The Discovery of the Ionosphere

Since Marconi's successful trans-Atlantic radio transmission in 1901, it was realised that radio waves could be transmitted beyond the horizon. Various theories existed as to how this could be so, with many people postulating the existence of some sort of reflective layer in the upper atmosphere. It was not until 1924 however that Appleton was able to prove that such a layer existed.

With the aid of the BBC, Appleton began research into the strength of waves received in Cambridge from the transmitter in London . His results showed regular changes in signal strength over night while the signal remained constant during the day. He attributed this to an extra wave being reflected from the atmosphere at night and being detected at Cambridge along with the wave transmitted directly from London . Since the two signals had travelled different distances, they arrived at different times and interfered with each other, making the signal strength vary.

Appleton proved the existence of the reflective layer by using the BBC transmitter at Bournemouth in a similar experiment using a signal in which the frequency was varied in a controlled way. By measuring the interference between the direct and reflected signals at Oxford during an experiment on the night of December 12 th 1924 , Appleton and Barnett were able to determine that the height of the reflecting layer was at an altitude of 100km and ionospheric science was born.

Ionospheric Research at RAL

Having proved the existence of the ionosphere, Appleton started to monitor it in order to learn more about its behaviour. These measurements continue today at the Rutherford Appleton Laboratory in Oxfordshire and form the longest sequence of measurements of their kind. These data are used in many different ways by many hundreds of scientists across the globe.

We now know that the ionosphere is an electrified layer created when ultra-violet and X-ray radiation from the sun is absorbed by the Earth's upper atmosphere. The ionosphere is very sensitive to changes in solar activity and the last 70 years of measurements have shown that the ionosphere is also sensitive to climate change. Current research is focussing on the role of the ionosphere plays in the global electric circuit.

"I don't think that Edward Appleton himself, would have appreciated the importance of the ionosphere 80 years after its discovery," says Dr Chris Davis of RAL's Ionospheric Monitoring Group.

Sir Edward Appleton

Edward Appleton (after whom RAL is jointly named) was born in 1892 in Bradford . When he was 18 he earned a scholarship to Cambridge University to study the natural sciences. The outbreak of the first world war in 1914 led him to join the Royal Engineers where he was first introduced to radio.

After the war he returned to Cambridge as a student of the renowned scientist J. J. Thomson (famous for discovering the electron). Appleton 's own research was focussed on radio waves and how they interact with the atmosphere.

After proving the existence of the ionosphere, Appleton worked with Robert Watson-Watt in developing technology that would lead to the invention of RADAR, a vital component of the UK 's defence during the second world war.

Appleton was knighted in 1941 for his contributions to British military research and received the 1947 Nobel prize for physics for his 'Investigations into the upper layer of the atmosphere'. He died in 1965.

For more information on the history of Ionospheric research the following links are available.

RAL Ionopsheric monitoring group

The history of the Radio Research Station (Ditton Park)

Biography of Sir Edward Appleton (BBC History)