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Whistler Waves

VLF Whistlers seen by Thunderstorm III

Whistlers are a type of electromagnetic wave which result from lighting strokes. When a whistler wave travels through the ionosphere, the low frequencies have a smaller group velocity than the high frequency portions of the signal, resulting in a wave which is dispersed in time. The periodograms below show how the frequency (y-axis) of several whistler waves decreases as time progresses to the right along the x-axis.

Notice the transmitter near the top of the periodogram. It's the horizontal line at about 19 kHz. If you listen to the 20k sound file, you'll hear the whistlers as well as a constant high-frequency hum. The real data was sampled at 40,000 Hz for 0.5 seconds. By playing it back at a lower frequency, humans can hear the full range of the recorded signal, but at an artificially lower frequency. After passing the data through a notch filter centered at 19 kHz, the hum (20k au file) (wav file) >and the horizontal line are gone

Better yet--

Zero-minus nose whistlers (22k PostScript)
coming soon to an issue of Geophysics Research Letters and the CEDAR conference in June 1996.

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    PLEASE send comments about these pages to Steve Baker . Last updated May 21, 1996