Passive ionospheric sounding is carried out using a time signal, which is transmitted by  radio station DCF77 located at Mainflingen in Germany. A working frequency of the station is  77,5 kHz.  Digital recording of signal amplitude is carried out using  a standard receiver and computer.

Propagating in Earth-ionosphere waveguide, the amplitude and phase of the radio signal  are regularly changed depending on year and day time. Physical properties of the D layer ionoshpere are also changed under the influence of solar X-ray flares and following flux of protons and electrons.  As the result,  short term disturbances of the radio signal are occurred.  Research and analysis of these disturbances allow us to estimate intensity and duration of active processes on the Sun.   

Passive ionospheric sounding  Beginning: March 19, 2002  20152014201320122011201020092008200720062005200420032002 JanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember 01020304050607080910111213141516171819202122232425262728293031