Conjugate observations of power line harmonic radiation onboard DEMETER

The effects of power line harmonic radiation (PLHR) on near-Earth space have been debated for many years. Since some ultrahigh voltage AC and DC power projects were added to the Chinese power grid, increasingly numerous radiations at 50 Hz and their harmonics have been caught by the microsatellite DEMETER in the geomagnetically conjugate topside ionosphere during the daytime of winter and spring. Distribution of SYM-H index has been studied, showing geomagnetic disturbances may be unnecessary for the occurrence of PLHR. The propagation of PLHR between hemispheres without much attenuation illustrates that PLHR can be amplified when crosses the equatorial zone. Eliminating seasonal and diurnal factors, dependence of PLHR propagation on the change in plasma density is disclosed. The abrupt variation of electron density gradient may create a favored condition for PLHR waves propagating between hemispheres.