Bill Steigerwald / Nancy Neal-Jones<br />NASA Goddard Space Flight Center February 22, 2006<br />Phone: (301) 286 5017 / 0039<br /><br />Release 06-17<br /><br />RADIATION "SAFE ZONE" MIGRATES WITH SOLAR CYCLE<br /><br />A "safe zone" in the radiation belts surrounding Earth moves higher in<br />altitude and latitude during peaks in solar activity, according to new<br />research by a NASA-led team. The safe zone offers reduced radiation<br />intensities to any potential spacecraft that must fly in the radiation<br />belt region.<br /><br />"This new research brings us closer to understanding how a section of the<br />radiation belt disappears," said Dr. Shing Fung of NASA's Goddard Space<br />Flight Center, Greenbelt, Md. Fung is lead author of a paper on this<br />research to appear in the on-line version of Geophysical Research Letters<br />today.<br /><br />The team based its results on measurements of high-speed particles<br />(electrons), which comprise the "Van Allen radiation belt", from the<br />National Oceanic and Atmospheric Administration's series of polar-orbiting<br />meteorological spacecraft during 1978 to 1999. As the spacecraft flew in<br />their polar orbits, they detected fewer radiation belt particles at a<br />certain latitude range, indicating safe zone passages by the spacecraft.<br />The researchers compared the data taken during relatively low solar<br />activity periods, called solar minimum, to data from peak solar activity<br />periods, called solar maximum. They noticed a shift in the safe-zone<br />location towards higher latitudes, and therefore altitudes, during solar<br />maximum.<br /><br />If the radiation belts were visible, they would resemble a pair of donuts<br />around the Earth, one inside the other with the Earth in the "hole" of the<br />innermost donut. The safe zone, called the "slot region", would appear as<br />a gap between the inner and outer donut. The belts are actually comprised<br />of high-speed electrically charged particles (electrons and atomic nuclei)<br />that are trapped in the Earth's magnetic field.<br /><br />The Earth's magnetic field can be represented by lines of magnetic force<br />emerging from the South Polar region, out into space and back into the<br />North Polar region. Because radiation-belt particles are charged, their<br />motions are guided by the magnetic lines of force. Trapped particles would<br />bounce between the poles while spiraling around the field lines.<br /><br />Very Low Frequency (VLF) radio waves and background gas (plasma) are<br />also trapped in this region. Just like a prism that can bend a light beam,<br />the plasma can bend the VLF wave propagation paths, causing the waves<br />to flow along the Earth's magnetic field. VLF waves clear the safe zone by<br />interacting with the radiation belt particles, removing a little of their<br />energy and changing their direction. This lowers the place above the polar<br />regions where the particles bounce (called the mirror point). Eventually,<br />the mirror point becomes so low that it is in the Earth's atmosphere. When<br />this happens, the trapped particles collide with atmospheric particles and<br />are lost.<br /><br />According to the team, the safe zone is created in a region where<br />conditions are favorable for the VLF waves to kick the particles. Their<br />research is the first indication that the location of this region can<br />change with the solar activity cycle. The Sun goes through an 11-year<br />cycle of activity, from maximum to minimum, and back again. During solar<br />maximum, increased solar ultraviolet (UV) radiation heats the Earth's<br />upper atmosphere, the ionosphere, causing it to expand. This increases<br />the density of the plasma trapped in Earth's magnetic field.<br /><br />Favorable conditions for the VLF wave-particle interaction depend on the<br />specific combination of plasma density and magnetic field strength.<br />Although plasma density generally decreases with altitude, expansion of<br />the ionosphere during solar maximum makes the plasma denser at the safe<br />zone's solar-minimum altitude, and forces the favorable plasma density<br />for the safe zone to migrate to a higher altitude. In addition, magnetic<br />field strength also decreases with altitude. To find the favorable<br />magnetic field strength for the safe zone at higher altitudes, one would<br />have to migrate toward the poles (higher latitudes), where the magnetic<br />field lines are more concentrated and thus stronger.<br /><br />"This discovery helps narrow down the search for the primary wave-particle<br />interaction region that creates the safe zone," said Fung. "Although no<br />known spacecraft uses the safe zone extensively now, our knowledge could<br />help planning and operations of future missions that want to take<br />advantage of the zone."<br /><br />According to the researchers, their discovery was enabled by a new data<br />selection and retrieval tool developed by the team, called the<br />Magnetospheric State Query System. The research was funded by NASA and<br />the National Research Council. The team includes Fung, Dr. Xi Shao<br />(National Research Council, Washington), and Dr. Lun C. Tan (QSS Group,<br />Inc., Lanham, Md.).<br /><br />For images and more information, refer to:<br /> http://www.nasa.gov/centers/goddard/news/topstory/2006/safe_zone_shif...