Tim Furniss/LONDON
Images of the earth's entire magnetosphere will be obtained for the first time next year
A new NASA spacecraft will next year capture the first image of the earth's entire magnetosphere - the region surrounding the planet in which the behaviour of charged particles is dominated by the world's magnetic field.
The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite will be launched on a Boeing Delta II booster from Vandenberg AFB, California, in February 2000. Numerous spacecraft have studied the magnetosphere for more than 40 years and its overall shape can only be deduced by the data returned. Orbiting in deep space and equipped with ultraviolet cameras, the IMAGE will be able to take images of the entire magnetosphere as it changes shape.
Scientists will be able to take a step back and watch events unfold. "It's exciting to be in a position where you know you're going to stumble onto things you didn't suspect were there," says Dr Dennis Gallagher at NASA's Marshall Space Flight Center, Huntsville, Alabama.
The IMAGE will be launched into an orbit that will loop from a perigee (low point) of 1,000km to an apogee of 45,000km. From this vantage point, scientists will have a view of the magnetosphere's inner structure and the boundary where it meets interplanetary space.
The key to this objective is the Far Ultraviolet Wideband Imaging Camera (FUWIC), which comprises three units. The Wideband Imaging Camera (WIC) will have a 17í x 17í field of view, about 34 times the apparent diameter of the moon, so it will have a full view of the earth and its surrounding space environment.
The FUWIC carries a spectrograph imager and geocorona photometers. The imager will measure different types of aurora and the photometers will examine the content of the solar wind, which is the main cause of interference.
The IMAGE will also carry neutral atom, ion and plasma imagers.
The nearest thing yet to an image of the magnetosphere was a photograph of the earth taken by a US Navy ultraviolet camera that was used to observe the stars from the moon's surface during the Apollo 16 mission in April 1972. The camera produced a striking photo of hydrogen in the magnetosphere around the earth.
The magnetosphere is an immense cloud of ionised gas, like an invisible fog. It is formed by the interaction of the solar wind - ionised or charged plasma particles, mainly protons and electrons, which stream out from the sun - with the earth's magnetic field. Interference in the magnetosphere disrupts terrestrial communications and can result in massive power outages on satellites, costing millions of dollars.
Predicting interference
The behaviour and extent of the magnetosphere is influenced by changes in the intensity and content of the solar wind. Scientists will better understand the solar-terrestrial effect by watching the magnetosphere change. This will lead to improved predictions of interference on the earth.
The solar wind moves at speeds of up to 900km/s and when it encounters the earth's magnetic field, the latter is greatly distorted, producing the magnetosphere. The magnetosphere begins about 60,000km from the sunward side of the earth. Here, there is a shock wave, or bowshock, where the solar wind meets the earth's magnetic field. Inside the bowshock is a turbulent region of ionisation generally referred to as the magnetosheath.
On the opposite side of the earth, the magnetosphere is drawn out to a great distance in the wake of the solar wind. Activity in the magnetosphere creates the aurora borealis, commonly known as the Northern Lights, with a similar phenomenon in the southern hemisphere, the aurora australis. Electrons and ions travel rapidly back and forth along the earth's magnetic field lines. When energised by a severe storm of solar wind, these particles can penetrate and hit the outer atmosphere, causing the aurorae.
The interaction of solar wind with the magnetic field also forms the famous Van Allen radiation belts, discovered by the USA's first satellite, Explorer 1, which was launched in January 1958. It was equipped with a Geiger counter provided by Dr James Van Allen.
The IMAGE will be a direct descendent of Explorer 1, being part of the NASA Medium Class Explorer Programme (MIDEX).
Unlike most previous science missions, all data from the IMAGE will be made freely available as soon as they arrive. NASA says the IMAGE mission has "a completely open data policy, with no periods of proprietary data rights''.
Gallagher says: "This is going to be an open data set."
"Our ambition is that, from the first data set that comes to the ground, starting about a month after launch, there will be browse data in the World Wide Web for anyone to look at via the NASA Deep Space Network.
"It's going to be an exciting time. Every time you figure out a new way to measure your environment-you see things you didn't think would be there. That's what research is about. You learn more about the environment in new ways."
Source: Flight International
