This spectrogram, recorded during the eclipse of 11 August 99, shows all the band with many kinds of signals, of natural origin and not. The nature radio signals can be a regular presence (like sferics) but not constant in intensity. Instead the artificial signal are regular, within the limits of propagation.
Detail of the first spectrogram, showing the band from 11 to 22 kHz. The signals are:
11.9 kHz ALPHA Russian Radio Navigation
B) 12.6 kHz ALPHA Russian Radio Navigation
C) 13.0 kHz weak tone (unknown origin, may be VL3DEF)
D) 14.9 kHz ALPHA Russian Radio Navigation
E) 16.0 kHz RTTY GBR, Rugby, UK, 45Kw ERP. R.Navy, MSK. Real center freq.is 15.980 kHz
F) 18.1 kHz RTTY UFQE, Matotchkinchar, Russia, 100kW OP rtty 50 hz shift
G) 18.2 kHz CW VTX3 (Indian Navy, Vishakapatnam Radio)
H) 19.6 kHz RTTY GBZ, Criggons, UK, 30kW ERP. MSK, 50 Bd
L) 20.5 kHz Tone (unknown origin, may be 3SA/3SB China?)
M) 21.0 kHz RTTY (maybe russian)
And following image, send me by Steve Olney VK2ZTO, shows the comparison on the same spectrum in Italy and Australia (the opposite part of the world). Left side in Italy and right side in Australia received with CHA antenna (see details at Steve Olney links in collaborators list). Two receptions are very similar.