电离层垂直TEC映射函数的实验观测与统计特性

利用GPS信标测量获得的电离层电子浓度总含量（TEC）是沿电波路径的斜向TEC.理论研究和实际应用中，常常需要通过映射函数将斜向TEC转换为垂直方向的TEC，这在当前主要采用对电子浓度分布模型的数值积分得到模型映射函数来实现.本文在考察现有不同模型映射函数的基础上，又提出了一种源于实际观测的实验映射函数的概念与估算方法.我们利用IGS的全球GPS观测站的斜向TEC和JPL提供的垂直TEC数据获得了2006年期间的实验映射函数，并对所得结果进行了初步统计分析.在卫星天顶角较小时，上述实验映射函数和模型映射函数之间相差甚微，均可很好描述垂直TEC与斜TEC之间关系；但卫星天顶角较大时，实验映射函数和常用的模型映射函数之间存在明显差异.本文认为，这种差异主要是因为现有模型映射函数中没有考虑到等离子体层的贡献.我们认为采用基于实验映射函数的模式，或者通过考虑等离子体层的贡献对现有模型映射函数进行改进，可以有效提高电离层TEC的估算精度.

Experimental observation and statistical analysis of the vertical TEC mapping function

The TEC data obtained from GPS is actually the slant TEC, i.e., the integration of electronic density along the GPS signal path. In many applications the slant TEC should be converted into vertical TEC. This is done by the so called mapping function which is usually determined by certain models of electron density profile. In this paper we proposed a new method to obtain mapping functions from TEC observation of GPS network. We first discuss the Single Layer Model (SLM), or the peak electron density height for the Chapman Model, and found that the value of the mapping function decreases obviously with the increase of the ionospheric height. We then estimated the'experimental mapping function' from the observation of vertical TEC (from JPL GIMs) and slant TEC (from IGS GPS data) during the whole year of 2006. After the comparison between both results, we find the values of the mapping functions from experimental data are much larger than that from the model with reasonable ionospheric height (e.g., 400 km SLM height), when the zenith angle is large enough. We attribute this to the effect of the plasmasphere which exists above about 1000 km altitude hence may increase the effective 'ionospheric height'. It is concluded that the present experimental results may be used to model or improve the vertical TEC mapping function. This might be helpful for retrieving TEC from GPS network.