It's Twins! Los Alamos Leads Team to Develop 3-D Magnetosphere Movies

Los Alamos National Laboratory
John Gustafson, 505-665-9197 / [email protected]

IT'S TWINS! LOS ALAMOS LEADS TEAM TO DEVELOP 3-D MAGNETOSPHERE MOVIES

LOS ALAMOS, N.M., Nov. 17, 1997 -- Early next century, watching the "weather" in Earth's magnetosphere could be as commonplace as watching satellite imagery of cloud fronts marching across the landscape on the evening news.

Los Alamos National Laboratory scientists are leading a national team developing a pair of unique instruments that will provide three-dimensional imagery of changing conditions in the magnetosphere, a protective envelope generated by Earth's magnetic field that repels most charged particles -- ions and electrons -- emanating from the sun and traps and circulates those able to penetrate inside this magnetic shield.

A variety of solar eruptions and space disturbances can inject particles and energy to the magnetosphere, creating conditions for storms that can wipe out satellites and even knock out electrical power grids on the ground. When the trapped particles rain down along magnetic field lines and hit the atmosphere they create the glowing, shifting aurorae.

The $18 million "TWINS" imagers funded by the National Aeronautics and Space Administration will ride on two separate satellites to create stereoscopic, three-dimensional movies of the churning activity in Earth magnetosphere.

"We believe the images obtained by TWINS will revolutionize the study of the magnetosphere," said Dave McComas, principal investigator for TWINS. "Scientists have an immense amount of data from the magnetosphere, but it's all tied to specific locations; you have to do statistical analyses to build up an average picture of the magnetosphere.

"TWINS will be the first time we'll be able to get a stereoscopic view of the overall magnetospheric behavior. Once we get that global view we expect to determine unambiguously the connections and causal relationships between activity in different regions," McComas said.

TWINS stands for Two Wide-Angle Imaging Neutral-atom Spectrometers and is a so-called "mission of opportunity," meaning NASA pays for the instruments but not the spacecraft. The instruments are scheduled to hitch rides on two non-NASA, U.S. government satellites scheduled for launch by early 2002 and by early 2004.

Using neutral atoms, which speed through the magnetosphere unimpeded by magnetic forces, to image the magnetosphere was an idea first realized by researchers at Johns Hopkins University's Applied Physics Laboratory in the late 1980s.

A major advance came with the 1996 launch of NASA's Polar satellite. A Los Alamos instrument, originally designed in the '80s and built during the early '90s, rides on Polar and has provided the first true images of the magnetosphere. Because the instrument also measures charged particles, it produces neutral atom images only when Polar is high above Earth's poles, but has proved the validity of the technique and produced several scientific discoveries.

"Around 1990 we really jumped on the bandwagon and invented a couple of technologies that let us study the range of energies appropriate for the bulk of the particles trapped in Earth's magnetosphere from any position in orbit," McComas said.

Charged particles in the magnetosphere are confined to travel along Earth's magnetic field lines and generally loop back and forth between the north and south poles along broad, arcing trajectories.

If the electric charge on the ions is neutralized, however, the particles are loosed from Earth's magnetic grip and shoot straight off through the magnetosphere. This can happen through a process called "charge exchange," in which a neutral atom at the tenous, outermost height of Earth's atmosphere collides with and gives an electron to a charged particle.

The air atom, having lost an electron, now is trapped by the magnetic field line while the neutralized ion zips off through space. If these neutral particles are captured and measured, as TWINS aims to do, they can be used to paint a picture of the populations of charged particles across different regions of the magnetosphere.

Los Alamos developed a technique for imaging neutral atoms with a wide range of energies. The technique incorporates a layer of carbon foil only a dozen or so atoms thick and microscopic gratings that let enough particles through to generate a good image but greatly suppress ultraviolet photons that would otherwise swamp the detector.

The imager starts with a collimator, a series of parallel, fan-shaped plates. These are used to reject incident charged particles and establish information about where in the instrument's field of view the neutral atom came from.

>From the collimator, the neutral atom passes through the carbon foil and causes the foil to eject an electron. The electron is used to start a clock for timing the neutral atom's passage through the instrument and also fixes a position where the atom entered.

The neutral atom next passes through a special grating formed from parallel, microscopic lines of gold-plated material, like a picket fence in shape but much thicker than the width of the slats. The openings in the grating are aligned with the collimator so a significant fraction of the neutral atoms pass through. But the vast majority of the ultraviolet light the instrument receives is eliminated as it passes through the grating.

The neutral atom finally collides with a device called a microchannel plate, which is a standard detector for particles.

Combining the time when the atom lands with the time when it entered the imager lets you calculate its velocity. Comparing the location where the atom hit the detector to the location where it passed through the foil gives information about the point on the sky from which the atom traveled. And the energy that the atom deposits into the detector when combined with its velocity provides information on whether it was hydrogen or oxygen, the most common elements found in the magnetosphere.

Los Alamos developed and tested in the laboratory a prototype instrument based on these technologies, the latest version of which is being developed for TWINS. "That really gave us the push to solve a lot of the technical issues," McComas said.

Nearly identical technology is being developed for IMAGE, another NASA mission to study the magnetosphere that is led by James Burch of the Southwest Research Institute in San Antonio. Los Alamos joined the IMAGE team after NASA made the award, and now many of the IMAGE team members have a role in TWINS.

IMAGE will provide observations of the magnetosphere from a single location at a time. The advantage of TWINS is that by combining measurements from two spacecraft it will be able to resolve sources of energetic particles in three dimensions.

"These stereo observations will provide an especially graphic means for visualizing the magnetosphere in action," McComas said. "That's why we think TWINS will be an outstanding tool for educational outreach and increasing public understanding of the connection of the magnetosphere to events on Earth."

Institutions involved in TWINS include Los Alamos, Aerospace Corporation, Applied Physics Laboratory, NASA's Goddard Space Flight Center, Southwest Research Institute, University of Southern California and West Virginia University.

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy. -30-