电离层春秋分不对称的地方时依赖

Local time dependence of ionospheric equinoctial asymmetry

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.

In this paper,the critical frequency of the F2-layer（foF2）and the critical frequency over the E-layer（fo_E）of global 203 ionosonde stations and Jet Propulsion Laboratory（JPL）total electron content（TEC）map data were used to investigate the characteristics of ionospheric equinoctial（March Equinox and September Equinox,ME and SE）asymmetry.By fitting thethree parameters foF2,foE and TEC in terms of a function of Day of Year（DoY）and the solar activity index F10.7,the three parameters were normalized to the low solar activity level of F10.7=80,the moderate solar activity level of F10.7=150,and the high solar activity level of F10.7=200.Thus,by this method,the actually observed seasonal and solar activity variations of ionospheric parameters,which are interlaced with each other,can be quantitatively separated to acquire more accurate characteristics of ionospheric equinoctial asymmetry.The equinoctial asymmetry index（AI）and the equinoctial difference（Δ=ME-SE）of the three ionospheric parameters at different local times were analyzed to get the global features and the solar activity dependence of equinoctial asymmetry.In general,fo_E is mainly higher at SE than at ME over the global ionosonde stations at sunrise,and the equinoctial asymmetry of foE almost disappears in the afternoon,while foE is higher at ME than at SE at sunset.foF2 is higher at SE than at ME over the most of the global ionosonde stations at sunrise,while it is higher at ME than at SE at other local times.With respect to TEC,the equinoctial asymmetry of higher TEC at SE than at ME exists over the global at low solar activity level but mainly occurs at low latitudes at moderate and high solar activity levels at sunrise,while TEC is mainly higher at ME than at SE at other local times.The dependence of the equinoctial asymmetry of fo_E is weak,while for foF2 and TEC,solar activity dependence of the equinoctial asymmetry is evident.The equinoctial asymmetries of the oxygennitrogen ratio（[O]/[N2],computed from the atmospheric model NRLMSISE-00）at the peak height of the F2-layer（hmF2）and hmF2 were investigated;they can be used to partially explain the equinoctial asymmetry of foF2.The amplitude and the phase of TEC annual variation were calculated.The phase of TEC annual variation can be used to account for the global distribution of TEC equinoctial asymmetry.