PHOTO:Edward Kennedy/ U.S. Naval Research Laboratory

24 October 2007—Ever since the Van Allen radiation belts were discovered, the U.S. armed forces have been interested in understanding—and maybe even controlling—how the belts influence wireless communication. For example, the U.S. Air Force, wanting to keep in touch with airborne fighter pilots at all times, would like to understand exactly how geomagnetic storms in the atmosphere will cause disruptions. Today, the armed forces are sponsoring two big experiments to gain more knowledge about the Earth's ionosphere.

The first of them is the High Frequency Active Auroral Research Program (HAARP), located in Gakona, Alaska, about 300 kilometers from Fairbanks.

Built on an old Cold War site meant to house an over-the-horizon radar, HAARP's main job is to produce radio waves to probe the ionosphere. Gakona is a particularly interesting location for HAARP, because “the Earth's magnetic field lines come down to Earth there,” says Paul Kossey, HAARP's program manager for the Air Force Research Laboratory.

One of the chief instruments at HAARP is a multimegawatt radio transmitter operating in the high-frequency (HF) range, known as the Ionospheric Research Instrument, which reached full power only last March.

The idea is to beam radio signals into the ionosphere and thereby stimulate or heat small, well-defined volumes of ionosphere. Back on the ground, an array of geophysical research instruments—such as low-frequency receivers, magnetometers, an ultrahigh-frequency (UHF) diagnostic radar, optical, and infrared spectrometers and cameras—try to see what happens to the ionosphere as a result of these signals.

“A lot of things we are doing are to mimic natural processes in a controlled fashion,” says Kossey. He says that the transmitter would be able to radiate about 3 to 4 megawatts—“about three times the power of Radio Moscow or Voice of America.” In the future, HAARP scientists hope to complete a UHF radar to allow measurement of electron and ion temperatures and electron densities, which are important to understanding the origins of satellite-damaging so-called killer electrons.

The other Air Force Research Lab ionosphere experiment is a spacecraft called the Demonstration and Science Experiments (DSX) satellite, which is set to launch in 2009. DSX is designed primarily to investigate the sometimes harsh radiation that environment satellites are subject to in a medium Earth orbit.

The satellite will also have an instrument designed to monitor very-low-frequency (VLF) transmissions in the magnetosphere—the magnetic shell surrounding the Earth—and will explore whether natural and man-made VLF waves, including those from HAARP, can reduce satellite-damaging space radiation. Several years ago, Stanford University electrical engineering professor Umran S. Inan theorized that low-frequency electromagnetic radiation injected into the lower Van Allen belt could cause the high-energy electrons there to prematurely rain out into the atmosphere, potentially ending a monthlong geomagnetic storm in a matter of days.

Lieutenant Colonel Jon Schoenberg, program manager for DSX, says that the satellite will be outfitted with instruments that measure electron energy over a wide range (20 electronvolts to 200 megaelectronvolts). The satellite will also test advanced electronics, solar panels, optical coatings, materials, and other components to see how they fare in such a harsh environment, so engineers will know whether to use them in future generations of spacecraft.

Schoenberg says that DSX's particular specialty will be the regions between the inner and outer Van Allen belts. The compact inner belt (discovered by a Geiger counter that was added to a U.S. satellite by University of Iowa physicist James Van Allen in 1958) lies 700 to 10 000 kilometers above the equator and is believed to be the by-product of cosmic radiation. In it are mostly energetic protons that can damage spacecraft instruments and human tissue; spacecraft tend to avoid it. The outer belt is thought to be caused by plasma trapped by the Earth's magnetosphere. It stretches from about 13 000 to 65 000 kilometers above the planet's surface. There are more charged particles in this belt, but most of them tend to be relatively low energy, although some high-energy electrons may also be found. Most of the magnetic storms that disrupt communications on Earth happen in this region. Scientists have also found that the regions between the belts are host to interesting and little understood physical phenomena.

“These regions fill with energetic particles during [magnetic] storms,” Schoenberg says, and the particles only gradually dissipate. “The storms are not easily predicted, and having a spacecraft in this region will help us understand what is going on.”