电离层电场的半年变化对F2区峰值电子浓度的影响

利用一个电离层理论模式,模拟了太阳活动低年、地磁宁静情况下,中低纬和赤道地区电离层F2区峰值电子浓度(NmF2)的半年变化规律,重点讨论了电离层电场对NmF2半年变化的影响.模拟结果表明,当输入的电场没有周年和半年变化时,磁赤道地区电离层NmF2本身就具有一定的半年变化特征,而在稍高的纬度上,NmF2半年变化的强度较弱.当输入的电场具有一定的半年变化时,电离层NmF2的半年变化强度有明显的改变,且这种改变随地方时和地磁纬度不同有明显的差别.在地磁赤道附近的电离层赤道槽地区,从上午到午夜的时间内,具有半年变化的电场对电离层NmF2半年变化的强度是减弱的作用,在其他的时间内,电场对电离层NmF2半年变化强度是加强的作用.而在稍高纬度的电离层驼峰地区,情况明显不同.从上午一直到翌日日出前,具有半年变化的电场对电离层NmF2半年变化的幅度都是加强的作用.在其他的时间内,电场对电离层NmF2半年变化的幅度是减弱的作用.同时,研究表明电离层电场对NmF2半年变化的作用和“赤道喷泉”现象强烈相关.     

2006年4月13～17日西太平洋地区电离层暴时特性研究

本文利用西太平洋地区的15个电离层台站的测高仪数据和国际GPS服务中心IGS 36个站提供的TEC数据，以及由美国喷气推进动力学实验室提供的Jason 1 TEC数据对2006年4月13～17日间一次由冕洞高速流所引发的磁暴所造成的电离层效应进行了分析.分析结果表明这次电离层暴呈现出显著的纬度效应，foF2和TEC等参量显示在磁暴主相期间对称分布的强正暴效应中心在磁纬±30°～±40°，且持续时间超过12 h.负暴效应被限制在中高纬地区，在磁暴进入恢复相时，开始向低纬渗透，且具有明显的地方时效应.TIMIED卫星测量的Σ［O/N2］显示磁暴发生后，暴时环流使得中低纬地区的Σ［O/N2］有大幅增加，而中高纬地区则显著下降.通过对hmF2的分析发现磁暴主相期间，有磁层电场向中低纬地区穿透，且持续时间较长为1～3 h.因此这次强正暴效应可能是由风场、电场和化学成分这三个因素的共同作用造成的.这次磁暴造成的电离层暴响应非常复杂，对造成各种正负暴的物理和化学机制还需要进一步的研究.     

基于COSMIC掩星数据分析全球NmF2周年和半年变化特征

使用COSMIC掩星提供的NmF2数据,利用傅里叶分析方法,研究全球F2层峰值电子密度的周年和半年分布特征,分析2010年LT12：00 14：00 NmF2周年和半年变化幅度及2008-2011年年平均值变化.结果显示,电离层NmF2周年和半年变化幅度在中高纬地区相对较大;在赤道和低纬地区相对较小,且NmF2以半年变化为主.太阳活动增强期间,NmF2年平均值增大.     

南极中山站电离层F2层临界频率变化特征

对南极中山站数字式电离层测高仪1995～2002年观测数据的月中值进行了统计分析,揭示了中山站电离层F2层临界频率（foF2）的主要特征：foF2存在明显的日变化和年变化;日变化存在“磁中午异常”现象;年变化中中午foF2在太阳活动低年不出现“冬季异常”,在太阳活动高年出现“半年异常”,即两分点高于两至点.本文结合中山站所处的地理位置,考虑太阳辐射电离、磁层的驱动和中性大气成分变化等因素,分析了这些现象的产生机理.     

GIM和IRI2012模式在中国地区的时空变化和扰动分析

在合成孔径雷达(SAR,Synthetic Aperture Radar)定位中,电离层延迟是不可忽略的误差源之一,需要选择合适的电离层模型进行校正.为了验证IRI2012模式和GIM模式在中国不同纬度地区的适用性以及中国地区电离层扰动与太阳耀斑、地磁场等因素的相关性,本文利用IRI2012模式计算的TEC预测值,并结合GIM模式给出的TEC观测值对2014太阳活动低年的中国不同纬度地区TEC时空变化特性进行了分析,包括周日变化、半年变化和年变化等.此外,由于电离层扰动引起的电离层延迟对星载低频SAR定位精度影响比较严重,以IRI2012模式为背景电离层,根据电离层平静期的GIM-TEC观测值确定电离层TEC扰动指数的阈值,在太阳耀斑和地磁异常发生时统计前后一定时间的电离层TEC扰动指数.实验结果表明:IRI2012模式和GIM模式在中国低纬和中高纬度地区的空间和时间变化均符合一般规律,在中国地区具有适用性;电离层扰动指数连续5小时超过阈值0.5判定为电离层扰动事件,当中国地区位于向阳面或处于日冕物质抛射区时,其上空电离层扰动与太阳磁暴和地磁暴紧密相关.     

通量管积分瑞利-泰勒不稳定性的半球不对称和随经度变化的研究

本文利用通量管积分方法,对磁南北半球分别沿磁力线积分,研究背景电离层南-北半球不对称以及中性风场和磁偏角随经度的变化对广义瑞利-泰勒不稳定性和电离层不规则结构生成和发展的影响.结果表明,通量管积分广义瑞利-泰勒不稳定性线性增长率存在显著的半球不对称,南北半球不对称的中性风场是导致电离层不规则结构呈南北分布不对称的重要因素;随经度变化的中性风场和磁偏角对瑞利-泰勒不稳定性的经度变化有重要影响,它们可能是导致不规则结构出现率经度变化的主要控制因素.     

电离层春秋分不对称的地方时依赖

利用全球203个电离层测高仪台站的F_2层临界频率(f_oF_2)和E层临界频率(f_oE),以及美国喷气推进实验室(JPL)提供的电离层总电子含量(TEC)地图数据统计分析了电离层春秋分(March Equinox and September Equinox,ME and SE)不对称的特点.基于电离层参量随年积日(Day of Year,DoY)和太阳活动指数F_(10.7)变化的傅里叶级数模型,对f_oF_2、f_oE及TEC数据分别进行最小二乘法拟合,将电离层参量归算到低太阳活动(F_(10.7)=80)、中等太阳活动(F_(10.7)=150)和高太阳活动(F_(10.7)=200)水平.该方法定量分离了实际观测数据中包含的电离层参量随季节和太阳活动的变化,因而得到了更为定量、精确的电离层春秋分不对称性特征.分析了不同地方时(LT)的春秋分不对称性指数(Asymmetry Index,AI)和春秋分差值Δ(=ME-SE)的全球分布特征与太阳活动依赖性.结果表明,foE日出时全球主要表现为9月分点值高于3月分点值,午后春秋分不对称性几乎消失,而日落时则反转为3月分点值高于9月分点值;f_oF_2日出时除少数地区外也主要表现为9月分点值高于3月分点值,而其他时段则相反;TEC日出时低太阳活动时的全球及中高太阳活动时的低纬地区表现为9月分点值高于3月分点值,而其他时段则相反.fo_E春秋分不对称性受太阳活动影响较弱,而f_oF_2和TEC的春秋分不对称随太阳活动有明显的变化,其3月分点值相对于9月分点值增加.计算了F_2层峰高(h_mF_2)处对应的氧氮浓度比([O]/[N_2],由大气模型NRLMSISE-00计算得到)和h_mF_2的春秋分不对称性,提取了TEC年变化的幅度及相位信息.氧氮浓度比和h_mF_2的春秋分不对称性能够部分解释电离层的春秋分不对称性,而TEC春秋分不对称的全球分布特征可以用TEC年变化的相位的全球分布解释.     

火星电离层电子密度对太阳辐射变化的响应

太阳辐射是火星电离层变化的重要控制因素.利用火星全球勘探者号(Mars Global Surveyor,MGS)电离层掩星探测数据,并结合一个火星电离层总电子含量(Total Electron Content,TEC)经验模型,研究了火星北半球高纬地区电离层电子密度对太阳辐射变化的响应特性.在考虑了火星掩星数据中电离层...     

利用半参数核估计法预报全球电离层总电子含量

本文将半参数平差模型引入电离层球谐函数系数的预报中,建立了半参数球谐函数模型(Semiparametric-Spherical Harmonic,Semi-SH)来预测全球电离层总电子含量.首先,通过快速傅里叶变换获得球谐函数系数的周期和振幅,将振幅高的主周期归入趋势函数,振幅低的剩余周期归入随机信号,建立了半参数模型,同时利用核估计方法拟合趋势函数,解算随机信号,并在时间域上进行外推,得到了预报时间的球谐函数系数,代入15阶电离层球谐函数模型,最后得出电离层总电子含量(Total Electron Content,TEC)的预报值.本文基于欧洲定轨中心(CODE)发布的球谐函数系数进行电离层TEC长期预报和短期预报分析,其中长期预报采用四年预报两年的模式对球谐函数系数进行预报,短期预报设计了三个算例,采用前30天预报后一天的模式,分别预报1天、滑动预报7天和滑动预报30天.实验结果表明:长期预报能够较好地反映全球电离层TEC的变化趋势和波动情况,Semi-SH模型对全球电离层TEC平均值(Mean TEC global,MTECglobal)的拟合值和预报值与MTECglobal实际值的相关系数分别为0.8743和0.8010,呈现出高度相关性.短期预报中,在太阳活动高年和太阳活动低年,Semi-SH模型在中纬度地区预报精度较CODE发布的电离层TEC 1天预报产品(CODE′S 1-Day Predicted GIM,C1PG)有较大提升,在高纬度与低纬度地区两种模型预报精度相当;Semi-SH模型在太阳活动高年和太阳活动低年30天滑动预报精度的均值均高于C1PG模型.实验结果说明了Semi-SH模型预报电离层TEC值的有效性.     

高纬电离层气候学特征研究--EISCAT雷达观测及与IRI模式的比较

利用1988～1999年欧洲非相干散射EISCAT（European Incoherent Scatter）雷达观测数据,对不同太阳活动周相、不同季节的极光椭圆区电离层F区电子密度进行统计分析,研究其气候学特征,并与IRI 2001模式比较.EISCAT观测到的电子密度显示出显著的太阳活动高年“冬季异常”和太阳活动低年半年变化等现象.EISCAT实测电子密度随时间和高度的平均二维分布和500 km高度以下总电子含量TEC,从总体来看与IRI 2001模式预测结果符合较好.但高年在TEC达到最大值前后,IRI 2001模式预测的电子密度高度剖面与EISCAT观测结果有显著差别：F2峰以上IRI 2001模式预测的电子密度过大,造成TEC明显高于雷达观测值.另外,在太阳活动下降相,EISCAT观测显示出明显的半年周期季节变化特征,但IRI 2001模式未能预测出此下降相季节变化.     

120°E赤道电离异常区电子浓度总含量分析与预测

基于国际全球定位服务中心(International GPS Service,IGS)提供的120°E 上空1999-2009年IONEX格式电离层电子浓度总含量(TEC)资料,分析赤道电离异常驼峰区TEC峰值Inc和Isc的年变化和季节变化以及与太阳、地磁活动的相关性.在11年时间尺度上,Inc和Isc与太阳辐射P指数的日均值有较好的相关性(r=0.90和r=0.84),而与地磁活动指数Dst、Kp和Ap日均值的相关性均不好.驼峰区TEC峰值Inc和Isc都是在北半球春、秋季出现极大值,而且冬季值大于夏季值,即Inc呈现"半年异常"和"冬季异常"现象,我们认为Inc和Isc相似的半年变化特征是与赤道上空电离层电急流相关的东向电场半年变化导致的.利用支持向量回归方法构建了EIA指数的预报模型,预报试验结果表明,该预报模型能较准确地描述Inc和Isc的变化,对南北驼峰TEC峰值预报的平均相对误差分别为22.96%和10.2%.基于支持向量机回归的预测方法为赤道电离异常特征指数预报的实现提供一条有效方法途径和好的应用前景.     

Characterization of low latitude GPS-TEC during very low solar activity phase

We present the mean diurnal, seasonal and annual variations in TEC during the lowest solar activity phase from low latitude Indian zone recorded at Udaipur (Geog. Lat. 24.6°N, Geog. Long.73.7°E, Geomag. Lat. 15.6°N) using a GPS receiver. Seasonal variations in daytime TEC show a semiannual periodicity, with a minimum in winter. Results of seasonal variations have been compared with that of the IRI-2007 model. Model calculations reveal significant seasonal as well as longitudinal differences in TEC. Seasonal variations in the nighttime TEC reveal an annual periodicity. Near the crest of the EIA, TEC shows a very good correlation with the solar flux. The results also point to weakening of the anomaly crest as well as its spatial and temporal contraction with declining solar activity.     

Seasonal variation of the total electron content, maximum electron density and equivalent slab thickness at a South-American station

Total electron content (TEC) and foF2 ionosonde data obtained at Tucumán (26.9°S; 65.4°W) from April 1982 to March 1983 (high solar activity period) are analyzed to show the seasonal variation of TEC, NmF2 (proportional to square of foF2) and the equivalent slab thickness EST. Bimonthly averages of the monthly median for January–February, April–May, July–August and October–November have been considered to represent summer, autumn, winter and spring seasons, respectively. The results show that the higher values of TEC and maximum electron density of F2-layer NmF2 are observed during the equinoxes (semiannual anomaly). During daytime, both in TEC and in NmF2 the seasonal or winter anomaly can be seen. At nighttime, this effect is not observed. Also, the observed NmF2 values are used to check the validity of International Reference Ionosphere (IRI) to predict the seasonal variability of this parameter. In general, it is found that averaged monthly medians (obtained with the IRI model) overestimate averaged monthly median data for some hours of the day and underestimate for the other hours.     

The features and a possible mechanism of semiannual variation in the peak electron density of the low latitude F2 layer

Ionospheric data observed in 30 stations located in 3 longitude sectors (East Asia/Australia Sector, Europe/Africa Sector and America/East Pacific Ocean Sector) during 1974–1986 are used to analyse the characteristics of semiannual variation in the peak electron density of F2 layer (NmF2). The results indicate that the semiannual variation of NmF2 mainly presents in daytime. In nighttime, except in the region of geomagnetic equator between the two crests of ionospheric equatorial anomaly, NmF2 has no obvious semiannual variation. In the high latitude region, only in solar maxima years and in daytime, there are obvious semiannual variations of NmF2. The amplitude distribution of the semiannual variation of daytime NmF2 with latitude has a “double-humped structure”, which is very similar to the ionospheric equatorial anomaly. There is asymmetry between the Southern and the Northern Hemispheres of the profile of the amplitude of semiannual variation of NmF2 and longitudinal difference. A new possible mechanism of semiannual variation of NmF2 is put forward in this paper. The semiannual variation of the diurnal tide in the lower thermosphere induces the semiannual variation of the amplitude of the equatorial electrojet. This causes the semiannual variation of the amplitude of ionospheric equatorial anomaly through fountain effect. This process induces the semiannual variation of the low latitude NmF2.     

Global scale annual and semi-annual variations of daytime NmF2 in the high solar activity years

The annual and semi-annual variations of the ionosphere are investigated in the present paper by using the daytime F2 layer peak electron concentration (NmF2) observed at a global ionosonde network with 104 stations. The main features are outlined as follows. (1) The annual variations are most pronounced at magnetic latitudes of 40–60° in both hemispheres, and usually manifest as winter anomalies; Below magnetic latitude of 40° as well as in the tropical region they are much weaker and winter anomalies that are not obvious. (2) The semi-annual variations, which are usually peak in March or April in most regions, are generally weak in the near-pole regions and strong in the far-pole regions of both hemispheres. (3) Compared with their annual components, the semi-annual variations in the tropical region are more significant.In order to explain the above results, we particularly analyze the global atomic/molecular ratio of [O/N2] at the F2 layer peak height by the MSIS90 model. The results show that the annual variation of [O/N2] is closely related with that of NmF2 prevailing in mid-latitudes and [O/N2] annual variation usually may lead to the winter anomalies of NmF2 occurring in the near-pole region. Moreover, NmF2 semi-annual variations appearing in the tropical region also have a close relationship with the variation of [O/N2]. On the other hand, the semi-annual variations of NmF2 in the far-pole region cannot be simply explained by that of [O/N2], but the variation of the solar zenith angle may also have a significant contribution.     

电离层电子总量水平梯度和相关系数

电离层电子总量水平梯度和相关系数是进行越境越障空间物理研究时的基本参量。本文分析了当同步信标卫星ATS-6停泊于东非地区上空时,在西欧观测到的太阳宁静年条件下电离层电子含量水平分布和空间相关性。指出纬向梯度、经向梯度、全梯度矢量的昼夜变化与季节特性。文中提出并论述中纬度TEC水平梯度矢量的指日效应:在北半球,冬季时梯度矢量指向的昼间进程是东南-南-西南,方位角变化小,幅值与幅值变化亦小;夏季时梯度矢量指向的昼间进程是东-南-西,方位角变化大,幅值与幅值变化亦大。梯度矢量箭头具有判然的指日性,即追随太阳方位角而变。梯度矢量幅值,在非低纬区条件下,随太阳仰角的增大而增大。文中还述及电子总量空间相关系数的表现规律并探讨了产生中纬度电离层这种时空行为的可能的动力学原因。     

Ionospheric slab thickness and its seasonal variations observed by GPS

The ionospheric slab thickness, the ratio of the total electron content (TEC) to the F2-layer peak electron density (NmF2), is closely related to the shape of the ionospheric electron density profile Ne (h) and the TEC. Therefore, the ionospheric slab thickness is a significant parameter representative of the ionosphere. In this paper, the continuous GPS observations in South Korea are firstly used to study the equivalent slab thickness (EST) and its seasonal variability. The averaged diurnal medians of December–January–February (DJF), March–April–May (MAM), June–July–August (JJA) and September–October–November (SON) in 2003 have been considered to represent the winter, spring, summer and autumn seasons, respectively. The results show that the systematic diurnal changes of TEC, NmF2 and EST significantly appeared in each season and the higher values of TEC and NmF2 are observed during the equinoxes (semiannual anomaly) as well as in the mid-daytime of each season. The EST is significantly smaller in winter than in summer, but with a consistent variation pattern. During 14–16 LT in daytime, the larger EST values are observed in spring and autumn, while the smaller ones are in summer and winter. The peaks of EST diurnal variation are around 10–18 LT which are probably caused by the action of the thermospheric wind and the plasmapheric flow into the F2-region.