利用高频天波返回散射反演电离层水平不均匀结构

高频天波返回散射探测作为重要的电离层探测手段，能够实现遥远区域电离层空间上的连续监测，探测获取的返回散射扫频电离图显示了探测频率-群路径-回波能量三者之间的关系.由于电离图包含了探测路径上的电离层状态信息，通过对其反演可以实时获取大面积范围的电离层参数.本文提出了一种基于解空间约束的返回散射前沿反演算法，能够重构电离层水平不均匀结构.针对反演非线性问题，采用Newton-Kontorovich方法进行求解，同时又引入了求解不适定问题的Tikhonov正则化方法，有益于解的稳定性和唯一性.利用模拟数据和实测数据分别对本文建立的算法进行了验证，并与Fridman和Fridman于1994年提出的反演方法进行了对比.结果表明，本文算法反演结果稳定，对返回散射前沿判读误差不敏感，与Fridman和Fridman 1994年方法相比，本文方法对电离层局部精细结构反演更加准确，具有较高的反演精度.本文提出的算法不但能够反演白天和夜间这种电离层较平稳时期的电离层状态，而且对于日出/日落时段等电子浓度分布变化较快情形下的电离层，也有很好的反演效果，表明了该算法在处理复杂多变的实际探测的返回散射电离图中的应用价值.

Reconstruction of horizontally-inhomogeneous ionospheric structure using HF sky-wave backscatter ionograms

High-frequency (HF) sky-wave backscatter sounding, as a powerful tool for detecting the ionosphere and studying the characteristics of radio channels, can be used to monitor the ionosphere continuously at a remote distance and to acquire the ionospheric parameters in a large area. Backscatter ionograms can be obtained showing the relationship between operating frequency, group path and echo amplitude. Since an ionogram carries the information of the ionospheric profile along the detection path, the ionospheric parameters can be evaluated through the inversion technique. An algorithm for backscatter sounding ionogram inversion is developed based on the restriction of solution space to reconstruct the horizontally inhomogeneous structures of the ionosphere. Furthermore, the Newton-Kontorovich method that generally treats nonlinear operator equations and the Tikhonov regularization method that generally deals with ill-posed problems are effectively combined to resolve the equations. The algorithm can get a stable and unique solution under the solution space limitation. In terms of comparisons between model results and observation data, the developed algorithm proves reliable convergence, insensitivity to measurement errors, and higher accuracy for the inversion of ionospheric structures than the method of Fridman and Fridman (1994). Our algorithm not only can inverse horizontal changes in electron density under quiet conditions (at night or during the daytime at mid-latitudes), but also can diagnose the horizontally inhomogeneous structures of the ionosphere during sunrise or sunset periods with high accuracy. This consequently demonstrates the application value of the proposed algorithm in the treatment of the complex and volatile measured backscatter ionograms.