我国极区冬季电离层加热实验研究

通过对非相干散射雷达观测数据的处理分析,研究了2008年1月我国在挪威Troms进行的冬季电离层加热实验效应.研究结果表明,电离层临界频率大于泵波频率的O波加热事件扰动效应明显,电子温度存在60%~120%的增强,扰动范围从150 km一直延伸到400 km,电子密度扰动不显著,最大可以观察到12%的密度衰减.受加热影响,离子声波频率有1~2 kHz的增加,离子线谱峰谷比增加,有时伴随有高阶谐振线出现.离子线和等离子线功率存在过冲现象,等离子线的功率剖面存在单峰、双峰和三峰结构,等离子线的功率增强幅度随频率负指数衰减.     

离子分布函数的20矩近似在非相干散射谱中的应用

根据Grad理论,以麦克斯韦分布函数为基,将离子分布函数展成20矩近似的形式,讨论了黏滞张量项与热流项对离子分布函数的影响.在电场比较弱的情况下,沿着E×B和E的方向上,热流矢量项和黏滞张量项是可以分别忽略掉的.离子温度的各向异性特征和分布函数的不对称性是由黏滞项和热流项引起的.利用Sheffield理论计算得到了非相干散射谱,并对13矩近似和20矩近似得到的非相干散射谱进行了对比.在电场比较弱的情况下,这两种近似基本一致,而当电场增强时,热流张量项的贡献变得很明显,温度各向异性的特征加剧.此时与13矩近似相比较,20矩近似能够更加准确地描述以温度各向异性为特性的非麦克斯韦分布等离子体.     

云南1月降水异常的气候成因分析

在云南旱季降水稀少的气候背景下发生的异常旱涝气候事件是短期气候预测面临的难点之一。开展旱季降水异常的气候成因分析,对于提高气候预测准确率具有重要意义。1月是云南隆冬季节的主要时段,降水变化具有明显的偏态分布特征,年际波动非常显著。云南1月降水偏多年与偏少年大气环流有明显差异,500 hPa高度场上有四个区域位势高度与云南1月降水显著相关,中高纬的三个区域分别与斯堪的纳维亚(SCA)型遥相关的三个活动中心对应,低纬阿拉伯海的高度异常与孟加拉湾印缅槽活动有密切联系。当SCA为正(负)位相且阿拉伯海高度负(正)异常时云南1月降水偏少(多)。海洋上两个显著海温影响区域分别位于东南太平洋和北大西洋中西部,海温异常主要通过影响东亚和南亚地区的环流进而对云南1月降水产生影响。利用高度场和海温场的关键区域的数据分别合成高度场和海温场的组合序列,组合序列与云南1月降水有更高的相关性和更好的距平符号对应关系。在高度场和海温场组合序列的散点图上,近80%的年份位于第一和第三象限,这两个区域高度场和海温场的配置对云南1月降水作用一致,落区位置与降水距平符号对应关系很好;其余约20%的年份位于第二和第四象限,高度场和海温场的作用相反,落区位置与降水距平符号对应关系较差。     

菲律宾宿务岛地区矿化蚀变图像识别与提取分析

文章通过遥感多波段数据的背景端元数M值的估计方法、回归偏度曲线分析和地物光谱反射特征来选择信息提取的特征波段;在综合分析的基础上,选择基于Crosta法的F因子与H因子分析方法,根据不同蚀变信息提取的波段选择,以自行开发的软件对律宾宿务岛地区遥感图像进行矿化蚀变信息检测与提取.提取结果表明,区内已知的矿床(点)和矿化蚀...     

卫星遥感影像有理函数模型优化方法

针对高分辨率遥感影像有理函数模型（RFM）在实际应用中存在过度参数化和定位精度不高的问题,提出基于离差阵和消去变换及残余系统误差补偿的高分辨率遥感影像RFM优化方法。试验结果表明,经过参数筛选后的RFM参数均为无偏估计值,拟合精度可以达到全部参数用于拟合时的精度,而且模型病态性基本消除,模型稳定性更高;残余系统误差补偿方法可以有效消除RFM拟合严格成像模型的残余误差,达到与严格成像模型一致的对地定位精度。     

Air–water two-phase flow modeling of turbulent surf and swash zone wave motions

Wave breaking and wave runup/rundown have a major influence on nearshore hydrodynamics, morphodynamics and beach evolution. In the case of wave breaking, there is significant mixing of air and water at the wave crest, along with relatively high kinetic energy, so prediction of the free surface is complicated. Most hydrodynamic studies of surf and swash zone are derived from single-phase flow, in which the role of air is ignored. Two-phase flow modeling, consisting of both phases of water and air, may be a good alternative numerical modeling approach for simulating nearshore hydrodynamics and, consequently, sediment transport. A two-phase flow tool can compute more realistically the shape of the free surface, while the effects of air are accounted for. This paper used models based on two-dimensional, two-phase Reynolds-averaged Navier–Stokes equations, the volume-of-fluid surface capturing technique and different turbulence closure models, i.e., k–ε, k–ω and re-normalized group (RNG). Our numerical results were compared with the available experimental data. Comparison of the employed method with a model not utilizing a two-phase flow modeling demonstrates that including the air phase leads to improvement in simulation of wave characteristics, especially in the vicinity of the breaking point. The numerical results revealed that the RNG turbulence model yielded better predictions of nearshore zone hydrodynamics, although the k–ε model also gave satisfactory predictions. The model provides new insights for the wave, turbulence and means flow structure in the surf and swash zones.     

极区电离层加热的数值模拟与实验对比

大功率无线电波能加热电离层等离子体,可以引起电离层电子温度和密度的扰动,实现电离层的人工变态.从电子的连续性方程、动量方程和能量方程出发,我们给出了地面人工大功率无线电波加热电离层的数值模型.通过对方程的数值求解,计算了极地电离层条件下,电子温度、电子密度的加热效应,讨论了泵波参数对加热效应的影响.研究结果表明,电子温度几乎在整个高度上表现为一致性的幅度增强,且在反射高度附近形成温度增强峰很平缓.电子密度在峰上高度附近形成密度谷,谷两侧存在密度增强.加热效应基本随加热功率的增大而增大,随加热频率的增大而减小.使用我国2008年1月在挪威进行的电离层加热实验的电离层参量作为仿真初值,对6个O波加热时刻进行了数值仿真,仿真结果与实验观测基本保持一致.     

锥束CT散射抑制方法综述

锥束CT严重的散射伪影极大地降低图像质量,成为限制其发展和应用的关键因素。国内外学者对散射抑制或校正方法已做了很多综述性的报道,但是随着锥束CT系统散射抑制研究的进一步推进,出现了一些新的技术。本文分析了X射线散射对CT成像的影响,对常用的散射抑制方法进行归纳总结,梳理散射校正框架,详细介绍基于投影补偿的散射校正技术和常用的散射估计方法,对目前散射校正的问题进行总结,重点讨论了散射校正对噪声的放大作用,为散射抑制的研究和发展提供借鉴作用。      

On the accuracy of the orbit of asteroid (99942) APOPHIS at the time of its encounter with the Earth in 2029

We estimate the effect of trajectory measurement errors on the orbital parameters of asteroid Apophis determined from improvements. For this purpose, based on all of the optical and radar observations available to date, we have computed a nominal orbit of the asteroid. The scatter ellipsoid of the initial conditions of motion has been obtained by two methods. In the first, universally accepted method, the scatter ellipsoid is calculated by assuming a linear dependence of the errors in the parameters being determined on observational errors. In the second method, the scatter region of the orbital parameters around the nominal-orbit parameters is determined by the Monte Carlo method. We show that the region determined by the latter method at the initial epoch differs only slightly from the scatter ellipsoid for the linear approximation. We estimate the sizes of the projections of the corresponding regions onto the target plane at the time of the closest encounter of the asteroid with the Earth in 2029. The projections are approximated by ellipses. Our computations have shown that the ellipse has the following sizes: 389.6 km for the semimajor axis and 16.4 km for the semiminor axis in the linear case and 330.0 and 11.1 km, respectively, in the nonlinear case.     

Comparison of Aerosol Single Scattering Albedo Derived from the Ozone Monitoring Instrument with Aerosol Robotic Network Observations

The single-scattering albedo (SSA), which quantifies radiative absorption capability, is an important optical property of aerosols. Ground-based methods have been extensively exploited to determine aerosol SSA but there were no satellite-based SSA measurements available until the advent of advanced remote sensing techniques, such as the Ozone Monitoring Instrument (OMI). Although the overall accuracy of OMI SSA is estimated to approach 0.1, its regional availability is unclear. Four-year SSA daily measurements from three Aerosol Robotic Network (AERONET) sites in China (Xianghe, Taihu, and Hong Kong) are chosen to determine the accuracy of OMI SSA in specific locations. The results show that on a global scale, the OMI SSA is systematically higher (with a mean relative bias of 3.5% and a RMS difference of ~0.06) and has poor correlation with the AERONET observations. In the Xianghe, Taihu, and Hong Kong sites, the correlation coefficients are 0.16, 0.47, and 0.44, respectively, suggesting that the distinct qualities of OMI SSA depend on geographic locations and/or dominant aerosol environments. The two types of SSA data yield the best agreement in Taihu and the worst in Hong Kong; the differing behavior is likely caused by varying levels of cloud contamination. The good consistency of the aerosol variation between the two SSA datasets on a seasonal scale is promising. These findings suggest that the current-version OMI SSA product can be applied to qualitatively characterize climatological variations of aerosol properties despite its limited accuracy as an instantaneous measurement.