ELF/VLF Radio Atmospherics
Measurements at Palmer Station, Antarctica
VLF and ELF Spectrograms of Radio Atmospherics
Measurements at Palmer Station, Antarctica
We record wideband ELF/VLF (~200 Hz to 20 kHz) data at Palmer Station,
Antarctica.
Extremely Low Frequency (ELF, 30-3000 Hz) and Very Low
Frequency (VLF, 3-30 kHz) are frequency bands to denote where the signals
we measure are in the electromagnetic spectrum. Our signals happen to
be in the audio range, so that if they were sound waves, you could hear them.
Most radio waves used for communication, including ham radio, AM and FM
broadcast radio, are far above the VLF band in frequency. VLF transmits
information very slowly, because of its narrow bandwidth, but VLF signals
can be heard nearly all the way around the world, limited
only by the Antarctic ice sheet or auroral zones.
VLF signals travel in the waveguide formed by the Earth and the ionosphere.
The ionosphere is the outer region of the Earth's atmosphere consisting of
an ionized gas, or plasma. For VLF frequencies, the equivalent height of
the ionosphere is 60 km (40 miles) during the day and 85 km (56 miles)
at night.
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VLF and ELF Spectrograms of Radio Atmospherics
Click on the image for a larger, clearer view!
Spectrograms are dynamic representations of the frequency content (Fast
Fourier Transform, or FFT) of the signals. Each vertical row of pixels
represents 16 milliseconds of data. The top spectrogram shows the VLF
from 5.5 to 9.5 kHz, and the bottom spectrogram shows the ELF spectrum
from nearly 0 to 1000 Hz. Sferics S1 and S2 were associated with optically
measured Sprites. Sferics A and B were produced by other lightning strokes
in the same storm which were not related to Sprites. All four lightning
strokes were detected by the National Lightning Detection Network, as
shown in the lower panel (data courtesy of MSFC DAAC).
Radio atmospherics ("sferics" for short) are impulsive signals launched
by lightning strokes that travel efficiently in the waveguide formed by the
Earth and the ionosphere. "Impulsive" means that they are brief in time
and broadband in frequency. That is why they appear as vertical lines in
the spectrograms above.
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Last update: October 24, 1996
scr@nova.stanford.edu
