Radio waves predict earthquakes, unveil secrets of ionosphere

Using very low-frequency radio waves, researchers have unravelled how solar flares affect and change the density of charged particles in the ionosphere^1,2. These radio waves carry signals that could be decoded to predict earthquakes, they say.

The researchers calculated the number density of electrons of each layer of ionosphere by reading the signals emitted by low-frequency radio waves that travelled all the way from India to Antarctica. Lead researcher Sandip Chakrabarti from Kolkata-based S. N. Bose National Centre for Basic Sciences says the radio-wave signals allowed them to measure how X-ray photons emanating from solar flares change the number density of electrons in the ionosphere. "This gives us insights into the chemical composition of the ionosphere", he told Nature India.

It is extremely difficult to study the properties of the D-region of ionosphere that lies in the upper atmosphere, between 60 and 90 km above the Earth's surface. This region is formed after sunrise and cannot be explored by sending balloons and satellites. The only way to study this region is by sending low frequency radio waves which when reflected back to earth suffer no attenuation.

To probe this specific region of ionosphere, the researchers installed radio signal receivers at Kolkata and the Indian station in the Antarctic, Maitri. These receivers were designed to receive radio waves from radio-wave transmitters in India, northwest Australia and Germany.

The researchers gathered about five weeks of radio-signal data from the transmitters in India, Australia and Germany. They deciphered the radio signals by connecting the receivers to the sound card of a computer loaded with special software.

At Maitri station, they detected three weeks of clear signals out of five weeks at 18.2 kilohertz. In these signals, they found evidence of the presence of the 24-hour solar radiation in the Antarctic region during local summer. A simultaneous observation was made in Kolkata, which is around 12000 km away from the Maitri station.

However, the signals from these two stations differed qualitatively. At Maitri station, the signals showed no signature of night-time fluctuations and the so-called night-time part in the diurnal variation. In Kolkata, the signals fluctuated during night with a sharp fall and rise of the signal amplitude following the local sunrise and sunset respectively. The sunrise and sunset terminator times are quite distinct and the signal shows the clean signature of appearance and disappearance of the D-region of the ionosphere.

By analysing these and other previously gathered radio signals, they found that the signal amplitude fluctuates anomalously three days before earthquakes^3,4. “From Antarctica, it is possible to detect many low-frequency radio signals from different countries simultaneously. The direction in which the anomaly in radio signals is the highest is likely to have earthquakes in a few days,” explains Chakrabarti. Statistical analysis has identified mainly three types of anomalies in the signals before earthquakes, showing that such anomalies peak in the direction which is closest to the epicentre, he says, adding that “it is possible to locate the epicentre provided we have a large number of receivers.”

The authors of this work are from: Indian Centre for Space Physics and S.N. Bose National Centre for Basic Sciences, Kolkata, India and University of Electro-Communication, Tokyo, Japan.