Ionosphere

Lore & Background

As early as 1839, Carl Friedrich Gauss postulated that an electrically conducting region of the atmosphere could account for observed variations of Earth's magnetic field. In 1902, Oliver Heaviside proposed the existence of the Kennelly–Heaviside layer, and Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties. In 1912, the U.S. Congress imposed the Radio Act of 1912 on amateur radio operators, limiting their operations to frequencies above 1.5 MHz, which led to the discovery of HF radio propagation via the ionosphere in 1923.

Between 1923 and 1925, Edward V. Appleton, Miles Barnett, Reginald Smith-Rose, and R.H. Barfield conducted radio sounding experiments, establishing the height of the E layer at between 80 and 90 km. In 1925, Gregory Breit and Merle Tuve used pulse-echo to measure ionosphere height, finding variations from 88 km to 211 km. In 1927, Appleton's frequency change experiments established the higher F layer. In 1962, the Canadian satellite Alouette 1 was launched to study the ionosphere, followed by Alouette 2 in 1965 and the ISIS satellites in 1969 and 1971.

Reader's Guide

The ionosphere's significance for radio propagation was demonstrated early through Marconi's 1901 trans-Atlantic signal, which would have required bouncing off the ionosphere twice, though Dr. Jack Belrose contested this based on theoretical and experimental work. The discovery of HF propagation via the ionosphere in 1923 followed from the 1912 Radio Act's restriction of amateurs to higher frequencies. Observations during a 1925 solar eclipse by Dr. Alfred N. Goldsmith and his team showed that short waves became weak or inaudible while long waves steadied, contributing to understanding of the ionosphere's role. The ionosphere enabled long-range skywave propagation, with the E and F layers refracting radio waves at different altitudes depending on solar radiation. The launch of Syncom 2 in 1963 enabled the first measurement of total electron content variation along a radio beam from geostationary orbit to an Earth receiver, a technique used by Elizabeth Essex-Cohen from 1969 to monitor the atmosphere above Australia and Antarctica. The ionosphere remains critical for long-distance communications and GPS signal propagation.

Did You Know?

• Guglielmo Marconi received the first trans-Atlantic radio signal on December 12, 1901, using a 152.4 m kite-supported antenna.
• The term 'ionosphere' was introduced by Robert Watson-Watt in 1926.
• The Canadian satellite Alouette 1, launched in 1962, was the first satellite to study the ionosphere.