Detection of low-frequency continuum radiation

The electromagnetic spectrum at low frequencies from ∼3 to 300 kHz is dominated by impulses from lightning discharges and anthropogenic radio transmissions used for communication. Electromagnetic waves generated in near-Earth space exhibit generally smaller amplitudes that are attenuated when travelling through the ionosphere before they can be observed at high and midlatitudes. Electromagnetic waves with yet smaller amplitudes contribute to the overall electromagnetic energy trapped within the Earth-ionosphere cavity. At this point, the electromagnetic waves from all possible sources blend into an unstructured continuum radiation near the instrumental noise floor, which is often considered to be a fundamental limit to scientific discovery. As a result, the sources of continuum radiation have been little studied and are essentially unknown. Here we show how low-frequency continuum radiation is detected and discriminated against known radio sources and instrumental noise by use of rigorous criteria inferred from novel precision measurements with an array of radio receivers. In particular, it is found that coherent continuum radiation from intermittent radio transmitters exhibits electric field strengths 0.5–0.7 µV/m, which are almost 2 orders of magnitude smaller when compared to the noise floor of the radio receivers ∼25 µV/m. Another part of the continuum radiation is found at local zenith above the array when it is caused by random instrumental noise. The results exemplify the possibility to extract sources from continuum radiation to study their origin and physical properties, which can contribute to an improved understanding of the impact of space weather and solar variability on the Earth's upper atmosphere.