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NASA Scientists Build First-Ever Wide-Field X-ray Imager

  Three NASA scientists teamed up to develop and demonstrate NASA's first wide field-of-view soft X-ray camera for studying "charge exchange," a poorly understood phenomenon that occurs when the solar wind collides with Earth's exosphere and neutral gas in interplanetary space.

The unique collaboration involved heliophysics, astrophysics, and planetary science divisions at NASA's Goddard Space Flight Center in Greenbelt, MD, and resulted in the first successful demonstration of the Sheath Transport Observer for the Redistribution of Mass (STORM) instrument and a never-before-flown X-ray focusing technology called lobster-eye optics.

Scientists first discovered the charge-exchange effect in the mid-1990s while observing comet Hyakutake. They found an intense source of soft X-rays at the comet's head, which was unusual because comets are cold objects and soft X-rays are associated with hot objects. How could balls of ice emit X-rays?

Scientists soon discovered that the X-ray emission was caused by the solar wind, a constantly flowing stream of charged particles that sweeps across the solar system at about a million miles per hour. When highly charged heavy ions in the solar wind collide with neutral atoms found in space, the heavy ions "steal" an electron from the neutrals -- an exchange that puts the heavy ions in a short-lived excited state. As they relax, they emit soft X-rays.

STORM potentially holds the answer for obtaining a more complete understanding of the physical process, giving scientists insights currently impossible with existing instruments, the scientists said. STORM gave scientists a global view. The wide-field-of-view camera imaged processes near Earth's magnetosphere, which until now was impossible.

Making the imagery possible was an emerging technology called lobster-eye optics. As the technology's name implies, the optics mimic the structure of a lobster's eyes, which are made up of long, narrow cells that each captures a tiny amount of light, but from many different angles. Only then is the light focused into a single image.

Pioneered by researchers at the United Kingdom's University of Leicester, a partner in STORM's development, lobster X-ray optics work the same way. Its eyes are a microchannel plate, a thin curved slab of material dotted with tiny tubes across the surface. X-ray light enters these tubes from multiple angles and is focused through grazing-incident reflection, giving the technology a wide-field-of-view necessary for globally imaging the emission of soft X-rays in Earth's exosphere.

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