Seeing the invisible
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"This is an immense volume," said Dr. Mark Moldwin of the Florida Institute of Technology in Melbourne, Fla. "We're doing single-point missions in a lot of regions and drawing cartoons to connect the dots."
Right: An artist's "cartoon" hints at the complexity and vastness of the magnetosphere. The structures present are shaped by the Solar wind, and magnetospheric "storms" can affect communications, power grids, and other important aspects of modern life on Earth.
Drawing the right cartoon and linking it to an accurate model that predicts what the magnetosphere does will help us understand not only what happens here at Earth, but in solar flares and even in magnetars. Moldwin was speaking to the New Millennium Magnetosphere Workshop which opened on Monday in Guntersville, Ala. It is sponsored by NASA and the National Science Foundation.
Mankind had always had a hint that outer space was more than "empty space." The aurora borealis, or Northern Lights, had entranced humans since the first people mover northward. In 1958, the Explorer 1 satellite carried instruments that revealed the presence of radiation trapped by Earth's magnetic field into two belts encircling the planet.
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Not until 1959 was the term "magnetosphere" coined by Dr. Thomas Gold of Cornell University, noted Dr. James Burch of Southwest Research Institute in San Antonio, Texas. Substorms - storms of particles that brighten the aurora, disrupt communications, and cause other effects - were not formally recognized until "the last page" in a set of three articles in Journal of Geophysical Research in 1968.
Meanwhile, public attention was readily taken by interplanetary missions that returned pictures of Earth and other worlds.
Right: This view of the Aurora Australis, or Southern Lights, from STS-39 in 1991 shows a spiked band of red airglow called a "Red Crown" above the Earth Limb. Calculated to be in the 80 - 120 km altitude region, auroral activity is due to excitation of atomic oxygen in the upper atmosphere by radiation from the van Allen Radiation Belts and is most common above the 65 degree north and south latitude range during the spring and fall of the year.
The United States and other nations have launched dozens of spacecraft to study the magnetosphere. Yet even with the most ambitious campaign - the International Solar Terrestrial Physics (ISTP) Program - has comprised fewer than a dozen spacecraft. From a scientific standpoint, it's been like setting up that many weather stations and then generalizing about weather and climate across the globe.
"When we talk about a global picture, we're really talking about a statistical picture," he said. "We're averaging over many events to get this picture."
This is because the magnetosphere is largely invisible.
Above: The Solar wind is shown interacting with Earth's complex magnetosphere. Major features include the polar cusp where the magnetic field lines are almost vertical and leave the Earth's poles exposed to space, and a neutral plasma sheet which extends outward from around the magnetic equator. Click for a.
"We are data poor," Moldwin said, even though several gigabytes of data have been returned in four decades of studies. "Where we can sample, we have a lot of information that we need to tie together."
More recently, the Dynamics Explorer 1 (launched in 1981) and Polar spacecraft (1995) have carried cameras that take visible, ultraviolet, and even X-ray images of the auroral regions from space.
Right: The first image taken of the geocorona, a halo of low density hydrogen around the Earth (center) photographed by Astronaut John W. Young, Apollo 16 commander, in April of 1972, with an ultraviolet camera. The geocorona is bright on the sunlit side. Exposure 15 seconds, with F/1 electronographic camera. The streaks and circular patch on right are instrumental. Click for a larger picture.
|Energetic Neutral Atom Images of Substorm Injections Los Alamos National Labs site
Scientists to explore what they know about space weather Announcement of the Workshop on the New Millennium Magnetosphere
www.Spaceweather.com - how does space weather affect Earth, headlines, introduction
More science headlines - NASA space science research
"What's missing now is how to map these images out into the magnetosphere," Burch said, because the images of the magnetosphere are still just cartoons drawn from countless data points collected along a narrow set of paths allowed a satellite.
In recent years, though, scientists have started developing some innovative tools - new methods and even new cameras - that would let them take a true picture of the entire magnetosphere.
An exciting approach that Burch and others described is taking pictures by using neutral atoms in space. Most of what is out there is electrified (stripped of one or more electrons) and is captive to the magnetic field. When one of these ions hits a hydrogen atom, it steals an electron back and becomes neutral, free to fly off in a straight line.
The problem is, the atom could have originated 10 meters or 10,000 kilometers away. It's a bit like looking through a fog.
Right: In 1996, Polar observed a burst of energetic neutral atoms that appeared suddenly and then slowly decayed away. The picture at right was constructed with a ray-tracing program to produce the images () and to define all the correct geometries, showing a promising method for "seeing" the shape of the magnetosphere. Courtesy of Los Alamos National Labs.
But it's promising. Burch showed an image, taken by the Imaging Proton Spectrometer (IPS) on Polar, looking down the magnetotail (the portion of the magnetosphere that the solar wind drags into deep space). He then showed how the image closely matched activities in the aurora.
The method is limited and will be expanded in the year 2000 with the launch of the IMAGE, the Imaging Magnetosphere Explorer, carrying an array of specialized cameras. Some will borrow technology from the Advanced X-ray Astrophysics Facility (AXAF). Optical cameras will use an array of filters and other tricks to produce images of the magnetosphere in visible and ultraviolet light, and X-rays. The "non-imaging" cameras will capture and measure neutral atoms to make more sophisticated depictions than Polar can now provide, and a 500-meter (1,640-ft) long dipole antenna will probe the magnetosphere with radio waves.
Still, scientists will face conflicting demands.
"We need to look at the big picture," Moldwin said, "and we need to look at the microscale physics." This is akin to watching global weather circulation while keeping an eye on small rainstorms.
To do this, NASA and other agencies are looking at several ambitious missions, including one that would orbit hundreds of microspacecraft to take data simultaneously at that many locations.
The Magnetosphere Constellation and Tomography (MAGCAT) mission would deploy a smaller number spacecraft to probe the magnetosphere with multiple radio beams and build an image much as a medical CT-scan shows a cross section of the body. Better understanding of how the magnetosphere works will have applications beyond better predictions of how magnetospheric substorms may affect communications and power grids on Earth. It will also help in exploring the planets - Jupiter has the strongest magnetosphere, one that even makes its moon, Io, light up in the dark. And it will become a tool for deciphering what is happening in the newest cosmic oddity, magnetars, highly magnetized neutron stars that give off blinding flashes of gamma radiation.
POLAR - spacecraft home page
IMAGE - spacecraft home page (Launch in 2000)
Sun-Earth connection - NASA's International Solar-Terrestial Physics mission
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Author: Dave Dooling
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