Research On Ionospheric Electron Density Inversion Based On Backscatter And Vertical Soundings

The ionosphere is the atmospheric ionization region nearest to the Earth's surface.It contains lots of free electrons and positive ions,which are important for the propagation of radio waves.Compared with neutral gases,charged particles in the ionosphere have a very small proportion,about 1%or smaller,but they can significantly change the electrical properties of the atmosphere,resulting in the current in the atmosphere carrying the current and affecting the propagation of radio waves from very low frequencies to microwave frequencies in which the high frequency wave propagation is significantly impacted.The ionospheric structure can be represented by the spatial distribution of electron density,ion density,electron temperature,and ion temperature in the ionosphere,while the most important is the spatial distribution of electron density.In this paper,the inversion and reconstruction of the ionospheric electron density is studied systematically.A robust inversion algorithm is developed to deal with vertical incidence ionograms,and an inversion algorithm based on backscatter ionograms is also established capable of acquiring the horizontally inhomogeneous structure of the ionosphere.The performance of these developed algorithms is further tested with experimental data.The major research contents and principal results are summarized as follows:1.The inversion algorithms of the vertical ionograms are studied,and the accuracy of the algorithms has been verified with experimental data.While the echo trace of incompletely developed F1 layer is common in vertical incidence ionograms,the existing ionospheric models and inversion algorithms generally deal with the completely developed F1 layer with the assumptions of a parabolic profile and an infinite slope at the peak of F1 layer,which however break down for the profile of incompletely developed F1 layer.An F1 layer electron density profile model based on the shifted Chebyshev polynomial is developed for the incompletely developed case of F1 layer by introducing a critical frequency parameter of F1 layer.By taking into account the profile smoothness,an inversion algorithm for electron density profile is proposed in terms of constrained optimization of F1 and F2 layer parameters.In addition,for overlapping polynomial inversion methods,the absence of large amounts of data,or data interpolation processing without ionospheric propagation characteristics can result in significant increase of errors in profile calculation or even incorrect results.To reduce the occurrence of such problems,an improved method for the profile inversion is presented with an emphasis on data preprocessing.For the two proposed inversion algorithms based on vertical ionograms,the validity of the models and algorithms are analyzed through simulations,and their efficiency is further verified in terms of comparisons between the synthesized vertical and oblique sounding traces and experimental data.2.To obtain the horizontally inhomogeneous structure of the ionosphere,a joint inversion algorithm is developed by using the leading edge of a backscatter ionogram measured through sounding in a given direction and of a vertical ionogram measured over the sounding station.A test platform is implemented as well to carry out the verification test.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 inversion equations.This algorithm can generate a stable and unique solution.By comparing the model results and observational data,the developed algorithm demonstrates reliable convergence,insensitivity to measurement errors,and higher accuracy for the inversion of ionospheric structures than the method of Fridman and Fridman(1994).The proposed algorithm can not only inverse horizontal changes in electron density under quiet conditions(at night or during the daytime at mid-latitudes)but also diagnose the horizontally inhomogeneous structures of the ionosphere during sunrise or sunset periods with high accuracy.Therefore,our developed method can be useful in inversion applications to treat complex and volatile backscatter ionograms.3.The joint inversion algorithm based on the leading edges from different layers on a backscatter ionogram is put forward,and the accuracy of the algorithm is examined by experiments.Traditional BSI inversion methods are unable to distinguish leading edges associated with different ionospheric layers,and simply utilize the minimum group path of each operating frequency,which generally corresponds to the leading edge associated with the F2 layer.Consequently,while the inversion results can provide accurate profiles of the F region below the F2 peak,the diagnostics may not be so effective for other ionospheric layers.In order to resolve this issue,we present a new BSI inversion method using leading edges associated with different layers,which can further improve the accuracy of electron density distribution,especially the profile of the ionospheric layers below the F2 region.The efficiency of the algorithm is evaluated by computing the mean and standard deviation of the differences between the model results and observational values from both vertical and oblique incidence sounding.Test results clearly manifest that our developed method can output more accurate electron density profiles due to improvements associated with the profiles of the layers below the F2 region.Our method can therefore act as an efficient improvement of the current BSI inversion methods on the reconstruction of 2-D electron density distribution in a vertical plane aligned with the sounding direction.