Seventy years of continuously monitoring the earth’s upper atmosphere

Mrs Jacky Hutchinson
CLRC

01235 44 6482
j.hutchinson@rl.ac.uk

Scientists have been celebrating seventy years of continuous data collection of measurements from the ionosphere, the outer-most layer in the Earth’s atmosphere. On 11 January 1931, a sequence of measurements was started by Sir Edward Appleton in order to understand recently discovered electrified layers high in the Earth’s atmosphere – later to be called the ionosphere. These observations began at the Radio Research Station at Slough, Bucks and are continuing today at the Rutherford Appleton Laboratory in Chilton, Oxfordshire. They represent the longest unbroken sequence of daily ionospheric measurements made anywhere in the world.

“A truly remarkable record”, admitted Dr Chris Davis of RAL’s Space Science & Technology Department, who is responsible for the current collection of this data.

The ionosphere is formed when sunlight is absorbed by the Earth’s atmosphere on the very edge of space. These layers refract radio waves and enable signals to be broadcast well beyond the horizon. Given the vital role of radio communications, the ionospheric observatory was established to monitor the variation in the ionospheric layers to establish their affect on radio reception.

It soon became clear that the strength of the ionospheric layers was controlled by the intensity of extreme ultraviolet and X-ray radiation emitted by the Sun. It had been known that the Sun went through an eleven year activity cycle, as seen in the number of sunspots visible on the solar disk. Only by monitoring the ionosphere for several decades did it become clear that the ionosphere, and therefore the radiation from the sun, underwent an eleven year cycle too.

Despite this general agreement, there are occasions when the ionosphere becomes very weak for several days at a time. During such an event magnetic compasses are disturbed, satellites can malfunction and brilliant auroral displays can be seen over Scandinavia and Antarctica (and occasionally in mid-latitudes sites such as the UK). Data from the ionospheric observatory has played a vital role in piecing together clues about what has become known as ‘Space Weather’. The Sun is continually throwing off vast clouds of energetic particles. If such a cloud reaches the Earth, it can be funnelled into the Earth’s atmosphere at the poles by the magnetic field. Here the particles heat the atmosphere and cause the spectacular aurora. For several days afterwards, winds high in the atmosphere sweep this heated air around the world, changing the chemical composition of the upper atmosphere; it is this which temporarily destroys the ionosphere.

Most recently, scientists have been using the data sequence from the Chilton ionospheric observatory, along with other stations around the world, to study the response of the Earth’s atmosphere to climate change. The ionosphere has proved to be a sensitive indicator of greenhouse warming. While an increase in greenhouse gases in the lower atmosphere causes it to warm, the upper atmosphere cools as less heat is being reflected from below. This cooling makes the upper atmosphere contract, lowering the height of the ionosphere by a tiny amount each year. Thanks to the long data sequences, the total effect over many decades has now be measured.

In addition to telling us about the Earth, ionospheric data have also been used to discover changes that have been taking place on the Sun. An experiment carried out during the eclipse over the UK in 1999 showed that the Sun’s atmosphere was in 1999 than during any similar measurements since the 1930s.

“We need to know how the Sun has changed in recent years in order to account for this fact in climate change models”, explains Dr Davis, who led this experiment.

It is unlikely that Edward Appleton could have envisaged the many and varied uses of the ionospheric measurements he started in 1931 but they have proved, and are continuing to prove, essential to our understanding of the planet.