Power line harmonic radiation (PLHR) and magnetospheric line radiation (MLR) are recognized as electromagnetic pollutants in the near-earth space, and their correlation has been a controversial issue. An identification procedure based on short-time discrete Fourier transform and Welch power spectrum estimation was applied to detect 114 PLHR events and 328 MLR events that occurred as DEMETER satellite flew over China from 2008 to 2010. PLHR events feature parallel horizontal lines with a frequency interval of 50/100 Hz in spectrogram, whereas MLR events feature multi-parallel spectral lines with frequency drift as a baseline for possible triggered emissions as well as triggered emissions with no baseline. We statistically compared the temporal and geographical distribution, Kp and Dst indices, and propagation characteristics between PLHR and MLR events. PLHR events showed more obvious variations annually than MLR events, and the former had a close relationship with the development of Chinese power system. Both PLHR and MLR presented similar diurnal and seasonal differences, owing likely to the ionospheric state; moreover, they appeared to have no significant connection with geomagnetic activity level. The geographical distributions of PLHR and MLR differed significantly. PLHR events occurred in the range of (20◦ N–۴۵◦ N), whereas MLR events were concentrated in southwestern low-latitude regions of (18◦ N–۲۸◦N). Furthermore, PLHR waves were right-hand circularly polarized, whereas MLR waves were approximately linearly polarized. In summary, MLR is more likely caused by natural radiation that is not directly related to PLHR, whereas PLHR is closely related to terrestrial power systems.

Introduction

China has developed an ultra-high-voltage (UHV) power system characterized by an alternating current (AC) grid of 1000 kV and a direct current (DC) grid of ±۸۰۰ kV. Its power consumption ranks first in the world [1]. Several new environmental problems such as electromagnetic interference caused by power systems of earthquake-monitoring stations, navigation systems of unmanned aerial vehicles (UAVs,) and near-Earth space has raised public concerns. Since the 1970s, events have been observed from two pairs of geomagnetically conjugate ground-based stations: Siple station in Antarctica and Roberval station in Quebec and Halley station in Antarctica and St. Anthony station in Newfoundland [2]–[۴]. These events have shown that the timefrequency spectrogram of the electric field strength has several parallel horizontal spectral lines separated by 50 Hz/100 Hz or 60 Hz/120 Hz, corresponding to the working frequency of the local terrestrial power system. Similar events have also been observed from satellites including ARIEL-3, OGO-3, AUREOL-3, DEMETER, C/NOFS, and Chibis-M [5]–[۱۰]. They are referred to as power line harmonic radiation (PLHR) and are generally considered as pollutants to the near-Earth space resulting from the power system [11].