The signals occur between 4 and 15 kHz, at the upper end of the VLF spectrum, and have been recorded for several hours a day in winter. They appear to be generated inside the Earth’s magnetosphere – but quite how they are generated, and how they propagate, is unknown.

"They were so unexpected," said Jyrki Manninen of Sodankylä Geophysical Observatory in Finland. "Their full explanation requires something of an academic breakthrough."

The natural VLF electromagnetic spectrum hosts a cacophony of signals. There are sudden signals known as atmospherics or "sferics", which resound thousands of kilometres beneath the ionosphere after every lightning strike; there is an unstructured hiss associated with the same charged solar particles that cause aurorae; and there are discrete signals known as chorus – named after the birdsong – that originate in instabilities in the magnetospheric plasma.

All these signals – as well as those in other frequency ranges – are interesting to scientists because they act as an interface between the distant interplanetary medium and the Earth’s magnetosphere. Solar energy incident on the magnetosphere influences many processes going on within the Earth’s atmosphere, from changes in the climate to the behaviour of honeybees.

Yet the VLF spectrum has not been wholly uncovered: sferics dominate most recordings at the higher frequencies in the range. Manninen and colleagues based at the Sodankylä observatory as well as other institutions in Finland, Russia and the UK were curious to know whether the sferics were hiding anything.

The researchers placed a sensitive battery-powered radio receiver dozens of kilometres away from civilization in Kannuslehto, northern Finland, and recorded emissions in the range 0.2–39 kHz over several winters. They filtered the emissions to remove communication signals, the 50 Hz hum from the electric grid and – using a newly designed digital filter – sferics.

To the researchers’ surprise, the remaining spectra contained bursts of signals. Each lasted from a fraction of a second to several minutes at frequencies above 4–6 kHz – even up to 15 kHz – and re-occurred for several hours. Previously, says Manninen, "VLF radio signals observed on the ground [were] considered to be generated near the equatorial plane of the magnetosphere only at frequencies below half the electron cyclotron frequency there – in other words, at frequencies less than 2.5 kHz for Kannuslehto."

The researchers believe that these "recently revealed emissions" (RREs) could originate close to the geomagnetic equator and begin to propagate downwards through plasma ducts, before escaping to higher latitudes. "Some ray-tracing calculations and other computations need to be done to investigate this suggestion further," said Manninen. "And it would be a good idea to investigate whether similar signals are observed aboard Earth-orbiting satellites when they cross [those] geomagnetic flux tubes that are near the ground-based receiving station."

For now, the precise nature of the new emissions remains a mystery. Manninen hopes that the observations will be repeated at other longitudes, and that models of their generation and propagation will be tested experimentally. "This may be possible in the next few years," he said.

The team published the study in Environmental Research Letters (ERL).

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