![]() The small aperture array determines the wave number vector (length and direction) of distant electromagnetic radiation sources, while a lightning detection network aims to determine lightning source locations inside, or in the proximity of, the network. Yet it was only recently suggested to investigate the context of these challenging observations with a novel analysis that uses a small aperture array. These observations thereby lend to the question what the spatial distribution of anthropogenic electromagnetic noise at a given location actually is.Īnother geophysical application for mapping electromagnetic noise sources in the sky is to identify areas with little or no electromagnetic noise to facilitate the detection and location of the weak natural electromagnetic emissions emanating from the growth and branching of streamers in sprite discharges above thunderclouds that have been predicted based on a theoretical analysis of transient luminous events. This conclusion was demonstrated by the efficiency of a Faraday cage to restore natural migratory behavior. For example, it was recently reported that European robins, Erithacus rubecula, have a receptor for anthropogenic electromagnetic noise that inhibits natural migratory behavior, i.e., well below the action limit for maximum permissible exposure. Answering this important question is a responsibility which falls squarely into the area of geophysical research in order to constraint speculation. However, global maps of electromagnetic noise sources do not provide any information on how electromagnetic noise sources would be perceived at a given location which is relevant for living beings on planet Earth. These low-frequency radio communication transmitters are widely used for geophysical exploration, for example, to study particle precipitation from the radiation belts into the Earth's atmosphere and the impact of lightning discharges on the upper atmosphere. For example, the electromagnetic radiation from transmitters for submarine communication and marine navigation escapes from the Earth ionosphere waveguide into near-Earth space in the Whistler mode. The locations of these electromagnetic noise sources are closely monitored and mapped around the world by the use of sensitive electromagnetic recordings on board of satellites. The observation of weaker natural electromagnetic signals is increasingly confined to remote areas because these signals are superimposed on disturbing electromagnetic noise sources from man made communication transmitters operating in relatively narrow frequency bands ranging from ∼100 Hz around ∼20 kHz to ∼10 kHz near ∼250 kHz, i.e., with a typical bandwidth of ∼10 % of the carrier frequency. The Earth's natural electromagnetic environment from ∼1 Hz to ∼300 MHz is dominated by broadband pulse sequences emanating from lightning discharges. The observed locations of the noise sources enable detailed observations of ionospheric modification, for example, caused by particle precipitation and lightning discharges, while the observed exclusion zones enable the detection of weak natural electromagnetic emissions, for example, from streamers in transient luminous events above thunderclouds. These results are obtained by a rigorous analysis of the residuals from the classic dispersion relation for electromagnetic waves using an array analysis of electric field measurements in the frequency range from ∼20 to 250 kHz. ![]() ![]() The sky also exhibits zones with little or no noise that are found toward the local zenith and the southwest of the array. It is found that the noise sources can be localized at elevation angles up to ∼60° in the sky, well above the horizon. The locations of electromagnetic noise sources are mapped on the hemisphere above a distributed array of wideband receivers that operate in a small aperture configuration. Here we report the first results from an electromagnetic noise survey of the sky. ![]() The Earth's natural electromagnetic environment is disturbed by anthropogenic electromagnetic noise. ![]()
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