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Ionospheric Sounding

Synoptic measurements of the terrestrial ionosphere have been made for nearly 70 years using the vertical incidence sounding radar (ionosonde), which utilizes high-frequency radiowaves to probe the ionosphere from about 90 to 800 km.

In its simplest form, the ionosonde transmits a HF radio wave (typically 1-30 MHz) vertically and records the time-of-flight of "echoes" reflected from the various ionized layers in the ionosphere. Since the radio wave suffers retardation when the transmitted frequency approaches the plasma frequency of the layer, the "virtual" range of the ionospheric echo increases and produces the characteristic cusps which appear in ionograms. When the transmitted frequency exceeds the plasma frequency of the layer, the radio wave is no longer reflected and travels into free space.

Approximately 150 ionosondes are operational worldwide and are used for both scientific and engineering applications in nearly every country. The ionospheric layers vary as a function of time-of-day, season, solar cycle and geographic latitude of observation. The study of the ionosphere from the Bear Lake Observatory has been an ongoing project since 1990.