We don't know who was the first one to discover the presence of these signals; but, given their maximal intensity around 5 kHz, the discover was connected to the use of these frequencies. Telephonic societies noticed that first, but they couldn't explain the origin of these interference (the long air wires which unawares were used as antennas). First telegraphists noticed by experimenting with low frequencies (thinking that to cover long distances were necessary long waves). First military spy electronic department noticed that by using triodes done to amplificate currents between two earth connections (this in order to intercept communications inserted in the ground). Maybe even us, we noticed that when unawares we have detached the ground in a microphone connected to an audio amplifier and we thought at that moment to hear our neighbor doing works at his home with his electric drill.
Despite the many occasional meetings with those signals their origin was finally discovered in 1957 after works done during the international geophysical year: during that period, when scientists of different countries did together a scientific research program just in order to study this phenomenon, 66 nations participated to the study of geomagnetic phenomenon, of meteorology of atmosphere, of cosmic rays and of many other phenomenas.
Begun on the 1st of July 1957 in correspondence with the maximum sun activity period, ended up december the 31st 1958 a lot of money was spent and many devices as observatory astronomic, probe balloons , meteorological missiles and satellites for the study of high layer of the atmosphere. After this study an answer has been given to the many questions up to that moment unanswered about the provenance of natural origin signals.