Pinning down the energy flows that drive space weather

For space physicist Brian Anderson, the Iridium communications satellite system has become a powerful research tool. But he isn't making phone calls.

His research team has found a way to use the satellites to track the electric power that drives the space weather surrounding Earth.

This energy flow lights auroras and kicks up magnetic storms that disrupt communications. Satellite and ground-based sensors see the auroral light. They track particles, X-rays, and other emissions from the electrified gas that swirls through the region where Earth's outer atmosphere meets interplanetary space. But they can't detect the underlying electric currents or energy flows. This has hampered efforts to understand and forecast space weather that can disrupt the electronic infrastructure on which the world increasingly depends.

Dr. Anderson says his team at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., now has learned how to use magnetic sensors on Iridium satellites to "make what is invisible, visible." These record the magnetic fields that invisible electric currents generate. Data on those magnetic fields, combined with data on electric forces already available from ground-based radar, let the Hopkins researchers calculate invisible electric power.

Explaining this last week during a meeting of the American Geophysical Union, Anderson said the satellites finally make it possible to keep tabs on space weather the way meteorologists track atmospheric weather closer to the ground. The 66 satellites traveling on polar orbits amount to a weather observing system in near Earth space. They already have produced the first global maps of electric currents and energy flows through the outer atmosphere around the polar regions. Robert Robinson from the National Science Foundation in Arlington, Va., says this hasn't come too soon. He notes that "space weather forecasting is where ordinary weather forecasting was a century ago." Yet our electronically-dependent world needs better forecasting now.

Space weather storms rage in what scientists call the magnetosphere, a region from the outer atmosphere to the zone where Earth's magnetic field forms a shield against the solar wind. This is a "wind" of electrically charged particles laced with magnetic fields, flowing from the sun. It blows against Earth at a million miles an hour, generating voltages of 200,000 volts. Sometimes, it contains massive clouds of particles that push back the magnetic shield and even penetrate it. The weather this roiling activity stirs up in the magnetosphere can induce electric currents that knock out power lines on Earth and upset satellites. It can heat and expand the thin outer atmosphere so it drags on low orbiting satellites. The Hopkins studies show electric power concentrating in narrow hot spots which expand the atmosphere locally in ways not previously suspected.

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