3D radar study of northern lights
The atmosphere is electric high above us. Now scientists want to see the stream of particles that make up the Aurora Borealis – in 3D.
The new Nordic space radar EISCAT 3D won’t just let scientists see which direction the charged particles are heading.
"We will also get a 3D map, a volume of the air space high above us, showing where the particles are,” says César La Hoz, of the Department of Physics and Technology at the University of Tromsø.
He leads the Norwegian contingent in its efforts to develop the new radar.
This will give a better understanding of the processes that can upset radio communications and GPS navigation.
Three of these radar facilities are likely to be constructed in the Nordic countries.
Monitoring the ionosphere
To grasp the advantages of the new 3D-radar you first need to understand what the current EISCAT radar can do. One of these installations is just ten kilometres east of Tromsø.
It consists of broad parabolic antenna dishes like the ones many people use for their TVs, only much larger.
“EISCAT monitors the atmosphere 70–1,000 kilometres above us,” says La Hoz.
Like a radio mirror
At such a high altitude the thin atmosphere is electrically charged by, among other things, solar radiation. The charged particles are free electrons and atomic nuclei, called ions. That’s why this thin level of the atmosphere is called the ionosphere.
The ionosphere is far more than the stage for some modern communications problems. It reflects radio waves like a mirror. This is why we can listen to short-wave and medium-frequency radio waves that are far below the horizon, but which are bounced down to us from the ionosphere.
EISCAT also makes use of this mirror effect.
La Hoz explains that EISCAT sends radar pulses up into the ionosphere. Up there, free electrons start to oscillate in the frequency of the radar waves and send electromagnetic radiation back to the ground, only much weaker.
“That’s why we need large antennas,” says La Hoz.
Scientists measure distances of radar echoes just as you can estimate the distance to a canyon wall or face of a mountain by taking the time it takes for an echo to return. Of course, the big difference is that radar waves return much faster than sound waves. It only takes a thousandth of a second to travel a distance of 150 kilometres.
This echo from ions high above us tells scientists something about the direction the ions are travelling. If they are moving toward the radar, down towards the Earth, the frequency rate will be a little faster.
This is called the Doppler Effect, which you can experience if an ambulance speeds by you. The pitch of its siren is higher as the vehicle approaches and lower as it drives away.
The signal also gives information about how closely packed the electrons and ions are up there, and also about their temperature.
So far, so good. We have the distance, the temperature and speed of the charged particles, measured along a line from the ionosphere down to the radar station.
But what if the ions aren’t moving directly toward or away from the radar station?
Then we need several radar stations, so we can measure the speed along several lines. By comparing these measurements we can calculate the speed and direction of the charged particles.
Four EISCAT stations have been set up: one near Tromsø, one at Longyearbyen on Svalbard, one in Kiruna in Sweden and one at Sodankylä in the north of Finland.
But these facilities have a major flaw. They can only measure the distance and speed of the ions in a little area at a time.
“If we want to chart a larger area in three dimensions we have to turn the antennas. This takes several minutes, at least,” says La Hoz.
Several minutes, that’s much too slow. Anyone who has seen the flickering and waving of the Northern Lights across the sky can testify to this.
The lights move fast across the night sky. The electromagnetic winds up there in the ionosphere gust around very quickly.
Many small antennas
Researchers need a radar that can move to match this pace – ones that can focus in on a spot in milliseconds. They need a whole new technology.
Well, not all that new. It has already been used in other contexts, both in satellites and ground stations. One of the latter is the Jicamarca radio observatory east of Lima, Peru, which is César La Hoz’s homeland.
“I’ve worked there,” he says. “Jimarca consists of 19,000 small antennas rather than a single large one.”
Small and speedy
What can thousands of small antennas do that one large one can’t? Answer: move around in a split second.
The antennas don’t really move around physically. But they are positioned in a multiple-phased array, which means they can emit radar signals at slightly different times.
All the individual radar signals are like waves that meet in one particular spot. This spot can be moved fleetingly by alternating the timing of the individual emissions.
“The planning of the 3D radar has been going on since 2008,” says La Hoz.
That’s when the project was selected by the EU organisation European Strategy Forum for Research Infrastructure (ESFRI), which is part of the programme Roadmap 2008 for Large-Scale European Research Infrastructures for the next 20-30 years.
Much of the new radar project is still in the blue. Its size depends on allocations the joint Nordic project receives from the member countries and probably from the EU as well.
“The field containing as many as 30,000 antennas will cover at least 100 x 100 metres,” says La Hoz.
One of such magnitude will improve the concentration of the beam nine-fold, which is nine times the resolution of each point on the picture put together, line by line, as the radar quickly scans the skies above it.
Skibotn is a strong candidate
As with today’s EISCAT, at least three of these facilities must be built to determine the direction of the ions in motion. The locations of these stations will be the result of scientific evaluations and a political tug-of-war among the Nordic countries.
“For now Skibotn in northern Norway is a strong contender as the main station,” says La Hoz.
The Finns and Swedes have their own candidates − Kilpisyärvi and the Esrange-launch centre at Kiruna are two of them.
A so-called heat transmitter will also be built at the main station. It has an effect like that of a giant microwave oven high in the atmosphere − the electrically charged particles are made to vibrate − which raises their temperature temporarily.
Researchers will learn about the nature and conduct of the ions by regulating this temperature and gauging how it drops back down.
“Once the location of the main station is decided we can pick the sites for other stations,” says the researcher.
A report that could go a long way toward settling the issue is likely to be completed this spring. If all goes according to the scientists’ wishes, construction will commence some time in 2015.
Translated by: Glenn Ostling