Numerical Study Of Electromagnetic Wave Propagation In Low-frequency Acoustic Disturbanced Troposphere

The effects of neutral atmosphere on radio wave propagation,are mainly the tropospheric effects with atmospheric density and meteorological processes complex changes.As an elastic longitudinal wave,acoustic waves is an important way of artificial disturbance atmosphere.Under the disturbance of acoustic wave with a certain frequency and sufficient intensity,the tropospheric atmospheric parameters such as density,pressure and temperature changes,so the tropospheric refractive significantly changed,forming local environment of artificial disturbance troposphere.The changes of atmospheric refraction index affect the propagation characteristics of radio wave,such as the radio wave refraction,reflection and scattering.The acoustic modulated atmosphere change the propagation behavior of radio wave,and having practical value,especially in military applications.The purpose of this paper is to study the radio wave propagation problems in acoustic wave perturbed troposphere by the numerical calculation method.The research contents are summarized as follows:(1)In the acoustic disturbances atmosphere,combining radio wave propagation theory in tropospheric disturbances medium,and using the stable and reliable time domain finite difference method(FDM),a coupling numerical model describing the electromagnetic wave propagation in acoustic disturbance medium is established,and the numerical format,boundary conditions and the time and space step of numerical model are given in details in this paper.Furthermore,the correctness of the model is verified through concrete examples.By using this numerical model,the tropospheric radio wave propagation behavior and its application in acoustic disturbance background are mainly studied.(2)For the influence of atmospheric wind and temperature on radio acoustic sounding system detection height,in the atmospheric temperature and wind field(obtained from the Wuhan MST radar)background,the propagation of a single frequency acoustic wave packet under different wind conditions is simulated,and the scattering propagation of radio wave packets corresponding to the acoustic scatterer are analyzed and compared.The numerical simulation results reveal that,the temperature profile mainly affects the propagation velocity of acoustic wave,while the presence of wind fields may result in a shift of acoustic wave fronts,reducing the strength and changing the trajectory of radio wave backscattering echo.The theoretical calculations with the acoustic wave scatterer at different heights under the same atmospheric wind field(strong wind)background demonstrate that,if the height of scattering point is reduced,the offset of the scattering echo "bunching point" at the same altitude will be greatly improved and the intensity will be enhanced,but it also means the decline of detection height.By comparing the numerical simulation results with measured data,it is verified that RASS detection height significantly decrease under the strong wind atmospheric background.(3)For the improvement of detecting height of RASS affected atmospheric wind,based on the numerical model describing the interaction of acoustic wave and radio wave,and in the horizontal wind(obtained from the Wuhan MST radar)and temperature(obtained from MSISE-00 model)background,the improvement situations of sounding height of RASS in windy atmosphere are simulated.The numerical simulation results suggest that,shifting radio emission source in the downwind direction and steering the radio beam pointing to appropriate direction is capable of compensating the migration of acoustic wave fronts effectively,reducing the horizontal offset and enhancing the intensity of scattering echoes.Thus,the detection height of RASS under the background of wind field is improved,and the comparation with the measured data also verify the numerical simulation results.(4)For the radio wave scatter propagation in acoustic disturbance background,by applying time domain finite difference method(FDM)for radio wave propagation in acoustic disturbance medium,a numerical model is introduced.The characteristics of radio wave scattering propagation in the artificial acoustic perturbations is investigated by this model.The numerical simulation results demonstrate that:(a)The radio wave scattered by acoustic scatterer has a characteristic of forward tropospheric scatter.When the radio waves are scattered,the scattered waves are in all directions,but the intensity distributions of radio wave are distinct in different directions;as the atmospheric acoustic perturbation is small amplitude perturbation,a majority of radio waves continues to propagate along the original direction,and only a small part of the energy is scattered.(b)For acoustic scatterer with the same scale at the same height,with the increasing of the acoustic intensity,the intensity of forward scattering radio wave increases,but the scale of scattered wave packets will not change significantly.For acoustic scatterer with the same amplitude intensity at the same height,with the increasing in the scales of acoustic wave packets,not only the intensity but also the scale of forward scattering wave packets are increased.(c)For the same acoustic scatterer,if merely changes the radio wave emission elevation,the horizontal spans of forward scattering radio wave packets centers at the same height,gradually decrease with increasing of emission elevations;and the energy of wave packet presents the trend that,increases firstly and then decreases with launching elevation,and will reach the maximum at a certain angle.If merely changes the wave emitting position,the horizontal spans decrease with increasing of emission positions,and the energy of wave packet also presents the trend that,increases firstly and then decreases with launch position,and will reach the maximum at a certain position.