Mapping stormy weather in the ionosphere.
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 Haystack 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, Evgeny D. Tereshchenko of the Polar Geophysical Institute 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 to monitor signals from radio beacons aboard the Russian Cicada 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 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.