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31.
基于整体经验模态分解和支持向量回归的北京市 PM 2.5预测 总被引:5,自引:0,他引:5
为了更好地掌握大气中PM2.5浓度的变化规律,利用EEMD-SVR混合模型对该地区的PM2.5浓度值进行了短期预测。首先,通过采用整体经验模态分解(EEMD)方法分析北京市PM2.5,把原始时间序列分解成多个固有模态函数和趋势项;然后,对各阶固有模态函数进行周期性分析,揭示了北京市PM2.5的周期性变化特点;最后,对经过EEMD分解后的各阶固有模态函数和趋势项用支持向量机回归(SVR)方法进行预测。结果表明,EEMD-SVR混合模型比单一的SVR模型预测精度更高。 相似文献
32.
利用南极中山站极光全天空摄相、地磁、地磁脉动数据和Wind卫星的行星际磁场IMF观测数据,分析了7个亚暴期间高纬黄昏—子夜扇区极光弧的短暂增亮现象.极光弧特征是,短暂增亮随后很快衰减,历时10—20min,基本沿着日-地方向,有明显黄昏方向运动.这些事件大都发生在IMFBz南转之后,亚暴增长相或膨胀相期间,极光浪涌到达之前10—73min消失.相应的IMFBx>0,IMFBy<0.这种极光弧和亚暴极光不同,它们与地磁活动及Pi2脉动不相关.这7个极光弧的形态和IMF特征表明,极光弧的增亮很可能由尾瓣重联产生,很快衰减归因于IMFBz南向条件,而黄昏方向运动受IMFBy控制. 相似文献
33.
Variation of phase speed of the magnetoacoustic surface waves on a magnetohydrodynamic (MHD), tangential discontinuity (TD) is examined against the variation of Alfvén speeds on opposite sides of the discontinuity with the acoustic speeds remaining constant. The discontinuous thermal profile of the plasma is employed to classify various surface wavemodes. The existence conditions for the surface waves manifest themselves through the appearance of critical values for various parameters restricting the propagation of such waves on the TD. The limiting situation of extremely low-β plasmas is studied to improve upon earlier results based on simplified calculations. Theoretical understanding of the basic structure of surface waves is then applied to specific observations of discontinuities in the interplanetary magnetic field (IMF) to arrive at simple estimates of phase speed of the surface waves on such discontinuities that may be tested in further observations. 相似文献
34.
M. Yamauchi Y. Futaana E. Kallio R. Lundin J.D. Winningham M. Holmström 《Planetary and Space Science》2008,56(8):1145-1154
In a previous paper, we showed a method for deriving the interplanetary magnetic field (IMF) orientation from the velocity distribution of ring-like distributed ions as measured by the Ion Mass Analyser (IMA) on board Mars Express (MEX). This method has been improved so that one can derive the IMF orientation from a very limited portion of the ring distributions, i.e., only the highest energy portion of the ring distribution. This method uses the maximum variance direction L instead of the minimum variance direction N, which are derived from manually selected ring data. Because IMA's count rate for a semi-persistent ring distribution is nearly proportional to energy squire, L is most likely aligned to the tangential direction of the ring distribution at its highest energy, and this tangential direction is parallel or anti-parallel to the electric field. A vector product of L and the solar wind direction (X) gives the IMF orientation projected to the Y-Z plane. The tilt angle of IMF toward the X direction from the Y-Z plane is the same as the angle between the X direction and the ring plane, and is obtained from two methods when the initial speed of the ring ions is estimated to be much smaller than the solar wind speed: (1) angle between the velocity of ring's maximum energy portion and the solar wind vector, and (2) energy ratio between the solar wind and the maximum energy of the ring. The present method is applied to the IMA data from 3 June 2005 (0605-0640 UT) when the Mars Global Surveyor (MGS) magnetometer data are available. Using these data, we also tried to determine the sign of the IMF direction by estimating the evolution direction of the ring ions. 相似文献
35.
The characteristics of latitudinal angles of solar wind flow (θv) observed near earth have been studied during the period 1973-2003. The average magnitude of θv shows distinct enhancements during the declining and maximum phases of the sunspot cycles. A close association of Bz component of IMF in the GSE system and the orientation of meridional flows in the solar wind is found which depends on the IMF sector polarity. This effect has been studied in typical geomagnetic storm periods. The occurrence of non-radial flows is also found to exhibit heliolatitudinal dependence during the years 1975 and 1985 as a characteristic feature of non-radial solar wind expansion from polar coronal holes. 相似文献
36.
利用1951—2004年我国北方348个测站的夏半年逐日降水资料,首先整理出各站点的极端干期日数和无雨总日数两个序列,得到极端干期日数的空间分布,并将北方划分为A、B、C 3个区,其中A、B 2个区相对湿润,C区则相对干旱和半干旱,这种分布与地理环境有密切的联系。然后建立这3个区夏半年极端干期以及无雨总日数的时间序列,采用经验模态分解(EMD)方法,分析极端干期序列的多尺度振荡结构以及多年变化的总趋势。结果显示,北方各区夏半年极端干期变化主要由前2个较高频的本征模态函数(IMF)分量构成,这表明3~4年和6~7年的变化对整个极端干期的变化起主要作用。对较低频分量和总趋势的分析表明,近54年来东北和华北大部分相对湿润的地区极端干期延长,有变干的趋势;北方主要干旱半干旱地区的极端干期变短,干旱有所缓解。近10~20年来整个北方极端干期日数增加,干旱化加剧,但西北局部干旱地区的极端干期在缩短,干旱有所缓解。这说明,分析的时—空尺度不同,得到的结果会有所不同。 相似文献
37.
提出了基于经验模式分解(EMD)的环境激励结构模态参数随机子空间识别(SSI)方法。该方法用设置间断频率的EMD将结构环境振动响应原始信号分解成若干个基本模式分量(IMF),使每一个基本模式分量仅为结构的某一阶固有模态,进而用随机子空间方法进行模态参数识别。实桥环境振动实验分析结果表明,该方法能有效地避免结构各阶模态之间的相互影响,能够更清晰方便地得到结构的模态参数。 相似文献
38.
Håvard Haarstad 《Geoforum》2009,40(2):239-248
Recent literature asserts that labour movements worldwide have been disempowered as a result of the various processes and discourses linked to economic globalisation. But there is a need to explore more carefully the mechanisms by which these processes and discourses affect resources for and constraints on effective union organisation. The purpose of this paper is to understand the construction of political spaces for organised labour and how these have been influenced by one such discourse: the policy discourse on foreign direct investments (FDI). It analyses the relationship between this policy discourse and political spaces for Bolivian petrolero unions through a case study of the period from 1984 to 2006. The policy discourse shifted the function of the state, fragmented symbols of collective identity, introduced flexible contracts, decentralised negotiations, rewarded individual achievement, and fostered competition rather than cooperation between employees. The shift towards an internationally integrated, FDI-driven economy shapes political spaces that allow collective action to be restructured from the workplace-based demands of unions to demands for democratic and indigenous rights of NGOs and other non-labour civil society organisations. 相似文献
39.
Guo‐Qiang Tang Ben‐Xun Su Qiu‐Li Li Xiao‐Ping Xia Jie‐Jun Jing Lian‐Jun Feng Laure Martin Qing Yang Xian‐Hua Li 《Geostandards and Geoanalytical Research》2019,43(4):585-593
Secondary ion mass spectrometry (SIMS) requires matrix‐matched reference materials to calibrate mass fractionation during oxygen isotope measurement. Over one thousand SIMS oxygen isotope measurements were conducted on eleven natural mineral samples (five olivines, three clinopyroxenes and three orthopyroxenes) in nineteen sessions using CAMECA IMS 1280 SIMS instruments to evaluate their potential as SIMS reference materials. The obtained results reveal oxygen isotope homogeneity of these samples. No matrix effect was measured for the same variety of mineral samples with limited Mg‐number variations (89.6–94.2, 90–91.9 and 90.1–92.1 for olivine, clinopyroxene and orthopyroxene, respectively). The recommended oxygen isotope compositions of these samples were determined using laser fluorination. These samples are therefore suitable to be used as reference materials for in situ oxygen isotope microanalysis. 相似文献
40.
亚暴期间高纬黄昏-子夜扇区极光弧增亮与衰减及其与IMF的关系 总被引:3,自引:0,他引:3
利用南极中山站极光全天空摄相、地磁、地磁脉动数据和Wind卫星的行星际磁场IMF观测数据,分析了7个亚暴期间高纬黄昏-子夜扇区极光弧的短暂增亮现象.极光弧特征是,短暂增亮随后很快衰减,历时10-20min,基本沿着日-地方向,有明显黄昏方向运动.这些事件大都发生在IMFBz南转之后,亚暴增长相或膨胀相期间,极光浪涌到达之前10-73min消失.相应的IMFBx>0,IMFBy<0.这种极光弧和亚暴极光不同,它们与地磁活动及Pi2脉动不相关.这7个极光弧的形态和IMF特征表明,极光弧的增亮很可能由尾瓣重联产生,很快衰减归因于IMFBz南向条件,而黄昏方向运动受IMFBy控制. 相似文献