Radiation "safe zone" migrates with solar cycle (Forwarded)

Bill Steigerwald / Nancy Neal-Jones
NASA Goddard Space Flight Center February 22, 2006
Phone: (301) 286 5017 / 0039

Release 06-17

RADIATION "SAFE ZONE" MIGRATES WITH 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 to appear in the on-line version of Geophysical Research Letters
today.

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.

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.

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.

"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."

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.).

For images and more information, refer to:
http://www.nasa.gov/centers/goddard/...one_shift.html