Air Force to loft low-cost probe to study northern lights

Technicians at Vandenberg Air Force Base in California are preparing to launch an economy satellite with a first-class mission. It's called HILAT, for high altitude research. It will enable meteorologists to explore the zone of the aurora borealis (northern lights) and the thin atmosphere of electrically charged particles above it. It's equipped to beam down the first television views of the entire ring of auroral light.

It also will help its sponsor, the US Defense Nuclear Agency (DNA), study the atmospheric effects of high-altitude nuclear bursts without having to explode a weapon. And if it successfully completes its nominal three-year mission, HILAT is likely to please administration budget managers by returning a wealth of scientific data at minimal cost.

HILAT is a modified navigation satellite - a leftover from the old US Navy Transit program. Developed in the late 1950s and early 1960s, the Transits gave Polaris missile submarines accurate navigational data. Mission scientist Thomas A. Potemra says that by using a spare satellite and other surplus equipment, such as radio receivers, a project such as HILAT costs ''perhaps . . . a hundredth of what it would cost'' otherwise.

Dr. Potemra is with the Applied Physics Laboratory of the Johns Hopkins University, which developed the Transit satellites. HILAT is the 48th satellite the APL has built and the second of the Transit family adapted to explore what meteorologists call the ionosphere. This is a region far above the stratosphere where the sun's ultraviolet (UV) radiation strips electrons from oxygen, helium, hydrogen, and other atoms and molecules in the air. This forms a mix of positively charged particles and negatively charged electrons called a plasma. The UV is said to ''ionize'' the atoms and molecules, hence the name ionosphere.

The region extends from a height of about 60 kilometers to 1,000 kilometers or more. Aurorae glow in its lower part. Radio waves used for long-distance communication are reflected from the electrically conducting plasma. But turbulence and other plasma disturbances can disrupt the messages.

Scientists at the APL, together with colleagues at other institutions, began using Transit family satellites to probe the ionosphere more than a decade ago. At first, this was a byproduct of the satellite's navigational mission. For example, the Triad satellite launched in September 1972 carried a magnetometer as routine equipment. The instrument's sensitivity was increased a thousand-fold for scientific work. This led to what Potemra calls ''a major discovery'' of the past decade - the existence of a vast cone-shaped sheath of electric currents flowing from space into and out of the auroral zone.

The sheath, with an area on the order of a million square miles, carries currents of 1 million to 10 million amperes. The region can dissipate enormous power - as much as a billion kilowatts. With such energy being released, Potemra says that this is the only place on Earth to study what amounts to a simulated nuclear burst as far as atmospheric effects are concerned. Even then, the simulation is puny - equivalent only to a 50-kiloton bomb. The DNA hopes to be able to extend its findings theoretically to estimate what the effect of a much larger weapon would be.

The DNA is also concerned about radio transmissions. In May 1976, a modified Transit called the Wideband Satellite was launched into a near-polar orbit 1,000 kilometers high. Its investigations began a detailed probing of the ionospheric weather that affects radio waves. Now HILAT will continue this kind of study. It is scheduled to be launched in late June into an 830-kilometer orbit that is inclined 82.2 degrees to the equator - not quite going over the poles.

While HILAT makes direct measurements of magnetic fields, electric currents, and the plasma through which it travels, ground-based radars will probe the plasma structure. However, HILAT's most dramatic result should be its TV pictures.

The auroral glow is hard to see in daylight because it is swamped by scattered sunlight. HILAT's TV will operate at UV wavelengths, where the aurora should always be visible.

Although HILAT, like Triad and Wideband, is a military-funded project, it is not secret. Most of its data will be freely available. Scientists from a number of other institutions, including the Air Force, are joining the APL team. Data will be collected in Canada, Greenland, and Norway as well.

''It's a military program,'' Potemra says. ''But it enables us to do a lot of good science, too.''

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