Observations of power line harmonic radiation by the low‐altitude AUREOL 3 satellite

During the past decade, most scientific satellites have been dedicated to the study of the auroral zones, but low‐altitude satellites also allow the study of the anthropogenic effects in the mid‐latitude regions. AUREOL 3 was such a low‐altitude polar‐orbiting satellite, with an apogee and perigee of 2000 and 400 km, respectively, which observed power line harmonic radiation (PLHR). The wave experiment named ARCAD 3 provided the waveforms of five components of the electromagnetic field (three magnetic and two electric) in a frequency range between O and 1.5 kHz. The telemetry was recorded at different stations. During the lifetime of the mission, 940 records, each lasting approximately 12 min, were processed in order to obtain spectrograms of the waveforms. On these spectrograms, five events with magnetospheric lines have been detected, which in all cases are separated by 50 Hz (with an accuracy of 1 %); lines are not at exact harmonics of the 50‐Hz power system, but drift in frequency with time. The drift, which is between 1 and 8 Hz/s, is not constant during an event. A common property of these five events is that they were recorded during periods of very low magnetic activity and were observed mainly in the night sector. Comparison of the characteristics of these PLHR with existing theories concerning the general problem of VLF triggered emissions and sideband stability of whistler mode waves, shows that the drift in frequency is not due to a Doppler effect, but most probably to a non linear interaction between electrons and coherent waves, occurring in the topside ionosphere.