Somewhere over the radar
UHF parabolic antennas
9 March 2012 by Ian McCrea
There's still plenty we don't understand about what's going on hundreds of kilometres above our heads, where the atmosphere meets space. Ian McCrea explains how a facility deep in the Arctic Circle is helping unravel the mysteries of space weather.
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'.
The aurora over the EISCAT facility
As we drive through the gathering gloom, 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 University of Lancaster. They're working with another EISCAT facility, the HF 'Heater' - a powerful short-wave transmitter which transmits radio waves which couple to other natural wave frequencies in the ionosphere. This lets them transfer energy into the upper atmosphere, perturbing it in a controllable way, and use the radar dishes to measure how it responds.
Radar dishes in Tromsø
As well as helping to make a radio programme, I will be assisting Steve and Andrew with their measurements. For the next three days, this involves the glamorous task of driving to a small shed in the woods, where they have set up a spectrum analyser to measure Stimulated Electromagnetic Emissions (SEE). SEE is another kind of radio wave, generated when the heater transmissions interact with the ionosphere.
Learning about SEE, and how it can be excited artificially, helps the Lancaster team understand how waves and charged particles exchange energy naturally in the Earth's environment. While they may sound obscure, these wave-particle interactions produce the very energetic particles found in the Earth's radiation belts - a major hazard for satellite operators, because of their ability to penetrate through spacecraft shielding into the sensitive electronic systems beneath. Similar 'space weather' processes are responsible for accelerating particles near the poles, causing them to cascade downward into the Earth's atmosphere, where they produce the dazzling light show known as the aurora.
The aurora. Neither of my journalist friends has ever seen it, and as the days go on they are becoming increasingly blasé about the sight of those big radars. All they want to know is, when will they see the aurora? Unfortunately, the sun is just awakening from one of the longest periods of low activity in its recent history.
When solar and geomagnetic activity are low, as they are during our stay, the auroral oval - the area within which the aurora appears - remains tantalisingly north of Scandinavia, and our night skies, although clear and starry, are free from the 'Northern Lights' which I foolishly promised they would probably encounter during their few days here.
This is good news for the Lancaster team, who can work without any interference from auroral processes which might confuse their results, but bad news for my radio crew, who are becoming increasingly despondent.
Finally, it is our last night. The skies are clear and starry again. The geomagnetic activity seems to be ticking upwards but there's still no aurora. About midnight, I bid the journalists a frustrated goodnight and we retire to bed. Within minutes, however, a call comes from Andrew that the All-Sky Camera is seeing a weak auroral arc.
We all rush outside, just in time to see the show beginning at last, with curtains of red and green light filling the northern sky. Despite the bitter cold, journalists and scientists alike spend the next hour outside, taking pictures, recording ourselves as we describe what we are seeing, and being genuinely awed by something that, however often you witness it, is still one of the world's most amazing natural wonders.
And so, finally, the Lancaster team return home with some good data to analyse, the BBC folks have done enough recording for about a day of radio, never mind a 30-minute programme - and I have the satisfaction of knowing that EISCAT, with a little help from Mother Nature, has given them at least part of what they all came for.
Dr Ian McCrea is head of the UK EISCAT Support Group at the Rutherford Appleton Laboratory.