Editing Ionosphere (section)

===Ionograms===
{{main|Ionogram}}
Ionograms show the virtual heights and '''critical frequencies''' of the ionospheric layers and which are measured by an [[ionosonde]]. An ionosonde sweeps a range of frequencies, usually from 0.1 to 30&nbsp;MHz, transmitting at vertical incidence to the ionosphere. As the frequency increases, each wave is refracted less by the ionization in the layer, and so each penetrates further before it is reflected. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma, or critical, frequency of the layer. Tracings of the reflected high frequency radio pulses are known as ionograms. Reduction rules are given in: "URSI Handbook of Ionogram Interpretation and Reduction", edited by [[William Roy Piggott]] and [[Karl Rawer]], Elsevier Amsterdam, 1961 (translations into Chinese, French, Japanese and Russian are available).