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Radiation "Safe Zone" Migrates During Solar Cycle
A "safe zone" in the radiation belts surrounding Earth moves higher in altitude and latitude during peaks in solar activity, according to new research by a NASA-led team. The safe zone offers reduced radiation intensities to any potential spacecraft that must fly in the radiation belt region.

"This new research brings us closer to understanding how a section of the radiation belt disappears," said Dr. Shing Fung of NASA's Goddard Space Flight Center, Greenbelt, Md. Fung is lead author of a paper on this research appearing in the on-line version of Geophysical Research Letters February 22.

Artist's concept of radiation belts and safe zone Image right: Artist's concept of the Earth's radiation belts and safe zone. The two donut shapes surrounding the Earth represent the Van Allen radiation belts. In the image, the front half of the radiation belts has been cut away to show detail. The safe zone appears as a gap between the inner and outer 'donut,' beginning about 7,000 km (4,350 miles) and ending about 13,000 km (8,110 miles) above the Earth's surface. Click on image for movie (no audio). Credit: NASA/Walt Feimer

The team based its results on measurements of high-speed particles (electrons), which comprise the "Van Allen radiation belt", from the National Oceanic and Atmospheric Administration's series of polar-orbiting meteorological spacecraft during 1978 to 1999. As the spacecraft flew in their polar orbits, they detected fewer radiation belt particles at a certain latitude range, indicating safe zone passages by the spacecraft. The researchers compared the data taken during relatively low solar activity periods, called solar minimum, to data from peak solar activity periods, called solar maximum. They noticed a shift in the safe-zone location towards higher latitudes, and therefore altitudes, during solar maximum.

If the radiation belts were visible, 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, called the "slot region", would appear as a gap between the inner and outer donut. The belts are actually comprised of high-speed electrically charged particles (electrons and atomic nuclei) that are trapped in the Earth's magnetic field.

Computer graphic of data showing safe zone Image left: This is a still image from a data-based visualization showing the Van Allen Belts pulsing from solar particles over ten days. Purple and white areas represent the radiation belts. The gap that appears between the inner and outer belts shows a cleared-out safe zone for satellites. The red ring represents the orbit of the IMAGE satellite, which dips into the safe zone every few days. Credit: NASA/Tom Bridgman Print-resolution copy (1 meg tif image)

The Earth's magnetic field can be represented by lines of magnetic force emerging from the South Polar region, out into space and back into the North Polar region. Because radiation-belt particles are charged, their motions are guided by the magnetic lines of force. Trapped particles would bounce between the poles while spiraling around the field lines.

Very Low Frequency (VLF) radio waves and background gas (plasma) are also trapped in this region. Just like a prism that can bend a light beam, the plasma can bend the VLF wave propagation paths, causing the waves to flow along the Earth's magnetic field. VLF waves clear the safe zone by interacting with the radiation belt particles, removing a little of their energy and changing their direction. This lowers the place above the polar regions where the particles bounce (called the mirror point). Eventually, the mirror point becomes so low that it is in the Earth's atmosphere. When this happens, the trapped particles collide with atmospheric particles and are lost.

Computer animation of radio waves clearing safe zone Image right: This is an artist's concept illustrating the theory that radio waves from lightning clear the safe zone. Without lightning's cleansing effect producing this protective safe zone in Earth's radiation belts, it would be dangerous for satellites. The camera pulls up through the clouds to a view from space. Radio waves from lightning (white glow) travel along the Earth's magnetic field and intercept particles in the safe zone region of the Van Allen Belts (red spiral). The radio waves deflect particles there, causing them to stream down the magnetic field line (blue) and impact Earth's upper atmosphere. This process occurs again until the safe zone is clear (represented by a dimming of the red spiral). Click on image for mpg format movie (no audio). Credit: NASA/Walt Feimer.

According to the team, the safe zone is created in a region where conditions are favorable for the VLF waves to kick the particles. Their research is the first indication that the location of this region can change with the solar activity cycle. The Sun goes through an 11-year cycle of activity, from maximum to minimum, and back again. During solar maximum, increased solar ultraviolet (UV) radiation heats the Earth's upper atmosphere, the ionosphere, causing it to expand. This increases the density of the plasma trapped in Earth's magnetic field.

Favorable conditions for the VLF wave-particle interaction depend on the specific combination of plasma density and magnetic field strength. Although plasma density generally decreases with altitude, expansion of the ionosphere during solar maximum makes the plasma denser at the safe zone's solar-minimum altitude, and forces the favorable plasma density for the safe zone to migrate to a higher altitude. In addition, magnetic field strength also decreases with altitude. To find the favorable magnetic field strength for the safe zone at higher altitudes, one would have to migrate toward the poles (higher latitudes), where the magnetic field lines are more concentrated and thus stronger.

Diagram of safe zone shift Image left: Migration of the radiation-belt safe zone occurs because the optimum wave-particle interaction region responsible for particle losses moves from its solar-minimum location (A) to a corresponding location (B) during solar maximum. The blue (dashed) and red (dotted) circles around Earth indicate schematically the altitudes of the same ionospheric density during a solar minimum and a solar maximum, respectively. Correspondingly, the safe zone position changes from closer to Earth (shaded blue) during a solar minimum to farther away from Earth (shaded red) during a solar maximum. Print-resolution copy (250 K JPG image) Credit: NASA/Shing Fung, Rob Kilgore

"This discovery helps narrow down the search for the primary wave-particle interaction region that creates the safe zone," said Fung. "Although no known spacecraft uses the safe zone extensively now, our knowledge could help planning and operations of future missions that want to take advantage of the zone."

Artist's concept of safe zone shift Image right: The video sequence shows the creation of the safe zone by scattering of trapped particles (green) by VLF waves (yellow), both bouncing along the Earth's magnetic field lines. The wave-particle interaction regions (red), characterized by a specific combination of plasma density and magnetic field strength, change locations over the solar cycle, causing the apparent migration of the radiation belt slot. Larger copy of still image Credit: NASA/Shing Fung, Vivek Dwivedi

According to the researchers, their discovery was enabled by a new data selection and retrieval tool developed by the team, called the Magnetospheric State Query System. The research was funded by NASA and the National Research Council. The team includes Fung, Dr. Xi Shao (National Research Council, Washington), and Dr. Lun C. Tan (QSS Group, Inc., Lanham, Md.).


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Bill Steigerwald
NASA Goddard Space Flight Center