Comparison of magnetospheric line radiation and power line harmonic radiation: A systematic survey using the DEMETER spacecraft

[1] Results of a systematic search for magnetospheric line radiation (MLR) observed by the DEMETER spacecraft since the beginning of the mission are presented. DEMETER is a French microsatellite (altitude of orbit about 700 km, inclination 98°) designed to study electromagnetic phenomena connected with seismic or man-made activity that has been launched in June 2004. An automatic identification procedure of possible MLR events has been used in order to analyze a large amount of measured data. It is shown that there are two principally different classes of events: (1) events with frequency spacing of 50/100 or 60/120 Hz (power line harmonic radiation, PLHR) and (2) events with a different frequency spacing. The first class of events is generated by power systems on the Earth's surface, with frequency spacing well corresponding to the fundamental frequency of the radiating power system. On the other hand, the second class is most probably generated in a completely natural way. All the detected events are thoroughly analyzed, and different properties of the two classes are statistically demonstrated. We have found that PLHR events occur both during low and high geomagnetic activity, with none of them significantly preferred. However, MLR events occur more frequently under disturbed conditions. Most of the PLHR events are observed at frequencies of 2 to 3 kHz. On the other hand, MLR events most frequently occur at frequencies below 2 kHz and seem to be more intense than PLHR. Additionally, PLHR events are more intense during the night than during the day, and there is about the same number of PLHR events observed during the day and during the night. On the contrary, no dependence of MLR peak intensities on magnetic local time was found, and more MLR events were observed during the day than during the night, although this difference is not statistically very significant. Finally, there is a group of MLR events with characteristics corresponding to the previous spacecraft observations of equatorial noise.