The Sun – Earth Workshop: A Summary of the Outcome

This special issue includes a set of papers that deal with extended solar activity, the launching of CMEs at the Sun, their propagation through interplanetary space, and the detection and study of the ejecta near Earth and of their interaction with the Earth’s magnetic environment. In particular solar events that occurred on 28 October 2003, 6 and 7 November 2004, and 20 January 2005, for which the related shocks arrived at Earth about two days later, are considered. The summary paper extracts the principal outcomes that were arrived at in the areas treated during the workshop and seeks to draw conclusions both on the progress made and on possible directions for future work in these areas.     

运用Geomagic Studio实现点云数据的曲面重建及误差分析

曲面重建是逆向工程的关键,为了评定曲面重建的精度要进行相应的误差分析。研究基于三维激光扫描仪获取得到的某塑像表面完整点云数据,运用Geomagic Studio软件对点云数据经过三角网格化、曲面划分等处理构建精确曲面,从而实现此塑像表面的重建。为对重建的曲面进行误差分析,通过平面拟合方程系数以及平面拟合的中误差,验证Geomagic Studio软件中的标准偏差即是测量中由残差计算得到的中误差,进而通过Geomagic Studio软件得到所建模型与点云的标准偏差,实现曲面重建的误差分析。     

球形标靶的固定式扫描大点云自动定向方法

根据目前地面激光扫描数据获取速度快、数据量大、测量距离远、专用特殊材料制作的标靶识别距离近、点云定向数据处理相对滞后、自动化程度低、不能适应远距离地形测量的现状,提出了从大点云中(每站1亿点以上)自动探测远距离标靶的点云定向方法。该方法首先根据标靶控制点的工程测量坐标信息,搜索到标靶所在点云环,然后对各点云环进行扇形分区,快速探测标靶,获取标靶中心扫描坐标,最后平差计算扫描仪位置参数和姿态参数,实现点云坐标到工程测量坐标的转换。该方法在普通配置的计算机上得到实现,并成功用于远距离山区地形测量,其中定向标靶半径0.162m,标靶到扫描站距离在180~700m之间。     

基于随机抽样一致性算法的稳健点云平面拟合方法

针对点云数据平面拟合过程中存在粗差及异常值等问题,文章提出一种基于随机抽样一致性算法(RANSAC)的稳健平面拟合方法。该方法以RANSAC算法为基础并结合特征值法,通过设置一定的准则,剔除点云数据中存在的粗差及异常值,达到获得理想平面拟合参数的目的。运用此算法对仿真数据及实测数据进行平面拟合,并与传统算法进行比较,结果表明该方法可以很好地适应于点云数据中存在粗差及异常值的情况,获得较好的平面参数估计值,是一种稳健的平面拟合算法。     

基于3维激光扫描的3维模型重建

作为一种新兴的空间信息获取手段,3维激光扫描仪的应用日益广泛。通过3维激光扫描仪获取的点云数据,可用于构建建筑物的整体点云模型,进而生成三角网模型和实体模型。本文结合工程项目中的实际操作,阐述点云数据采集以及模型构建的主要方法。     

Star formation in the LMC: Comparative CCD observations of young stellar populations in two giant molecular clouds

This work deals with a CCD imaging study at optical and near‐infrared wavelength oftwo giant molecular clouds (plus a control field) in the southern region of the Large Magellanic Cloud, one ofwhich shows multiple signs of star formation, whereas the other does not. The observational data from VLT FORS2 (R band) and NTT SOFI (Ks band) have been analyzed to derive luminosity functions and color‐magnitude diagrams. The young stellar content of these two giant molecular clouds is compared and confirmed to be different, in the sense that the apparently “starless” cloud has so far formed only low‐luminosity, low‐mass stars (fainter than m_Ks ∽ 16.5 mag, not seen by 2MASS), while the other cloud has formed both faint low‐mass and luminous high‐mass stars. The surface density excess oflow‐luminosity stars (∽2 per square arcmin) in the “starless” cloud with respect to the control field is about 20% whereas the excess is about a factor of 3 in the known star‐forming cloud. The difference may be explained theoretically by the gravo‐turbulent evolution of giant molecular clouds, one being younger and less centrally concentrated than the other (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dynamic evolution model of isothermal voids and shocks

We explore self-similar hydrodynamic evolution of central voids embedded in an isothermal gas of spherical symmetry under the self-gravity. More specifically, we study voids expanding at constant radial speeds in an isothermal gas and construct all types of possible void solutions without or with shocks in surrounding envelopes. We examine properties of void boundaries and outer envelopes. Voids without shocks are all bounded by overdense shells and either inflows or outflows in the outer envelope may occur. These solutions, referred to as type void solutions, are further divided into subtypes and according to their characteristic behaviours across the sonic critical line (SCL). Void solutions with shocks in envelopes are referred to as type voids and can have both dense and quasi-smooth edges. Asymptotically, outflows, breezes, inflows, accretions and static outer envelopes may all surround such type voids. Both cases of constant and varying temperatures across isothermal shock fronts are analyzed; they are referred to as types and void shock solutions. We apply the ‘phase net matching procedure’ to construct various self-similar void solutions. We also present analysis on void generation mechanisms and describe several astrophysical applications. By including self-gravity, gas pressure and shocks, our isothermal self-similar void (ISSV) model is adaptable to various astrophysical systems such as planetary nebulae, hot bubbles and superbubbles in the interstellar medium as well as supernova remnants.      

Fitting power-law distributions to data with measurement errors

If X , which follows a power-law distribution, is observed subject to Gaussian measurement error e , then   X + e   is distributed as the convolution of the power-law and Gaussian distributions. Maximum-likelihood estimation of the parameters of the two distributions is considered. Large-sample formulae are given for the covariance matrix of the estimated parameters, and implementation of a small-sample method (the jackknife) is also described. Other topics dealt with are tests for goodness of fit of the posited distribution, and tests whether special cases (no measurement errors or an infinite upper limit to the power-law distribution) may be preferred. The application of the methodology is illustrated by fitting convolved distributions to masses of giant molecular clouds in M33 and the Large Magellanic Cloud (LMC), and to H  i cloud masses in the LMC.     

Statistical assessment of shapes and magnetic field orientations in molecular clouds through polarization observations

We present a novel statistical analysis aimed at deriving the intrinsic shapes and magnetic field orientations of molecular clouds using dust emission and polarization observations by the Hertz polarimeter. Our observables are the aspect ratio of the projected plane-of-the-sky cloud image and the angle between the mean direction of the plane-of-the-sky component of the magnetic field and the short axis of the cloud image. To overcome projection effects due to the unknown orientation of the line-of-sight, we combine observations from 24 clouds, assuming that line-of-sight orientations are random and all are equally probable. Through a weighted least-squares analysis, we find that the best-fitting intrinsic cloud shape describing our sample is an oblate disc with only small degrees of triaxiality. The best-fitting intrinsic magnetic field orientation is close to the direction of the shortest cloud axis, with small  (∼24°)  deviations towards the long/middle cloud axes. However, due to the small number of observed clouds, the power of our analysis to reject alternative configurations is limited.