An artist's concept of an Iridium satellite. Motorola

Researchers at The Johns Hopkins University Applied Physics Laboratory, Laurel, Md., (APL) are now able to simultaneously measure the magnetic and electrical fields over large areas of the ionosphere above the Earth's polar regions, providing the first continuous monitoring of electric currents between space and the upper atmosphere and generating the first maps of electric power flowing into the polar upper atmosphere. These advances will allow greatly improved understanding and forecasting of global space weather and help prevent disruption of communication and power systems when electromagnetic storms strike the nation.

The work, sponsored by the National Science Foundation, makes use of magnetometers carried on each of the 66 satellites of the Iridium System satellite constellation operating as a global satellite communications network. Circling the globe in 470-mile-high, polar orbits, they are providing continuous measurements of the magnetic fields above the Earth's poles. Scientists at JHU/APL have developed techniques to extract the signatures of electrical currents flowing between the atmosphere and space from the magnetic field readings. Maps of the electric current in space are then constructed in much the same way that normal weather maps are made from weather station readings.

At the same time, SuperDARN - the Super Dual Auroral Radar Network, a multinational network of a dozen radars spread around the poles to study the ionosphere, sponsored by NSF and NASA and led by APL scientist Dr. Raymond A. Greenwald - is bouncing radar signals off the same regions to measure the electric field and its minute-by-minute variations.

An illustration of the Iridium satellite constellation, which features 66 primary spacecraft in low-Earth orbit. Motorola

"By combining Iridium System and SuperDARN data, we're able for the first time to continuously map the powerful currents flowing between space and the Earth's upper atmosphere," says Brian J. Anderson, who leads APL's research effort. "This is a major achievement because monitoring this environment is extremely difficult due to its enormous volume, which can vary by a factor of 10 in one hour. The Iridium orbits are ideal for monitoring this big system because the current is funneled to the polar regions, where the satellites detect it."

Based on extensive experience working with magnetic field data from satellites, APL scientists were able to develop sophisticated signal processing techniques for automatically extracting needed signals from Iridium data so they could be combined in a useful way with SuperDARN data. "This was an essential part of the effort," says Anderson. "With so many satellites involved, any hands-on analysis of the data would have been impossible."

The maps of electrical current show dramatic shifts due to changes in the solar wind. These results will allow scientists to test computer models of Earth's space environment far more accurately and exhaustively than ever before. Preliminary maps of the power flow have revealed "hot spots" of energy flowing into the atmosphere at high altitudes, creating pockets of hot air that rise and create drag on spacecraft flying through them at altitudes below 300 miles.

"Timely, accurate space weather forecasts will give advance warning of electromagnetic storms that in the past have shown their ability to disrupt communications, degrade GPS accuracy, cripple electrical power grids, and menace astronauts, satellites and aircraft with dangerous levels of radiation," says Anderson.

Anderson presented his findings at the 2000 Fall Meeting of the American Geophysical Union in San Francisco.