3D 'movies' of the ionosphere

A conventional hospital X-ray photograph is a 2-dimensional shadow of a 3-dimensional subject. If one bone lies in front of another, that second bone might not show up on the photo. ‘Tomography’ overcomes this problem by making multiple scans from different directions, which can then be assembled as a sophisticated 3-dimensional image.

What if tomographic images could be made of the sky . . .?

The University of Bath’s Department of Electronic and Electrical Engineering has pioneered a technique that makes tomographic measurements of the high atmosphere, just like x-ray beams scanning a patient; only in this case the beams are radio waves, and the patient under observation is the Earth’s ionosphere. Radio signals passing between overhead space satellites and the ground provide the ‘start’ and ‘end’ vertical values for electromagnetic pulses, which are then collated and analysed to provide a picture of the free-electron concentrations. The system produces not just pictures but 'movies.' We can watch the ionosphere's behaviour in real time.

There is a big problem to overcome first. Unlike a hospital patient, the ionosphere cannot easily be subjected to millions of finely controlled measurements—and almost all the horizontal data is missing. Fortunately, the gaps in the information can be partly compensated for by making realistic assumptions about the distribution of electron density. A little expert guesswork is allowable because it is based on previous knowledge of ionospheric behaviour.

There are still some basic questions we want to answer. For example, on a quiet day, the ionosphere’s maximum ionisation levels occur at about 300 km altitude. During storms, that height can suddenly rise to over 600 km, and at the same time the ionisation levels increase. Afterwards, parts of the high altitude and high-density ionosphere ‘flow’ across the polar cap, from Canada all the way to Europe. These events can distort GPS signals, leading to errors in navigational data, typically of a few meters but sometimes more. Occasionally an entire GPS signal can be disrupted.

Knowledge of the ionosphere is essential to operators of Earth observation radars (for example when monitoring deforestation or ice thickness). It also has far-reaching implications for high-frequency (HF) military radio transmissions; and an amateur ‘ham’ radio operator’s day can be ruined by space storms too.