The majority of Earth's natural radio emissions occur in the extremely low frequency and very low frequency (ELF/VLF) radio spectrum, frequencies between approximately 100 to 10,000 cycles per second (0.1 - 10 kHz). Natural radio waves are vibrations of electric and magnetic energy (radio waves) which, though occurring at the same frequencies as sound, require a radio receiver to make them audible.
Global lightning strikes number around 100 times per second and are the source of most natural VLF emissions on Earth. Lightning strokes emit a broadband pulse of radio waves at all electromagnetic frequencies simultaneously, from DC to Light. This constant crackling (static) is known as 'sferics', an abbreviation of 'atmospherics'.
VLF crackles can be detected from storms thousands of kilometers away. Radio waves can propagate such distances by bouncing back and forth between our planet's surface and the ionosphere, a layer of the atmosphere ionized by solar ultraviolet radiation.
Sferics that travel far enough through this reflector become 'tweeks', a musical ricochet sound. This is due to delay dispersion within the atmosphere. The frequency where half a wavelength will fit between our planet's surface and the bottom of the ionosphere is around 3 kHz. The closer a wave is to this cutoff, the slower it travels. High frequency components reach the receiver before the low frequencies resulting in a tweek.
Some lightning pulses follow magnetic field lines to the magnetosphere, 10,000 km or more above Earth's surface, and return more greatly dispersed. 'Whistlers' are heard as descending tones. The dispersal of the signal is due to travelling great distances through magnetized plasmas (an ionized gas), which are strongly dispersive media for VLF signals.
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