Mapping stormy weather in the ionosphere.
Solar outbursts can roil Earth's ionosphere ionosphere (īŏn`əsfēr), series of concentric ionized layers forming part of the upper atmosphere of the earth from around 30 to 50 mi (50 to 80 km) to 250 to 370 mi (400 to 600 km) where it merges with the magnetosphere, the region , rapidly changing the distribution of electric charge at high altitudes Conventionally, an altitude above 10,000 meters (33,000 feet). See also altitude. . In turn, these disturbances can damage orbiting satellites, disrupt radio communications, and cause harmful surges in electric power lines.
Although researchers have long used radar to study storms and other features of the ionosphere, they have lacked the tools needed to monitor and map it on a regular basis. A recent experiment has now furnished important data that will help scientists develop and refine a novel technique for producing global "weather" maps of the ionosphere.
Known as radio tomography, the technique involves the reconstruction of the three-dimensional distribution of ionospheric electric charge from its effect on radio signals sent from orbiting navigation satellites to ground-based receivers. It requires the same kind of mathematics used in medical tomography to construct three-dimensional X-ray images of biological tissue.
"Radar systems are too costly to build and run to do long-term, global monitoring of the upper atmosphere," says John C. Foster of the MIT MIT - Massachusetts Institute of Technology Haystack Observatory Haystack Observatory is a group of astronomical observatories owned and operated by Massachusetts Institute of Technology. It is located in Westford, Massachusetts (USA). It is the home of the Millstone Hill Observatory. in Westford, Mass. Radio tomography offers a relatively inexpensive way of achieving such coverage.
To test the validity of ionospheric radio tomography, Foster, Vyatcheslav E. Kunitsyn of Moscow State University Moscow State University, at Moscow, Russia, officially M. V. Lomonosov Moscow State Univ.; founded 1755 as Moscow Univ. by the Russian scientist M. V. Lomonosov, renamed Moscow State Univ. after the Russian Revolution, and renamed after its founder in 1940. , Evgeny D. Tereshchenko of the Polar Geophysical Institute The Geophysical Institute of the University of Alaska Fairbanks conducts research into space physics and aeronomy; atmospheric sciences; snow, ice, and permafrost; seismology; volcanology; and tectonics and sedimentation. It was founded in 1946 by the United States Congress. in Murmansk, Russia, and their coworkers set up a joint experiment to see how well tomographic reconstructions stack up against radar measurements. The project, called the Russian-American Tomography Experiment (RATE), also allowed them to compare rival mathematical schemes for constructing the images.
Over a 10-day period beginning Oct. 29, 1993, the researchers used pairs of portable radio receivers at various locations in North America North America, third largest continent (1990 est. pop. 365,000,000), c.9,400,000 sq mi (24,346,000 sq km), the northern of the two continents of the Western Hemisphere. to monitor signals from radio beacons aboard the Russian Cicada cicada (sĭkā`də), large, noise-producing insect of the order Homoptera, with a stout body, a wide, blunt head, protruding eyes, and two pairs of membranous wings. and U.S. Transit satellites. This period coincided with a severe ionospheric storm caused by a "solar bullet" of charged particles emitted by the sun.
"We predicted this event 3 or 4 months before it took place, so we scheduled our experiment to bracket it," Foster says. "We needed ionospheric structure -- a real storm -- to really test the method, and we got it."
Using computers, researchers created tomographic images of the ionosphere, which they compared with radar measurements made at the same time as the radio observations (see illustration). In general, the best mathematical algorithms presently available picked up the same ionospheric features seen in the radar data, though not the details.
"The tomographic algorithms developed by Kunitsyn and the Moscow group are the most sophisticated and most accurate I've seen," Foster contends. He and his colleagues describe the RATE project in the recently released Summer 1994 INTERNATIONAL JOURNAL OF IMAGING SYSTEMS AND TECHNOLOGY.
Several groups in the United States United States, officially United States of America, republic (2005 est. pop. 295,734,000), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area. and elsewhere are working to develop computerized radio tomography into a viable method of mapping the ionosphere. At the same time, scientists studying the ionosphere have an unusually well documented storm to ponder.