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VLF receiving - Earth sounds - Sferics, Tweeks, Whistlers, Dawn Chorus
There is a variety of VLF (Very Low Frequency) signals. This page gives a brief overview
including information for "when to listen" with your VLF receiver and how the signals sound like.
In general, 2 hours before sunrise and 1 hour after sunrise is the best time to listen to all sort
of VLF signals.
Sferics (short for "atmospherics"):
| Source: |
Impulsive signals emitted by lightning from up to a few 1000 km ducted between the surface of Earth and the ionosphere |
| Sounds like: |
twigs snapping (also cracks and pops) or short sizzling like frying bacon |
| Time: |
Anytime; very common sound |
Tweeks:
| Source: |
Sferics ducted in the earth-ionosphere waveguide from distances >> few 1000 km up to halfway around the Earth (20000km).
Higher frequencies arrive before the lower ones (dispersion) and therefore it sounds like a short ping or short ringing |
| Sounds like: |
pings and rings |
| Time: |
Anytime; scarcer sound |
Whistlers:
| Source: |
Sferics travel out of the atmosphere and return by following a magnetic field line. Higher frequencies
arrive before the lower ones (dispersion) and therefore, due the much longer path, it sounds like a descending musical tone. |
| Sounds like: |
descending musical tone for 1/2 second up to over 4 seconds |
| Time: |
local midnight to one hour after sunrise, but also from dusk to midnight and sometimes from a few hours before sunset |
| Best time: |
2 hours before sunrise to one hour after sunrise |
More Whistlers:
| Pure Note Whistler: |
A whistler traveling along only one magnetic field line. The sound is a very clear decending tone. |
| Diffuse Whistler: |
A whistler traveling along several magnetic field line with different length. The result is a "breathy" or "swooshy" sound.
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| 2-hop Whistler: |
A lightning stroke near the receiver (within about 2000km) produces a strong "local" sferic. The signal travels along a magnetic
field line, bounces off on the other hemisphere on the ionsphere and is heard as a whistler 1-2 seconds later.
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| Whistler Echo Train: |
One sferic bounces back and forth between magnetic conjugate points several times with some signal leaking thru the
ionsphere each time. This leakage can be heard as whistler each time. The descending tone gets slower each time
due the increased path of each hop (dispersion). |
Dawn Chorus:
| Source: |
many short short-path whistlers occuring at the same time |
| Sounds like: |
many birds calling in turn |
| Best time: |
Middle latitudes: from sunrise to one hour afterwards
High latitudes: extends to noon or even mid-afternoon, especially
during geo-magnetic storms |
Manmade signals:
- 50/60Hz hum
- LORAN navigation signals: sounds like rapid clicking
- Russian ALPHA navigation signal: repearting tone at constant frequency
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VLF Index