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A region between radiation belts surrounding the Earth was filled with concentrated radiation during the ferocious "Halloween" solar storms of October and November 2003. The radiation surge was the most intense ever observed in this region, called the "Van Allen Radiation Belt Slot."
The Van Allen Belt Slot region is thought of as a safe zone -- prime real estate for satellites in "middle-Earth orbits," because they will be exposed to relatively small doses of radiation. The new observation reveals that the safe zone is not as benign as previously thought.
High radiation occasionally forms in the safe zone during solar storms, but usually is not intense and dissipates in a few days. This is because various radio waves generated in or carried by the Earth's electrically charged outer atmosphere, or plasmasphere, disperse radiation belts by scattering their particles into denser layers of the Earth's atmosphere.
During the Halloween solar storms, billion-ton eruptions of electrified gas from the Sun slammed into the Earth's magnetic field at millions of miles per hour. The impact generated powerful electric fields that forced much of the plasmasphere into interplanetary space, eroding it to an unprecedented degree -- to the point where it was below the safe zone. Since the plasmasphere was below the safe zone, an intense radiation belt powered by the Halloween solar storms was able to form in the region.
"The region between the Van Allen belts, normally devoid of particles, became the location of highest radiation-belt particle intensities," said Daniel Baker, director of the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder and lead author of a paper on the research to be published in the Dec. 16 issue of the journal Nature.
"Space weather matters -- we now know that no matter what orbit we choose, there is the possibility that a spacecraft could get blasted by a significant dose of radiation. We need to take this into account when designing spacecraft," said Baker. "We also need the ability to continuously monitor space weather so satellite operators can take protective measures during solar storms."
Baker reported the latest findings in a news briefing at the Fall 2004 meeting of the American Geophysical Union Dec. 13 to Dec. 17 in San Francisco.
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 Earth in the "hole" of the innermost donut. The safe zone would appear as a gap between the inner and outer donut, beginning about 4,350 miles and ending approximately 8,110 miles above the Earth's surface.
The belts are comprised of high-speed electrically charged particles (electrons and atomic nuclei) trapped in the Earth's magnetic field.
The Solar, Anomalous and Magnetospheric Particle Explorer, or SAMPEX, satellite flies through these belts, taking measurements of the particle types and their energy and abundance. SAMPEX observed the formation of the new belt in the safe zone on Oct. 31, 2003. The radiation remained intense for about two weeks and then gradually dissipated.
NASA's Imager for Aurora to Magnetopause Global Exploration, or IMAGE, satellite observed the loss of the plasmasphere during the Halloween 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 that the plasmasphere was at its lowest point on Oct. 31, 2003. The plasmasphere then 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, or SwRI, in San Antonio, a co-author on the paper. "Radiation in the safe zone remained strong for a few weeks - a lot longer than usual."
"This was an extreme event -- a natural experiment that will be used to better understand how radiation belts work," said Baker. "We were fortunate to have a suite of spacecraft in place to observe this event. This is why it's important to systematically and continuously observe space weather, because there is always the potential to be surprised by nature."
The research was funded by NASA and used theory, modeling and observations from work sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration. The research team includes Baker and CU-Boulder's Shri Kanekal, Xinlin Li and Steven Monk as well as Goldstein and James Burch of SwRI.
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