NWC通信台在电离层中激发电磁响应的时变特征

本文利用DEMETER卫星VLF频段电场和磁场频谱数据对DEMETER卫星运行期间2005年至2009年澳大利亚甚低频(Very Low Frequency)通信台NWC发射的通信信号造成的电离层电磁响应的日变化、季节变化及年变化特征进行了统计分析,统计结果表明电磁响应日变化显著,夜间电场强度明显增强可达40dB,磁场变化略小也可为15dB左右,而季节变化不显著,年变化主要受太阳活动的影响,太阳活动越强,电磁响应越小.为解释数据分析结果,对地-电离层电磁波传播过程采用传递矩阵方法进行了模拟计算,模拟结果与数据分析的结果一致.我们认为这种随时间变化的特点可能由250km以下电离层电子密度分布特征导致,因此研究250km以下的电离层电子密度变化可能对寻找地震电离层电磁异常有重要意义.

Temporal variations of electromagnetic responses in the ionosphere excited by the NWC communication station

The characteristic of electromagnetic field anomalies in the ionosphere excited by earthquake has been studied by many researchers using satellite observation data in recent years. This study explores what are the differences of the electromagnetic field anomalies with different ionospheric backgrounds with the help of the known VLF signal transmitted by communication station on the ground, which can be observed by satellite in the ionosphere.VLF electric and magnetic field spectral data from DEMETER satellite is used to analyze the diurnal, seasonal, and annual variations of ionospheric electromagnetic responses from 2005 to 2009, which were excited by the NWC communication station located in Australia. The Maxwell equation in the ionosphere is solved using the transfer matrix method to simulate the propagation characteristics of VLF electromagnetic waves excited by NWC transmitter. Compared with the observation result and simulation result, the electromagnetic response is obtained with different ionospheric backgrounds.The result shows the electric field at daytime is almost 40 dB larger than at nighttime, and the magnetic field is 15 dB. According to the data analysis result, it is found that seasonal and annual variations of the electromagnetic field is not very significant and solar activity has some effect on the response, which is smaller when the solar is more active. The simulation results are consistent with the data analysis. The variation of the response may be caused by the distribution of electron density below 250 km in the ionosphere. If the electron density is higher, the electromagnetic response will be lower.The variation of the electric field is much more obvious than the magnetic field, which means finding earthquake anomalies from electric data of satellite may be more efficient than the magnetic field. Getting precise electron profiles below 250 km will be very helpful for studying the electromagnetic anomalies in the ionosphere related with earthquakes.