First Map of Coherent Low-Frequency Continuum Radiation in the Sky

Abstract

Lightning discharges and radio transmitters emit low‐frequency (∼3–300 kHz) electromagnetic waves with large electric field strengths and stable phases. This phase stability makes it possible to map the source locations of lightning and transmitters in the sky. Electromagnetic waves with smaller electric field strengths generally exhibit a reduced phase stability, caused by numerous simultaneous physical processes that blend into an underlying continuum radiation trapped inside the Earth‐ionosphere cavity. It is therefore currently not known whether the source locations of continuum radiation can be determined. Here we show the first map of coherent continuum radiation in the sky above an array of high‐precision radio receivers. The source locations of the coherent continuum radiation are found at elevation angles ∼30∘−60∘ above the horizon. The identified source locations are attributed to intermittent radio transmitters that emit electromagnetic waves with electric field strengths ∼2 orders of magnitude below the instrumental noise floor. The results demonstrate that it is possible to simultaneously map the signals from coherent continuum radiation, lightning discharges, and radio transmitters in the sky. This work thereby lays the foundation toward the discovery of many more coherent source locations of low‐frequency electromagnetic waves in the sky. It is expected that the identified source locations vary with time as a result of the impact of solar variability on the D ‐region ionosphere. Future studies have therefore the potential to contribute to a novel remote sensing and an improved understanding of the D ‐region ionosphere, influenced by the near‐Earth space environment.