Radar Backscatter from Underdense Meteors and Diffusion Rates

Many meteoroids burn up between about 120 km and 70 km, deposit metals and dust and form ionized trails which are detected by radars. Model studies about the influence of neutral or positively charged background dust on the ambipolar diffusion indicate that significant smaller decay times should be observed for weak meteor echoes compared to strong meteor echoes which can affect the estimation of temperatures. The variation of meteor decay times in dependence on echo strength, height, and season was studied using radar observations at 69° N, 22° S, and 67° S. Significantly reduced decay times were found for weak echoes below about 88 km at low latitudes throughout the year, and at high latitudes with the exception of summer. In summer at high latitudes, decreasing decay times of weak and strong meteors are observed at altitudes below about 85 km during the appearance of noctilucent clouds. The impact of reduced decay times on the estimation of neutral temperatures from decay times is discussed.     

北京雷暴大风气候特征及短时临近预报方法

北京地区雷暴大风的预报准确率低而且时效短.为了提高对这种灾害性天气的预警能力,在气候统计的基础上,研究了潜势预报方法和临近预报算法.对1998-2007年134个雷暴大风过程的统计结果表明,北京地区绝大多数的雷暴大风具有下击暴流特征,而且冰雹的落区附近也是大风的爆发区之一.因此,负浮力的作用和对冰雹具有指示性意义的因子是研究雷暴大风预报方法应主要考虑的因素.500hPa环流背景分析表明,尽管绝大多数雷暴大风爆发时对流层中层有干空气侵入,但是还有少数个例产生在偏南暖湿气流中.目前,对后一类大风产生的机制仍然不清楚.研究表明,当对流层中层有干空气侵入时,有利于雷暴大风出现的环境条件是:下沉气流具有较大的不稳定性,同时对流层低层环境大气的温度直减率较大.此外,还讨论了经验指数--大风指数在北京地区的应用.基于上述的研究,形成了北京地区雷暴大风短时潜势预报方法,还使用相关分析和多元回归分析技术建立了基于雷达观测和环境条件的雷暴大风临近预报方程.个例分析表明,临近预报方程对于飑线和弓形回波等带来的地面大风具有一定的预报能力.     

Extragalactic Astronomy with the VLTI: a new window on the Universe

Interferometry in the optical and near infrared has so far played a marginal role in Extragalactic Astronomy. Active Galactic Nuclei are the brightest and most compact extragalactic sources, nonetheless only a very limited number could be studied with speckle interferometry and none with long baseline interferometry. The VLTI will allow the study of moderately faint extragalactic objects with very high spatial resolution thus opening a new window on the universe. With this paper we focus on three scientific cases to show how AMBER and MIDI can be used to tackle open issues in extragalactic astronomy. This revised version was published online in July 2006 with corrections to the Cover Date.     

X-Ray Interferometry

For the astronomer, X-ray interferometry is the theory and practice of building dilute aperture telescopes for studying celestial X-ray sources. The short wavelengths and high surface brightness of X-ray sources will make the eventual scientific payoff very high, with direct imaging of the event horizons of black holes as the centerpiece. In this article, we review the history of X-ray interferometry and discuss the recent technical developments toward astronomical applications. We present several mission concepts and show they are achievable with todays technology.     

国际地球自转服务(IERS)评介

回顾了建立国际地球自转服务(IERS)的历史过程,简述了IERS机构各部门的设置及其作用。着重介绍了IERS重建的理由和过程以及新IERS机构的组成。最后给出IERS成果对现有科学研究的作用和意义。     

Potential of the VLTI for linking stellar frames to ICRF

The question of positioning the optical counterparts of the ICRF quasars is outlined in the perspective of future space astrometry missions, which ultimately will bring a new realization of the ICRS in the optical range. Ground-based interferometry with a dual-field observing mode (PRIMA/VLTI),together with the missions DIVA and FAME, will have a key role in building an extragalactic reference frame in the optical/near-IR range with about the same accuracy as that of the present (VLBI) primary frame. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Effect of Tidal Interaction with a Gas Disk on Formation of Terrestrial Planets

We have performed N-body simulation on final accretion stage of terrestrial planets, including the effect of damping of eccentricity and inclination caused by tidal interaction with a remnant gas disk. As a result of runway and oligarchic accretion, about 20 Mars-sized protoplanets would be formed in nearly circular orbits with orbital separation of several to ten Hill radius. The orbits of the protoplanets would be eventually destabilized by long-term mutual gravity and/or secular resonance of giant gaseous planets. The protoplanets would coalesce with each other to form terrestrial planets through the orbital crossing. Previous N-body simulations, however, showed that the final eccentricities of planets are around 0.1, which are about 10 times higher than the present eccentricities of Earth and Venus. The obtained high eccentricities are the remnant of orbital crossing. We included the effect of eccentricity damping caused by gravitational interaction with disk gas as a drag force (“gravitational drag”) and carried out N-body simulation of accretion of protoplanets. We start with 15 protoplanets with 0.2M⊕ and integrate the orbits for 10⁷ years, which is consistent with the observationally inferred disk lifetime (in some runs, we start with 30 protoplanets with 0.1M⊕). In most runs, the damping time scale, which is equivalent to the strength of the drag force, is kept constant throughout each run in order to clarify the effects of the damping. We found that the planets' final mass, spatial distribution, and eccentricities depend on the damping time scale. If the damping time scale for a 0.2M⊕ mass planet at 1 AU is longer than 10⁸ years, planets grow to Earth's size, but the final eccentricities are too high as in gas-free cases. If it is shorter than 10⁶ years, the eccentricities of the protoplanets cannot be pumped up, resulting in not enough orbital crossing to make Earth-sized planets. Small planets with low eccentricities are formed with small orbital separation. On the other hand, if it is between 10⁶ and 10⁸ years, which may correspond to a mostly depleted disk (0.01-0.1% of surface density of the minimum mass model), some protoplanets can grow to about the size of Earth and Venus, and the eccentricities of such surviving planets can be diminished within the disk lifetime. Furthermore, in innermost and outermost regions in the same system, we often find planets with smaller size and larger eccentricities too, which could be analogous to Mars and Mercury. This is partly because the gravitational drag is less effective for smaller mass planets, and partly due to the “edge effect,” which means the innermost and outermost planets tend to remain without collision. We also carried out several runs with time-dependent drag force according to depletion of a gas disk. In these runs, we used exponential decay model with e-folding time of 3×10⁶ years. The orbits of protoplanets are stablized by the eccentricity damping in the early time. When disk surface density decays to ?1% of the minimum mass disk model, the damping force is no longer strong enough to inhibit the increase of the eccentricity by distant perturbations among protoplanets so that the orbital crossing starts. In this disk decay model, a gas disk with 10⁻⁴-10⁻³ times the minimum mass model still remains after the orbital crossing and accretional events, which is enough to damp the eccentricities of the Earth-sized planets to the order of 0.01. Using these results, we discuss a possible scenario for the last stage of terrestrial planet formation.     

Wave Height Estimation from SAR Imagery

This paper presents a method developed for estimating wave height from synthetic aperture radar (SAR) imagery without prior assumption of noise distribution. It is based on two-dimenslonal ocean wave spectra retrieved from fully calibrated SAR images. Wen‘s spectrum was used as first-guess wave spectrum in the retrieval process. Comparison of the estimated wave height obtained by this method from two ERS-1 SAR subimages dated 23 July 1994 with in-situ measurements showed that the method works well.     

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江西北部岩溶地貌陆生贝类物种多样性

作者于2013年5月至6月调查了位于江西北部地区岩溶地貌陆生贝类资源,共采得陆生贝类63种及亚种(含5个未定种),分别隶属于12科21属,其中1个新种,即石钟山弯螺(Sinoennea shizhongshanensis sp.nov),优势种为雪土鸥螺(Georissa niva)、细纹喇叭螺(Boysidia gracilis)、囊喇叭螺(Boysidia dorsata)和灰尖巴蜗牛(Bradybaena ravida ravida)。区系成分以东洋界成分为主,占总种类数的61.90%。计算了江西北部地区岩溶地貌陆生贝类多样性指数,其中Margalef丰富度指数(dMA)为2.205~4.273,Shannon-Wiener多样性指数(H′)为1.960~3.374,Pielou均匀度指数(Jsw)为0.448~0.681。与邻近自然保护区比较,江西北部地区岩溶地貌陆生贝类较为丰富,与其他自然保护区物种相似性较低。