Radar, GPS technology, and high-frequency radio would not be possible without a groundbreaking discovery made nearly a century ago. In the mid-1920s, Sir Edward Appleton, an English physicist, proved the existence of the ionosphere: the layer in Earth’s atmosphere that is ionized by solar and cosmic radiation.
His work made it possible for Robert Watson-Watt to develop military radar systems that helped England win the Battle of Britain and prevent Germany from invading the United Kingdom in 1940. Appleton worked out the distance to areas of the ionosphere that reflected signals from radio transmitters. Watson-Watt then adapted the idea to create a radar system that could detect aircraft.
For this contribution, Appleton was knighted in 1941, and he was then awarded the Nobel Prize in physics in 1947. Appleton also helped found the IEEE United Kingdom and Ireland Section.
This month marks the 50th anniversary of Appleton’s death, but his contributions to the field of atmospheric science continue to resonate today.
UP IN THE AIR
The discovery of the ionosphere in 1924 was the culmination of almost a century of discoveries. In 1839, German mathematician and physicist Carl Friedrich Gauss came up with the idea that an electrically conducting region of the atmosphere could account for the observed variations of Earth’s magnetic field. Nearly 40 years later, Scottish physicist Balfour Stewart published a theory about an electrified atmospheric layer that could distort Earth’s magnetic field.
Finally, in 1902, English scientist Oliver Heaviside and Arthur E. Kennelly—1898 president of the American Institute of Electrical Engineers, one of IEEE’s predecessor societies—independently concluded that an atmospheric layer capable of conducting electricity must exist.
It was Appleton, however, who actually proved that there was something up there. In April 1924, Appleton, then a professor of physics at King’s College in London, began a series of experiments to investigate the strength of the radio signals he received from the London BBC station. He discovered that the signals’ strength was constant during the day but varied during the night, rising and falling in an almost regular manner. He theorized that at night he was receiving two waves: one that traveled directly along the ground and the other that was reflected back from a layer in the upper atmosphere.
In December, Appleton tested his hypothesis. He had the BBC send another radio signal into the air, this time from Bournemouth, on England’s southern coast. He wanted to check that it was an atmospheric layer and not the hills in southern England that were reflecting the second radio wave. From the interference patterns, Appleton concluded the reflective layer that Heaviside and Kennelly discovered was about 100 kilometers above Earth’s surface.
Appleton’s tests also showed the layer was heavily ionized by electrons—hence the term ionosphere—which allowed it to conduct electricity. He subsequently discovered the layer could be penetrated by waves above a certain frequency and that these penetrating waves could also be reflected back but from a much higher layer. This layer, 250 to 350 km above Earth’s surface, had a high electron density and reflected back shorter wavelengths. It reflected the waves back during the day as well as at night. This, he deduced, was the layer that reflected shortwave radio signals so that they could travel around the world.
Appleton didn’t want the layer of the ionosphere he discovered named after him, according to Chris Scott, associate professor of atmospheric physics at Reading University, in England. “He thought this would just confuse people,” he says. “He wanted to use something easier to remember and chose to call it the F layer because it was discovered after the E layer, the name given to the Heaviside-Kennelly layer.”
LEAVING A LEGACY
During World War II, Appleton was secretary of the United Kingdom’s Department of Scientific and Industrial Research, in London. He then moved on to Edinburgh University, in Scotland, and continued publishing work on the ionosphere throughout his life. In 1944 he founded the American Meteorology Society’s Journal of Atmospheric and Solar-Terrestrial Physics.
In 1947, for his contributions to atmospheric science and its application to radio, Appleton received in addition to the Nobel Prize in physics the U.S. Medal of Merit and the Norwegian Cross of Freedom; he was also made an officer of the French Legion of Honor. In 1948, Pope Pius XII appointed him to the Pontifical Academy of Science, in Vatican City.
Appleton was an honorary member of AIEE. In 1962, he received the Institute of Radio Engineers’ Medal of Honor for his work on the ionosphere. Later that year, when AIEE and IRE merged to form IEEE, Appleton helped found IEEE’s United Kingdom and Republic of Ireland Section.
He died on 21 April 1965 in Edinburgh. In 1974, the Rutherford Appleton Laboratory, a scientific research center in Oxfordshire, was named after him, as well as academic buildings at the University of Bradford, in Yorkshire, England, and the University of Edinburgh. There’s also a crater on the moon named in his honor. The highest ionized layer of the ionosphere is still referred to as the Appleton-Barnett layer. Miles Barnett, a physicist from New Zealand, was Appleton’s student in the 1920s.
“To efficiently use GPS, satellites, and high-frequency radio, we need to understand how the ionosphere blurs radio waves,” says Scott. “We also need to know that the ionosphere is sensitive to long-term change. To do all this, we rely on those measurements that Appleton set in motion all those years ago.
“Appleton not only discovered the ionosphere, but he investigated its nature in an effort to understand it, made complex mathematical calculations to work out how the layers formed, and then provided information for people who wanted to use the ionosphere for radio communications.”