Abstract
Electromagnetic wave data measured by the low-altitude Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft sometimes exhibit intense emissions at discrete, harmonically spaced frequencies. We present a comprehensive analysis of this type of phenomenon, and we demonstrate that it is caused by very low frequency (VLF) transmitters in Europe.
All available data were investigated for a possible presence of the events. Altogether, 87 events were identified, occurring exclusively during the nighttime.
The occurrence of the events does not show any clear relation to the AE and Dst geomagnetic activity indices. The events are clearly localized, occurring either close to Great Britain or in the vicinity of its geomagnetically conjugated region.
The frequency spectra of the events typically consist of up to four spectral peaks at multiples of about 1.3kHz. Additional weaker spectral peaks at frequencies of about 1.9kHz and 3.2kHz are occasionally present.
The events observed in the Northern Hemisphere tend to have larger-frequency bandwidths than the events observed in the Southern Hemisphere. The occurrence of these events is correlated with the simultaneous detection of signals from VLF transmitters.
A bicoherence analysis is employed to demonstrate that wave-wave coupling takes place. Finally, it is shown that the occurrence of the events is associated with a significantly increased precipitation of energetic electrons in a wide range of energies.