Thus, low frequency radio waves do not propagate as well as high frequency waves. In the Ionogram Handbook, a black out is defined as a condition in which no trace is observed in the ionogram. There is no interference. In a black out, the absorption condition becomes stronger. The disappearance of trace starts very quickly from the lower frequencies.
Recovery usually occurs slowly from the upper frequencies.
The letter gB h indicates all ionospheric parameters traced from an ionogram during a black out. If a black out occurs, no signals appear in any of the ionograms of four Japanese stations. The figure F10.7cm solar flux during 1968-1997 shows that the occurrence of black outs are associated with solar activity. This phenomenon depends on conditions such as neutral wind, electron temperature, and electron density in the mesosphere. Until a few years ago, however, we were not able to obtain much information about the mesosphere because the mesosphere is too high to be observed by airplanes and balloons, and too low to be observed by spacecraft.