Seasonal evolution of the transverse thermohaline asymmetry in the Ria de Pontevedra (northwestern Spain)

A quasi-permanent transverse asymmetry in the thermohaline properties of the Ria de Pontevedra (northwestern Spain) was measured from October 1997 to October 1998 during 23 cruises. On average, the density increased form north to south. This asymmetry was maintained by salinity during the wet season, and by salinity and temperature during the dry season. The density difference between coasts (Δσ_t = σ_tsouth − σ_tnorth) and the river discharge were positively correlated (correlation coefficient, approximately 0.6) throughout the year. The density difference was negatively correlated with the upwelling index during the wet season (correlation coefficient, approximately −0.45) and positively correlated during the dry season (correlation coefficient, approximately 0.55).     

Annual Cycle of Zooplankton Biomass, Abundance and Species Composition in the Neritic Area of the Balearic Sea, Western Mediterranean

Abstract. Seasonal changes in zooplankton biomass, abundance and species composition were studied at a neritic station in the Balearic Sea between April 1993 and May 1994. Sampling was carried out every 10 days in a zone influenced by the main current circulating through the Mallorca channel. Three main peaks of zooplankton biomass and abundance were observed: (1) at the beginning of summer when the thermocline developed, (2) in autumn when the thermocline broke down, and (3) in early spring. The smaller zooplankton fraction (100–250 μm) comprised on average 32 % of the total biomass and 73 % of total abundance. Copepods were the predominant group (64 % of the total abundance) with Clausocalanus, Oithona and Paracalanus being the most abundant genera. Paracalanus parvus, Clausocalanus furcatus, Acartia clausi, Oithona plumifera, Temora stylifera, Centropages typicus and Oncaea mediterranea were found to be the most important species in the area. Other abundant groups were cladocerans (15 %) and meroplankton larvae (12 %), both of which were particularly numerous during the stratified period. The copepod community was characterized by the above‐cited perennial species, which were abundant during the cycle studied. However, the influence of the hydrological conditions of the Balearic Sea, such as the Atlantic water influx and the physical structure of the water column (stratification and mixing), promoted the observed variability in zooplankton as well as the appearance of characteristic species during the annual cycle.     

二维动画地图表示方法的研究与应用

本文根据动画地图表示内容和常用动画技术的特点,对二维动画地图的表示方法进行了探讨。归纳了基于空间分布特征的表示方法、基于动画地图用途的表示方法和基于动画技术的表示方法及其分类,并讨论了影响选择表示方法的因素和几种表示方法相互配合的应用规律,对动画地图可视化的理论与技术的研究有一定参考。     

一种时空数据静态可视化表达方法

本文简要介绍了时空数据可视化的必要性,以及目前人们采用的时空数据可视化方法,并分析了这些方法的局限性,提出了一种将年轮(annual ring)作为时空载体,结合视觉变量对面状地物的属性数据进行静态可视化的方法。该方法将面状地物的同心环按照类似树木年轮的生长特征进行划分,作为空间和时间的载体,不同时期属性数据映射到相应的同心环中,同心环的厚度和色彩的差异揭示了属性数据的变化。最后以武汉市城市管理网格化平台的统计评价数据为例实现了对比率量表数据的时态可视化表达。本文所提出的静态时空可视化方法将面状地物的空间、属性和时间紧密地结合在一起,克服了以往静态可视化方法中时间-空间、时间-属性相分离的不足,试验证明了这种方法在实践中的可行性。     

Low frequency flickering of TT arietis: Hard and soft X-ray emission region

Using archival ASCA observations of TT Arietis, X-ray energy spectra and power spectra of the intensity time series are presented for the first time. The energy spectra are well-fitted by a two continuum plasma emission model with temperatures 1 keV and 10 keV. A coherent feature at 0.643 mHz appeared in the power spectra during the observation.     

TheROSAT survey of extreme UV sources: a new window on the universe

This paper discusses the first all-sky suvey of cosmic extreme ultra-violet sources, discovered by theROSAT Wide Field Camera. Details of the instrument and the survey are presented, with comparisons made to previous selected surveys in the X-ray regime. The subsequent optical identification program is described, and the major results summarized. I then discuss the main classes of EUV emitters: active chromosphere stars and hot white dwarfs, and describe the importance of EUV obserevations in understanding the astrophysics of these objects. Many bright, and relatively nearby, sources have been identified with hitherto unrecognized active stars, representing the extremes in chromospheric and coronal activity, be it binary or age related. Many new hot DA white dwarfs have also been indentified, and the most exciting result in this area is the discovery that significant traces of heavier elements (e.g. C, N, O, Si, Fe & Al) exist in their atmospheres, substantially increasing their EUV opacities. The importance of hot white dwarfs as standard candles in probing the local interstellar medium is also discussed. Miscellaneous counterparts (AGN, PNN, O-B stars and CVs) that make up the rest of the sample of EUV sources are also briefly mentioned. I conclude the paper with a discussion of the new magnetic cataclysmic variables discovered at SAAO, which have been the subject of intensive follow-up observations.     

Hamiltonian perturbation theory on a manifold

This paper deals with Hamiltonian perturbation theory for systems which, like Euler-Poinsot (the rigid body with a fixed point and no torques), are degenerate and do not possess a global system of action-angle coordinates. It turns out that the usual methods of perturbation theory, which are essentially local being based on the construction of normal forms within the domain of a local coordinate system, are not immediately usable to study perturbations of these systems, since degeneracy makes impossible to control that the system does not fall into a singularity of the coordinates. To overcome this difficulty, we develop a global formulation of Hamiltonian perturbation theory, in which the normal forms are globally defined on the phase space manifold. The key for this study lies in the geometry of the fibration by the invariant tori of an integrable degenerate Hamiltonian system, which is described by some generalizations of the Liouville-Arnol'd theorem and is reviewed in the paper. As an application, we provide a global formulation of Nekhoroshev's theorem on the stability for exponentially long times.     

The circumstellar shells of carbon miras

The relation between mass loss rate and pulsation period in carbon Miras is discussed. The dust mass loss rate is very low (about 2 × 10^–10 M/yr) up to aboutP = 380 days, where there is a sudden increase. ForP > 400 days there is a linear relation between logM andP. The change in the mass loss rate near 380 days may be related to radiation pressure on dust becoming effective in driving the outflow.     

GIS支持下青海湖地区草地蝗虫发生的地形分析

草地蝗虫发生的地形分析是建立草地蝗虫发生预报模型的基础，以青海湖地区为实验区，在Arc/Info和ArcView地理信息系统的支持下，进行草地蝗虫发生与各地形变量（包括海拔高度、坡度与坡向）的叠置分析，提取草地蝗虫发生的地形信息数据库，然后，分别采用T－检验（对连续变量－海拔高度和坡度）和卡方（Chisquare)检验（对类变量－坡向类型）进行各地形变量对草地蝗虫发生的差异显著性检验，结果表明，海拔高度和坡度对草地蝗虫发生的影响极为显著（显著性水平p=0.000)，而坡向地草地蝗虫发生的影响不是特别明显（显著性水平p=0.039)。     

塔里木河中游天然植被的数量分类与排序研究

通过塔里木河中下游天然植被带的调查研究，应用数量分类(TWINSPAN)和排序(CCA)方法，对塔里木河中下游地区天然植被类型进行了划分，并探讨了决定该地区天然植物群落类型的主要环境因子。该地区天然植被可分为4个植被型组，4个植被型，6种植被亚型，9个群系，15个群丛。通过对8个环境因子的CCA排序分析，结果表明制约塔里木河中下游天然植被组成和结构的主导环境因子为地下水位、地下水矿化度、地下水酸碱度。通过CCA二维排序图将16种植物对干旱、盐碱的适应性划分5类型。21个样地在CCA二维排序图上可聚集成9个植物群落类群，即胡杨(Populus euphratica)群落、铃铛刺(Halimodendron halodendron)群落、库尔勒沙拐枣(Calligonum Kuerlese)群落、多枝柽柳(Tamarix ramosissima)群落、黑果枸杞(Lycium ruthenicum)群落、盐穗木(Halostashys caspica)群落、花花柴(Karelinia caspica)群落、疏叶骆驼剂(Alhagi sparsifolia)群落、罗布麻(Apocynum venetum)群落，与TWINSPAN结果中的群系分类单位一致。CCA连续排序与TWINSPAN分类结果吻合较好。9种植物群落类型中，能耐受最大盐胁迫的为盐穗木群落，能耐受最大干旱胁迫的为铃铛刺群落，能耐受最大地下水碱胁迫的为黑果枸杞群落。