Statistical Study of Wave Properties of Power Line Harmonic Radiation at the Fundamental Frequency and Its Third Harmonic in the Topside Ionosphere

The wave properties of the power line harmonic radiation (PLHR) at the fundamental frequency 60 Hz and its third harmonic 180 Hz in the topside ionosphere are investigated statistically using the magnetic field data of China Seismo-Electromagnetic Satellite in the year of 2023. The spatial distributions of the wave properties including the polar angle, the azimuthal angle, and the polarization are obtained from the automatically identified events and their dependence on various factors are analyzed based on the statistical results. We find that the polar angles of PLHR at 60 Hz are generally larger at low magnetic latitudes (|MLAT| < 30°) than that at higher magnetic latitudes (|MLAT| > 30°). The large polar angles at high magnetic latitudes away from the high-occurrence region may be associated with the wave reflection of PLHR at 60/180 Hz. The wide longitudinal distribution of PLHR at 60/180 Hz with low occurrence rates outside of the high-occurrence region meridian planes may be explained by the azimuthal propagation of PLHR indicated by the large variation of the azimuthal angles. The latitudinal, diurnal and monthly variations of the polarization of PLHR at 60/180 Hz may be associated with the altitude of the local crossover frequency surface dependent on the ambient ion fraction and geomagnetic field strength. The occurrence rates of the left-handedly polarized PLHR are significantly higher at 180 Hz than that at 60 Hz, which may be due to the lower altitude of the crossover frequency surface at 180 Hz.