Research On The Effects Of Nonlinear Modulation In The Ionosphere

Modern wireless communication,satellite positioning and navigation and satellite detection technologies all rely on the propagation of electromagnetic waves in the ionosphere.The nonlinear effects of ionospheric plasma can have a serious impact on the propagation of electromagnetic waves.Heating the ionosphere with high-power electromagnetic waves is an effective means to study the characteristics of the ionosphere.Influence of ionospheric heating and ionospheric heating on electromagnetic wave propagation,the main studies in this theme are as follows:Firstly,The theoretical analysis of the cross-modulation phenomenon in the ionosphere and the calculation method of cross-modulation degree is deduced and numerical simulation are carried out.The effects of parameters such as the modulation frequency,modulation coefficient,power,and frequency of the amplitude modulated electromagnetic wave on the degree cross-modulation are analyzed.It is found that the cross modulation phenomenon is most likely to occur when the power of the electromagnetic wave is large,the modulation frequency and the frequency of the wave are small.Then,based on the ohmic heating theory of the ionosphere,a self-consistent heating model of the low ionosphere is established according to the electron energy equation and the electron continuity equation,and the model is combined with the empirical formula of the absorption and loss of electron energy.The background ionospheric parameters were introduced from the international ionospheric reference model IRI-2016 and NRLMSISE-00 atmospheric model.Under different heating time and heating power,the ionospheric heating was simulated and the effects of ionospheric heating on electron temperature and electron density at different heights of the ionosphere were analyzed.The absorption of electromagnetic waves by the ionosphere is weakened with increasing frequency,and the ionospheric peak height that causes electromagnetic wave absorption is shifted downward,but the heating effect is still mainly in the range of 70-100 km.Finally,the VLF electromagnetic wave propagation model based on FDTD is established in the spherical coordinate system,and UPML is used as the absorption boundary condition.Take the ionospheric parameters under natural conditions and heated by high-power electromagnetic waves as initial conditions respectively to simulate the propagation process of electromagnetic waves in the ionosphere.Comparing the phase change and propagation attenuation of electromagnetic waves in two cases,it is found that the ionosphere after heating has a greater influence on the phase of the electromagnetic wave.In addition,due to the different trends of the absorption rate of electromagnetic waves at different heights after heating the ionosphere,the absorption of electromagnetic waves at low heights is reduced,the absorption at high places is increased,and the change with time is also more obvious.It shows that the high-power electromagnetic wave heating ionosphere will have different effects on the propagation characteristics of electromagnetic waves of different heights in the ionosphere.As the heating progresses,the collision frequency and electron density increase successively,and the phase and attenuation of the VLF wave also show different variations at different heights.