基于张衡一号卫星波动观测的人工甚低频台站信号特征统计分析

地基人工甚低频(10~30 kHz)台站信号可以穿透电离层, 甚至泄露进地球磁层, 从而导致内辐射带高能电子的沉降.研究人工甚低频台站信号的特征对于研究辐射带电子的损失具有重要的科学意义.基于张衡一号卫星2019—2022年的电场观测数据, 本文对全球10个人工甚低频台站在电离层中的信号特征进行了研究, 统计分析了电离层中人工甚低频台站信号的昼夜差异、季变规律、对地磁活动水平的依赖性, 及其在地磁共轭区的分布特征.结果表明, 人工甚低频台站信号会传播至台站上空电离层及其共轭区, 信号辐射范围及强度与台站的发射功率呈正相关.电离层电子密度对甚低频信号的传播具有重要影响, 在夜侧和当地冬季, 电离层电子密度相对较小时, 泄露进入内磁层的人工甚低频台站信号较强, 而信号强度受地磁活动影响很弱.在台站上空, 信号增强区域以穿刺点为中心呈现近圆形分布, 并存在明显的波模干涉效应, 在共轭半球, 信号增强区域中心相对共轭穿刺点会发生极向漂移(L＜2的台站)或赤道向漂移(L＞2的台站), 但所有台站的电波能量都被限制在1/2赤道电子回旋频率磁壳以内的区域.这些统计观测特征表明, 发射台站位于较低L值(L＜1.4)的人工甚低频台站信号在内磁层中主要以非导管模式传播到共轭半球, 而发射台站位于较高L值(L＞2.6)的人工甚低频台站信号主要以导管模式传播.

Statistical study of the distribution properties of very-low-frequency transmitter signals based on CSES wave measurements

Very-Low-Frequency (VLF) signals from ground-based transmitters could penetrate through the ionosphere, and even leak into the Earth's magnetosphere, leading to the precipitation of inner radiation belt electron. Therefore, detailed information about the distribution characteristics of VLF transmitter signals in geo-space is of great importance for in-depth understanding of their driven radiation belt electron loss processes and consequences. Using the electric field measurements from the China Seismo-Electromagnetic Satellite (CSES) during the period from 2019 to 2022, we distinguish the VLF signals in the ionosphere emitted from 10 ground-based transmitters, and statistically investigate their diurnal differences, seasonal variations, geomagnetic activity dependence, and distribution characteristics at the geomagnetic conjugate region. The results indicate that, in terms of propagating into the ionosphere and its conjugate region, the VLF transmitter signals have their radiation radii and amplitudes positively correlated with the transmitted powers of the corresponding stations. The ionosphere electron density has a significant effect on the propagation of VLF transmitter signals. On the night-side and during the months of local winter, the ionosphere electron density is relatively smaller, leading to stronger VLF transmitter signals in the ionosphere. In contrast, the amplitudes of these signals are weakly affected by the level of geomagnetic activity. At the overhead region of the VLF transmitter stations, the radiation patterns show a nearly circular distribution centered on the puncture point, and there are obvious wave mode interference effect. However, at the conjugate region, the center of the enhancement area has the tendency of polar drift (for the stations with L < 2) or equatorial drift (for the stations with L > 2) relative to the conjugate puncture point. Furthermore, the VLF transmitter signals for all stations are very likely confined to the regions within the 1/2 equatorial electron cyclotron frequency of the specific magnetic shell. These features strongly support the hypothesis that VLF transmitter signals prefer to propagate to the conjugate hemisphere in the non-ducted mode when the transmitter is located at lower L shells (L < 1.4) but in the ducted mode at higher L shells (L > 2.6).