What are they?
The spectrographs linked to the calling page show wideband spectrum data. LF*EM samples the data from each of its receiver stations for about 15 seconds of every 10 minutes toensure that the station is working properly and to look at qualitative aspects of the data. These plots are a by-product of their data quality monitoring process.
What do they show?
The spectrographs show the variation of signal frequency with time. Generally, the plots show two different sorts of signals, manmade and natural. The manmade signals are more-or less constant frequency with time (ie, the horizontal lines). At lower frequencies (<5kHz) these are dominated by harmonics of the AC electrical transmission frequency (50Hz in New Zealand and Australia). Above ~10kHz the horizontal signals are signals from VLF transmitters. For example, in the Perth spectra one can clearly see the 13 kHz Australian Navy transmitter (located near the town of Sale in Victoria), the very strong 19.8 kHz US Navy transmitter on Australia's North West Cape, and a 22.2 kHz Chinese transmitter.
If you played the lower-frequency signals through a speaker they would sound like constant tones (as most people's hearing only goes as high as ~16 kHz, many of the transmitters cannot be heard). The dominant natural signals in all the spectra are lightning, seen as short-lived vertical lines (pulses) - lightning produces a very short pulse which simultaneously covers a very wide frequency band. An individual lightning discharge sounds like a "click" when played through a speaker. As there are approximately 100 lightning flashes over the entire globe every second, and we detect lightning from at least one-third of the globe, the over all effect sounds a bit like frying fatty bacon.
Adapted from email from Craig Roger, LF*EM, 21/9/2001
