VLF/LF propagation as noted above provides some insight on the way ionization builds up and decays, and provides a routine monitor for the detection and time change of solar-geophysical disturbances. The Fig. 6b illustrate the diurnal-seasonal variations of amplitude on DHO/23.40 kHz radio signal in Belgrade for three-month period during “quiet” (2009) and “active” (2014) solar condition. White areas were missing data caused by: transmitter off-air, archival data or failures at the receiver site.