A NASA-funded study found a region between radiation belts
surrounding the Earth is not as benign as once thought. The region
was considered a safe zone for satellites in “Middle Earth”
orbits, because of relatively small amounts of radiation.

The observations revealed the Van Allen Radiation Belt Slot, often
considered a safe zone, filled with concentrated radiation during
the ferocious solar storms of October and November 2003. The
radiation surge was the most intense ever observed in this region,
according to researchers.

“Space weather matters. We know that no matter what orbit we
choose, there is the possibility a spacecraft could get blasted by
a significant dose of radiation,” said Daniel Baker, Director of
the Laboratory for Atmospheric and Space Physics at the University
of Colorado, Boulder, and lead author of the study. “We need to
take space weather into account when designing spacecraft. We also
need the ability to continuously monitor space weather, so
satellite operators can take protective measures during solar
storms,” he said.

High radiation occasionally forms in the safe zone during solar
storms. This radiation is usually not intense and dissipates in a
few days. During the 2003 solar storms, billion-ton eruptions of
electrified gas slammed into the Earth’s magnetic field at
millions of miles per hour. This generated powerful electric
fields that forced much of the plasmasphere into interplanetary
space. It was eroded to an unprecedented degree, to the point
below the safe zone. Since the plasmasphere was below the safe
zone, an intense radiation belt powered by the solar storms formed
in the region.

If the Van Allen radiation belts were visible from space, they
would resemble a pair of donuts around the Earth, one inside the
other with the planet in the hole of the innermost one. The safe
zone would appear as a gap between the inner and outer donut,
beginning about 7,000 km (4,350 miles) and ending approximately
13,000 km (8,110 miles) above the Earth’s surface. The belts are
comprised of high-speed electrically charged electrons and atomic
nuclei trapped in the Earth’s magnetic field.

The Solar, Anomalous and Magnetospheric Particle Explorer (SAMPEX)
satellite flies through these belts, taking measurements of the
particle types, their energy and abundance. SAMPEX observed the
formation of the new belt in the safe zone on October 31, 2003.
The radiation remained intense for about two weeks, and then
gradually dissipated.

NASA’s Imager for Aurora to Magnetopause Global Exploration
(IMAGE) satellite observed the loss of the plasmasphere during the
solar storms. Since solar ultraviolet radiation energizes helium
ions in the plasmasphere, causing them to glow in ultraviolet
light, the Extreme Ultraviolet Imager instrument on IMAGE can
record the extent of the plasmasphere by observing this glow.

IMAGE discovered the plasmasphere was at its lowest point on
October 31, 2003. The plasmasphere slowly expanded past the safe
zone, replenished by particles from Earth’s upper atmosphere.
After its expansion beyond the safe zone, SAMPEX observed the
dissipation of the new radiation belt.

“We were surprised to see the radiation belt persist so long after
the plasmasphere expanded past the safe zone,” said Jerry
Goldstein of the Southwest Research Institute (SwRI), San Antonio,
a co-author of the paper. “Radiation in the safe zone remained
strong for a few weeks, a lot longer than usual,” he said.

The research used theory, modeling, and observations from work
sponsored by the National Science Foundation and the National
Oceanic and Atmospheric Administration. The research team included
Drs. Shri Kanekal, Xinlin Li, and Steven Monk also of the
University of Colorado and Dr. James Burch of SwRI.

The research data was presented today during the American
Geophysical Union Fall meeting in San Francisco, and it appears
tomorrow in the journal Nature. For more information about the
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