Sferics, tweeks and whistlers
Stories from Physics for 11-14 14-16
In addition to the radio waves produced by human beings’ transmitters, low frequency radio waves from a number of natural sources can be detected in the Earth’s atmosphere. Lightning strikes generate bursts of radio waves known as radio atmospherics, or sferics, which last only a few milliseconds but can travel for thousands of kilometres in the wave guide formed by the Earth and ionosphere.
Sferics typically have frequencies in the Very Low Frequency (VLF) range of 0.1-10 kHz. When sferics propagate over long distances, the different frequencies penetrate the ionosphere to different depths, disperse and arrive at different times, creating a sound similar to bird song. Hence waves in the 1-7 kHz range are called tweeks.
Radio waves from lightning strikes may escape the ionosphere and pass into the magnetosphere. Here, they interact with free electrons travelling along magnetic field lines to create VLF radio signals of decreasing frequency which are called whistlers. Samples of these different radio bursts can be heard at the NASA Space Weather website: spaceweather.com/glossary/inspire.html.