I recently bought and assembled the INSPIRE VLF 3 Kit and have had some success capturing what is sometimes referred to as the natural radio of the Earth’s magnetosphere. (It should be noted that I’m only becoming acquainted with these phenomena, and my understanding of the physics behind VLF emission is patchy at best. Over time I’ll hopefully improve on that!)
These emissions occur within the VLF band (3 -30kHz) and are produced by electromagnetic activity caused by phenomena such as lightning, aurorae, and solar flares within the Earth’s ionosphere.
There are generally three types of VLF signals caused by lightning: sferics, tweeks, and whistlers. The characteristics of each of these sounds are a result of the distance around the earth that the signal has had to travel.
Sferics are perceived as clicking and popping sounds, not too far removed from the sound of a crackling fire. Sferics can travel from thousands of kilometers around the globe, bouncing back from the Earth’s ionosphere to the ground and back again.
Along with the sferics, there are also tweeks on this recording which sound a little like birds chirping. Due to the fact that they have likely traveled from a greater distance than the sferics, there is a displacement across the frequency spectrum due to the higher frequencies traveling slightly faster than low-frequency components (apparently?).
Finally, there are some whistlers in this recording which can be heard at 1:42 and 2:40. Whistlers exhibit a greater frequency dispersion resulting in the characteristic whistling sound.
There is quite a lot of man-made sound on this recording which can be observed on a spectrogram as steady horizontal lines. One such sound that is hard to escape is the ever-present 50Hz hum generated by mains electricity. Although I was probably at least 500m from the nearest power line I still had to remove the hum and associated harmonics from the recording.
There are also some curious signals between 16kHz and 25kHz. I’m not sure what they are exactly but I’m going to do a little more reading to find out.
This recording was made around 23:30, 2/7/21 just above Limekiln Beach in Wicklow.
