Nightfall comes early in the Arctic in November, and it’s already getting dark as I drive along the narrow road which runs uphill
from the small village of Fagernes at the end of Ramfjord. We are at 69 degrees latitude in northern Norway, about 40km from the city of Tromsø.
My passengers are two radio journalists from the BBC, making the final programme in a series about the Earth’s atmosphere. The first was about the troposphere, below 10km, where all weather happens, while the second focused on the ozone layer, at 40km, which protects our planet from solar ultra-violet radiation and powers the dynamics of the middle atmosphere. Now their final programme will explore the ionosphere and thermosphere – the outermost reaches of the upper air, where our atmosphere gives way to the vacuum of space. So why come to such an out-of-the- way location?
We’re heading for the main site of the EISCAT Scientific Association, an international organisation with seven member countries, including the UK, whose subscription is funded by NERC. For 30 years, EISCAT has provided world-leading facilities for researchers studying the upper atmosphere and the space around Earth, known as ‘geospace’.
As we drive through the gathering gloom, Incoherent scatter
my companions talk about the places they have already visited for their series, including their recent trip to Antarctica. They seem to have been everywhere, and I’m thinking they’ll be hard to impress. Yet the conversation stops as we turn off the highway onto the gravel track leading up to the site.
Through the low trees, two huge radar dishes appear – the first a parabolic antenna 32 metres across, the second an enormous cylindrical antenna the size of a football field. I turn to the presenter in the passenger seat beside me and see that she’s grinning broadly. ‘OK, this is cool’, she says.
The reason the dishes have to be so big, I explain, is that we’re looking for a very weak signal coming from the ionosphere – the electrically charged layer of the Earth’s atmosphere, extending from heights of 80km upwards to more than 500km above our heads.
EISCAT works by transmitting high- frequency radio waves with a power of a few megawatts, in the form of coded pulses lasting about a millisecond. As these pulses travel through the ionosphere, they interact with electrons, causing each one to act like a tiny transmitter, re-radiating a minuscule fraction of the power that was transmitted. The power that makes it back to the radar is less than a million-millionth of that transmitted, but this is still enough to be detected by EISCAT’s very sensitive receivers.
The electrons that re-radiate this energy are controlled by the ions of the upper atmosphere, whose motion is, in turn, controlled by small-scale ‘ion acoustic’ waves. A radar like EISCAT is sensitive to those waves whose wavelength is half that of the transmitted signal – in this case, a few tens of centimetres. Such waves move randomly in all directions, but the radar is only sensitive to those moving towards and away from the radar along the direction of the beam.
The waves also lose energy to the particles they are composed of, in the same way that an ocean wave gives up some of its energy to a surfer. This means the frequency spectrum of scattered signals seen by the radar shows two broadened peaks, corresponding to
the Doppler shift of the approaching and receding waves.
These peaks’ size and shape are very sensitive to some fundamental properties of the upper atmosphere, such as the density of electrons, the temperature of the electrons and ions, and the speed at which the atmosphere is moving. So analysing the peaks tells us about these parameters, which are very difficult to measure in any other way. This makes EISCAT’s radar technique, known as incoherent scatter, a uniquely powerful tool for studying the upper atmosphere.
We’re not just here to record sound bites for Radio 4. In EISCAT’s prefabricated accommodation block, affectionately known as ‘The Hilton’, we meet up with Dr Andrew Senior and Dr Steve Marple, from the
There’s still plenty we don’t understand about what’s going on hundreds of kilometres above our heads, where the atmosphere meets space. But we do know it can be crucial for life down here, causing phenomena that range from magnificent polar auroras to the violent solar storms that can knock out communications satellites and damage power grids. Ian McCrea explains how a facility deep in the Arctic Circle is helping unravel the mysteries of space weather.
Somewhere over the
14 PLANET EARTH Spring 2012