A Weatherman in Space...
October 29, 1998: Over the past few years, TV audiences have become accustomed to weathermen showing them radar pictures of storms marching across the viewing area. In a little more than a year, space scientists hope they'll be able to do the same as space storms hit the Earth's magnetosphere. IMAGE - the Imager for Magnetosphere-to-Aurora Global Exploration spacecraft - scheduled for launch in 2000, will carry several instruments to paint pictures of the heretofore invisible regions of the inner magnetosphere.
Space "storms," seen on the ground as aurorae, or the northern and southern lights, can disrupt satellite communications, power grids, and even telephone conversations. The storms' source is in the magnetosphere - a vast and complex region encompassing Earth - where the solar wind interacts with Earth's magnetic field. A region of inner magnetosphere, the plasmasphere, consists of relatively dense ionized gas which rotates with the Earth.
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"The plasmasphere has some well-guarded secrets," said Dr. Donald Carpenter of Stanford University. "We hope that some of these fine instruments on IMAGE will reveal them."
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Left: Whistlers and "roars" of radio emissions from the magnetosphere have been recorded. Listen to a sample "Whistlers" Audio file (93KB, AU file)
The plasmapause changes with the solar wind and other events in space. A few of its average properties are well known, but the manner in which it changes or relates to other geophysical boundaries in space remains a mystery.
"We have some ideas, but we won't really know the physics until we can see the whole system," he added. "This is a huge volume of space [from three to five times wider than that Earth] and it's hard to visualize without some sort of powerful imaging technique."
Above: A diagram of a portion of the magnetosphere. The plasmasphere is a region circling the equatorial plane of Earth, and the plasmapause is the boundary surrounding the plasmasphere. (full image, 59KB JPEG)
So, almost four decades after his whistler discovery, Carpenter remains enthusiastic, and characterizes IMAGE and other missions as "an exciting new era of discovery."
With RPI on IMAGE, scientists will be able to reach out and probe the plasmasphere, producing pictures that reveal it and some of its secrets. "For the first time, because we can image all the time, we will be able to see it immediately when the solar wind changes the magnetosphere," said Dr. Bodo Reinisch, the RPI principal investigator at the University of Massachusetts.
RPI will work like Doppler radar, sending radio pulses into the magnetosphere, and measuring the echoes. Like Doppler radar slicing through a storm, the RPI should reveal plasmas moving into the magnetosphere from the solar wind, and how the magnetosphere reshapes itself as energy flows through it. It will even allow Carpenter to generate his own whistlers to study the fine structure of the plasmasphere.
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Unlike weather radar, the RPI won't use dish antennas. It will comprise two 500 meter (1,640 ft) antennas that are so thin they'll take a month to extend under the centripetal force produced by the rotating IMAGE spacecraft. A third 20-meter (65-ft) antenna will round out the array so return signals can be analyzed in all three dimensions.
Then the science team will go to work, trying different radio frequencies to see which ones will produce the clearest images.
"It's the first time such a radio imaging sensor has flown," Reinisch said, "so we don't know what frequencies to use."
To get an echo, the frequency will have to match the frequency of the plasma, and even then hit a flat "surface" - actually, a change in density - rather than an edge. Even so, the echoes will help scientists start to put flesh on what has been a ghostlike apparition in space.
Right: Both the northern and southern lights
are space storms of the inner magnetosphere. Here, the southern
lights were viewed and photographed by STS-39 astronauts in 1991.
A charged plasma glow around the aft end of the orbiter can also
"If we were to look down on this system as it evolves," Carpenter explained, "we'd learn great deal." That "look down" would be done by several cameras aboard IMAGE, as well as the Radio Plasma Imager, as IMAGE's lopsided orbit takes it up to 45,000 km (28,125 mi; 7 times the Earth's radius) above the Earth. As IMAGE descends across the magnetic equator, RPI will be able to take images from different angles and even inside the plasmasphere and other regions, so a complete anatomy can be drawn.
One of four "Mysteries of the Plasmasphere" that Carpenter wants to solve is the formation of the plasmapause at a new location after a substorm. "No one has ever observed the plasmapause in the process of forming," he said. "It's a transient phenomenon that has managed to evade us." The reforming comes after a storm literally strips away the outer layers and diminishes the size of the plasmasphere.
Other mysteries are:
- Why do dense "troughs" of plasma sometimes form deep within the plasmasphere?
- Why do dense plasma blobs, out near the magnetopause, last much longer than expected in the afternoon-to-dusk portion of the magnetosphere?
- Are subauroral ion drifts important in eroding the plasmapause during substorms? These fast, narrowly channeled drifts form during substorms before local midnight in regions where new plasmapause boundaries are believed to form.
Space physicists are optimistic that with better monitoring of the near-Earth environment they can answer these questions and others. As our understanding of the magnetosphere improves, so will our ability to forecast space weather and prepare for those storms that affect day-to-day life here on Earth.