VLF : very low frequencies is a range of frequencies below 22Khz, those frequencies receive also electromagnetic activities. VLF recording is also out of cities (no electricity) a way to catch sound from stratospheres activities. We usually say : Radio astronomy.
On the first recordings you can hear a soft chorus in the background, it was recorded between 7 and 8 AM on 25th of march, about 15 km north of Hemavan. It's quite difficult to describe, sometimes it sounds a bit like birds singing at dawn or faraway whistles. Maybe this chorus was related to an aurora borealis that happened further north during the night. I didn't saw any during my stay there. The other recordings are examples of discrete emissions, they are short signals, lasting a few seconds and appearing irregularly. They are also due to the interaction of the solar wind and magnetosphere. Some of them are called “risers” (they are ascending tones, you can recognize them on spectrograms) and others are “combinations”, more complex emissions. Those signals have been recorded on the frozen Tarnaby Lake between 7 and 8 AM on 27th of march.
Radio Nature, Renato Romero, http://www.vlf.it/
BBB4 Receiver schematic and http://www.auroralchorus.com/bbb4rx3.htm
VLF recorded with an electric field receiver near Jura mountains, eastern France - 05°58'05” E - 47°58'05”
On the recording from 23rd may 2008 at 10 PM you can hear some whistlers and a background of tweeks and statics. On 14th august 2008 around midnight I also recorded tweeks and man-made radio signals. Some of them can be heard as sequences of tones at high frequencies : this is the Alpha russian radio-navigation system. Some other signals can be seen on the spectrogram as bold horizontal lines at the top of the picture : these are military radio stations (RTTY) from different countries.
tweeks & alpha spectrum