Appleton and Barnett Detect the Ionosphere (1924)

In 1924 Edward Appleton and Miles Barnett determined that radio waves reflect off the bottom edge of a layer of the atmosphere at about 60 miles altitude, thus confirming the hypothesis that a conducting layer surrounds Earth. This layer is now named the ionosphere and it is known to be composed of charged particles that react to solar variations, thus creating a space weather region around Earth.

Over the course of the 18th and 19th centuries, various clues led scientists to believe that Earth’s atmosphere had some kind of an electrified layer, known today as the ionosphere. This layer was officially confirmed in 1924 by British physicist Sir Edward Appleton (1892-1965) and his student New Zealand scientist Miles Barnett (1901-1979) when they detected radio waves bouncing off the ionosphere some 60 miles up in space.

The possibility of this electrified layer was considered as early as the 18th century, when experimenters realized that electrical glows produced in a vacuum looked much like the aurora. They suggested the upper atmosphere itself might similarly conduct electricity.

Various theories were subsequently developed. American scientist Benjamin Franklin (1706-1790) formulated a theory that accumulations of electrical charge at polar latitudes caused auroras. German physicist Carl Friedrich Gauss (1777- 1855) suggested that electric currents might flow in the upper atmosphere. Around 1880, Scottish physicist Balfour Stewart (1828-1887) proposed that currents observed using ground-based magnetometers could be driven by upper atmospheric winds. In 1908, Norwegian scientist Kristian Birkeland (1867- 1917) suggested that strong currents in the auroral ionosphere during magnetic disturbances are caused by charged particles from distant space forced to flow primarily along geomagnetic field lines until they reach the high-latitude ionosphere. Italian inventor Guglielmo Marconi’s (1874-1937) demonstration in 1901 that radio waves could propagate across the Atlantic led to suggestions by American engineer Arthur Kennelly (1861-1931) and English engineer Oliver Heaviside (1850-1925) that the waves might be reflected by a conducting layer in the upper atmosphere.

With such theories mounting, Appleton and Barnett started a series of experiments in 1924 to investigate this electrically charged layer in the atmosphere. With the aid of the British Broadcasting Corporation, Appleton began research into the strength of waves received in Cambridge, England, from a transmitter in London. The results showed that the signal remained constant during the day, but regularly changed in signal strength at night. He theorized that during the night, as the waves traveled, some of the waves reflected off a layer of the atmosphere. Since the two signals travelled different distances, they arrived at different times and interfered with each other, making the signal strength vary.

To prove this theory, Appleton used the BBC transmitter at Bournemouth, England, in a similar experiment but varied the signal in a controlled way. By measuring the interference between the direct and reflected signals on the night of Dec. 12, 1924, Appleton and Barnett were able to determine that the height of this reflecting layer was at an altitude of 60 miles, thus providing direct experimental evidence of the ionosphere. At the same time in the United States, Ukrainian physicist Gregory Breit (1899-1981) and American Merl Tuve (1901- 1982) developed an independent method to determine the existence and height of the conducting layer, which they reported in the same year as Appleton and Barnett. Ionospheric science was born.

Appleton received the 1947 Nobel Prize for physics "for his investigations of the physics of the upper atmosphere…” which included his further research on his discovery of the Appleton Layer, which corresponds to an increase in the ionospheric density near 150 miles altitude.

Today, scientists continue to research the ionosphere. In fact, since the beginning of the space age (just after World War II), thousands of rocket and satellite experiments have been carried out to study the ionosphere in detail, often in collaboration with the hundreds of ground-based radio sounding instruments set up to provide continuous information about this conducting layer. The layer absorbs ultra violet and x-ray radiation from the sun and is very sensitive to changes in space weather – indeed, electric currents in the ionosphere can disrupt communications and GPS satellite signals.

Resources/References:

• RAL Space. 80 Years of Appleton’s Ionosphere. (link)
• Appleton, E. V. and M. A. F. Barnett. (1925) On Some Direct Evidence for Downward Atmospheric Reflection of Electric Rays, Proc. R. Soc. Lond. A, Vol. 109, pp. 621-641.
• Suess, Steven T. and Tsurutani, Bruce T., Editors. (1998) From the Sun: Auroras, Mangetic Storms, Solar Flares, Cosmic Rays, Washington, DC: American Geophysical Union, p. 37.
• Benson, Robert F. (2010) Four Decades of Space-Borne Radio Sounding, URSI, The Radio Science Bulletin, No. 333, pp. 24-44.
• Radio JOVE: Solar & Planetary Radio Astronomy for Schools. (link)
• Interactive NASA Space Physics Ionosphere Radio Experiments. (link)
• This animation shows how the ionosphere changes between daytime and nighttime, based on the Coupled Ion Dynamics Investigation (CINDI) observations, a joint NASA/US Air Force project. Credit: NASA GSFC. (link)