洪水事件及其气候背景提供了科学依据,本区两期显著的土壤侵蚀事件的发现,揭示了东亚季风系统的不稳定性,表现为千年到百年尺度上的快速变化。     

环境演变研究时间序列的建立方法评述

对环境演变时间序列建立的几种主要方法的纪年原理、应用状况及应用的优点、范围、局限性进行了阐述和比较，包括长寿命宇宙成因核素在内的同位素纪年方法主要判别第四纪以来大时间尺主的沉积物的地质年龄，且能很好地应用于全球变化的研究，但百余年来沉积物的年龄及侵蚀历史变化的研究只能通过^210Pbex、^137Cs等半衰期相对较短的核素来进行；指出^210Pbex可望在现代流域侵蚀环境演变以及其效应评价的研究中获得新的突破。     

Microscopy and rock magnetism of fine grain-size titanomagnetite from the Jacupiranga Alkaline Complex,Brazil: unearthing Ti-magnesioferrite nanoparticles

Very fine samples from the mineralized zones of the Jacupiranga complex at the Cajati mine were selected for crystallographic identification of Ti-magnesioferrite (TMf) nanostructures embedded in titanomagnetite (TM) using high-resolution transmission electron microscopy (TEM). A magnetic concentrate obtained of pyroxenite samples (sites 4 to 7) was reduced and divided into fractions of distinct range sizes: 26±2 μm, 19±1 μm, 13±1 μm, 9±1 μm, 6±1 μm and 6-0.1 μm. The mineralized samples of carbonatite and pyroxenite were characterized by X-ray diffraction, transmitted and reflected light microscope, and scanning electron microscope with multielemental analysis. The finest magnetic concentrate sample (MC₆) was analyzed under high-resolution transmitted electron microscopy (TEM) and high angle annular dark field and Raman spectroscopy. Magnetic properties were measured for the distinct granulometric fractions, showing drastic changes when grain sizes go beyond the frontier from micro to nanometer sizes. Frequency-dependent magnetic susceptibility percentage (÷_fd%) report higher values (10.2%) for the finer fractions (6±1 μm and 6-0.1 μm) attributed to dominant fractions of superparamagnetic particles. Nanometer and < 6 μm grain size TMf in TM particles require a magnetic field up to 249 mT to reach saturation during the isothermal remanent magnetization experiment. Coercivity and remanent magnetization of these samples increase when the particle size decreases, probably due to parallel coupling effects. Magnetic susceptibility versus temperature experiments were conducted two times on the same (< 35 nm) sample, showing that the repetition during the second heating is probably due to the formation of new TMf nanoparticles and growth of those already present during the first heating process.     

地面核磁共振信号相位求取电阻率

椭圆极化现象是电磁场在导电介质中传播时产生的,为得到准确的相位信息,在计算激发磁场时应考虑椭圆极化。本文研究了地面核磁共振(SNMR)信号相位求取电阻率的基本方法,利用SNMR信号通过广义逆反演法得到电阻率未知情况下的含水率及层位分布,作为相位反演电阻率的先验值,通过模拟退火法实现对电阻率的反演。对SNMR的研究解决了通过相位对电阻率的求取,有利于进一步利用相位信息提高含水层和含水率反演的精度。     

青少年胫骨结节撕脱性骨折影像学诊断

目的:探讨急性青少年胫骨结节撕脱性骨折的影像学诊断价值。方法:回顾性分析经手术证实的6例(都为男性)急性青少年胫骨结节撕脱性骨折患者资料,其中2例右侧,4例左侧。除1例为跌倒致伤外,其余5例为运动损伤。X线平片检查6例,CT平扫2例,MRI平扫3例。结果:X线平片表现,根据Ogden’s分型,1例ⅠB型,1例ⅡA型、2例ⅡB型、1例ⅢA型、1例ⅢB型;6例均有髌骨上移征象。CT表现,2例ⅡB患者显示骨碎块移位,髌骨往上移位,其中1例合并髌骨陈旧性骨折。3例合并髌韧带撕裂,1例ⅡB型骨折合并髌韧带撕裂患者CT、MRI显示髌韧带肿胀,扭曲,T1WI、T2WI髌韧带信号增高。结论:X线平片是诊断急性胫骨结节撕脱性首选的影像方法,CT对骨折分型也有一定的帮助。怀疑有合并髌韧带、半月板等软组织损伤时,MRI具有重要诊断价值,对治疗方案的选择具有指导作用。     

南海西北次海盆的磁条带重追踪及洋中脊分段性

由于缺少有效钻孔资料,对于南海扩张的时间一直存在较大的疑问.在南海三大海盆中,西北次海盆面积最小、磁条带特征不明显,因此对其扩张年代的争议最大.最新采集的高密度(小于10 km测线间距)船测地磁资料清晰地显示了西北次海盆磁条带的存在.在OBS和多道地震资料的约束下,利用船测地磁资料,本文对西北次海盆的地壳年龄进行了重追踪.根据定量的比较,西北次海盆的主体扩张始于35.8 Ma(C16n,2n),在34.7 Ma(C15)时其西南部开始扩张,扩张最终同时终止于33.2 Ma(C13n),整体的全扩张速率在40~50 mm/a之间.这表明南海的扩张可能首先起源于西北次海盆,在其结束扩张后,东部次海盆才开始打开(约30 Ma).得益于数据精度和密度的提高,利用化极后的磁力异常以及反演的磁化强度可以对西北次海盆进行二级中脊段的划分.我们共划分出六个中脊段和一个明确的转换断层.中脊的分段性与OBS反演的地壳厚度的变化相一致.转换断层东侧,中脊主体分为四个中脊段,每个中脊段长度均在30 km左右.转换断层西侧,存在一个长约50 km的中脊段和一个不确切的中脊段.中脊段上磁化强度的变化幅值和中脊段长度在整体上成正比.每个中脊段中央的磁化强度弱于中脊段两端的磁化强度,这与扩张速率相近的大西洋中脊的磁化强度特征一致.     

骨旁脂肪瘤的影像学特征及诊断价值

目的:分析骨旁脂肪瘤的影像学表现特征,提高对该病的认识及术前影像诊断水平。方法:回顾性分析经病理证实的我院2例骨旁脂肪瘤的临床及影像学表现。结果:2例骨旁脂肪瘤,男女各1例,分别位于右侧股骨上段、左侧胫骨中下段骨旁软组织内。X线表现为骨旁软组织内的透光肿块;CT、MRI表现为骨旁脂肪密度或脂肪信号、有包膜的椭圆形分叶状肿块,边界清楚;邻近骨皮质形成突向肿瘤内部的珊瑚礁状或树枝状骨性突起。结论:骨旁椭圆形脂肪性肿块,伴有骨皮质突向肿块内的珊瑚礁状、树枝状骨性突起为骨旁脂肪瘤的影像学特征,具有重要的诊断价值。     

老挝波乔省某磁铁矿区磁法勘探

勘查区位于老挝西北部之波乔省,地表风化严重、浮土掩盖厚、深林茂密、基岩露头少,虽然局部地段出露有大量的铁帽,但不敢判断该区是否有大规模的磁铁矿.为此,我们采用1∶10000地面高精度磁法测量以了解磁铁矿的有无、范围、延伸、规模.后期勘探阶段开展2.5D人机交互正反演工作以了解深部磁铁矿的有无.结果表明,在老挝西北部地质勘查程度低的地区开展高精度磁测扫面工作是寻找磁铁矿产行之有效的方法,能够解决基础地质问题,矿山勘查问题,后期运用2.5D人机交互正反演能够解决一定的矿层预测工作.