双频双波束加热电离层激发甚低频/极低频辐射理论分析

Theory of very low frequency/extra low frequency radiation by dual-beam beat wave heating ionosphere

基于高电离层质动力非线性加热理论,引入差频双波束概念,建立双频双波束加热电离层激发甚低频/极低频(VLF/ELF)辐射理论仿真模型,通过对已有实验参数进行仿真计算,验证了模型的正确性.据此模型,全面分析了不同纬度、有效辐射功率、加热频率、极化模式、频率差、实验时段等对激发VLF/ELF辐射强度的影响,并对比分析了中低纬度地区双频双波束和幅度调制两种方法激发VLF/ELF信号的差异.分析结果表明:VLF/ELF辐射效果随着地磁倾角的增加而增强,随着系统有效辐射功率的增大而增强;X波模式优于O波模式;实验时段冬季最好,夜晚优于白天;加热频率和频率差存在最优值选取问题.对于背景自然电流较弱的中低纬度地区,相对现有幅度调制方法,利用双频双波束方法激发VLF/ELF辐射更加有效,两者相差10 d B以上.

At high latitudes, the proposed dual-beam beat wave method has become the focus of research because of its relative amplitude modulation method. The very low frequency/extra low frequency（VLF/ELF） radiation intensity does not depend on the background of the current, and can be used as an importantparameter of VLF/ELF radiation in poor background conditions. As for the background of weak natural current in middle and low latitude regions,the amplitude modulation method stimulates the VLF/ELF radiation effect poorly, therefore, at the low latitudes, the relative amplitude modulation method, dual-beam beat waves method may be more effective. In this paper, according to the pondermotive nonlinear heating theory in the upper ionosphere, a simulation model is presented about VLF/ELF radiation generated by dual-beam heating of the ionosphere via beating waves. This model is validated by calculations and the available experimental parameters. Based on this model, a comprehensive analysis is performed, involving the dependences of radiation intensity on various parameters such as latitude, effective radiation power（ERP）, heating frequency, polarization, experimental times, and beating frequency difference. Then, we compare the dual-beam beat wave method with the amplitude modulation in stimulating VLF/ELF signals in the low and moderate latitude regions.Several conclusions are drawn as follows. First, the increases of geomagnetic declination and ERP of the heating facility may lead to a considerable improvement in radiation efficiency. Second, the X-mode polarization is more efficient for radiation than the O-mode polarization. Third, the most remarkable radiation effect may appear at winter night.The optimal heating frequency and the beating wave difference could be found under certain conditions. Stimulations of VLF/ELF radiation with the dual-beam beating wave method are more effectivethan the available amplitude modulation,the difference in VLF/ELF radiation intensity between the two methods is about 10 dB.