Lightning Sferics

Lightning sferics

 

Lightning is striking the Earth constantly. Each bolt generates powerful radio waves that traverse the atmosphere and travel around the globe. Here, radio energy is shown in a plot of frequency from 0.1 to 15 kHz (vertical) measured over the space of 15 seconds (horizontal). Intensity of signal is colour coded from blue (lowest) through cyan, green and yellow to red (highest). Notable are several curved shapes known as ‘whistlers’. These drop in frequency over a second or two, analogous to the falling whistling note commonly used to indicate a falling object. This is in fact due to the dispersion of the radio signal as it passes through the ionosphere before returning to Earth. The apparently random background signals are due to atmospheric effects, abbreviated to ‘sferics’.

Image © Drdan14 at English Wikipedia Commons

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