The study on the distribution properties of NWC transmitter signals based on CSES observation

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. Based on data from DEMETER, CSES and Van-Allen Probes, the VLF signals emitted from NWC transmitter located in Australia, were analyzed firstly to validate CSES data. The we distinguished the NWC signals in the ionosphere and statistically investigate the day-night asymmetry, geographic distributions, seasonal and geomagnetic activity dependence, and wave propagation features, using the electric field measurements from CSES during the period from 2019 to 2022. The results indicated that, on the night-side and during the months of local winter, VLF transmitter signals are stronger due to the smaller ionosphere electron density. In contrast, the amplitudes of these signals are weakly affected by the level of geomagnetic activity. The distribution properties of NWC signals at the conjugate region, showed that the signals propagate to the conjugate hemisphere both in the non-ducted mode and ducted mode.