Two giant donuts of charged particles called the Van Allen Belts surround Earth. Credit: NASA/T. Benesch, J. Carns
day-to-day lives exist in what physicists would call an electrically
neutral environment. Desks, books, chairs and bodies don’t generally
carry electricity and they don’t stick to magnets. But life on Earth is
substantially different from, well, almost everywhere else. Beyond
Earth’s protective atmosphere and extending all the way through
interplanetary space, electrified particles dominate the scene. Indeed,
99% of the universe is made of this electrified gas, known as plasma.
giant donuts of this plasma surround Earth, trapped within a region
known as the Van Allen Radiation Belts. The belts lie close to Earth,
sandwiched between satellites in geostationary orbit above and
satellites in low Earth orbit (LEO) are generally below the belts. A new
NASA mission called the Radiation Belt Storm Probes (RBSP), due to
launch in August 2012, will improve our understanding of what makes
plasma move in and out of these electrified belts wrapped around our
discovered the radiation belts in observations from the very first
spacecraft, Explorer 1, in 1958” says David Sibeck, a space scientist at
NASA’s Goddard Space Flight Center in Greenbelt, Md., and the mission
scientist for RBSP. “Characterizing these belts filled with dangerous
particles was a great success of the early space age, but those
observations led to as many questions as answers. These are fascinating
science questions, but also practical questions, since we need to
protect satellites from the radiation in the belts.”
inner radiation belt stays largely stable, but the number of particles
in the outer one can swell 100 times or more, easily encompassing a
horde of communications satellites and research instruments orbiting
Earth. Figuring out what drives these changes in the belts, requires
understanding what drives the plasma.
seethe with complex movement. They generally flow along a skeletal
structure made of invisible magnetic field lines, while simultaneously
creating more magnetic fields as they move. Teasing out the rules that
govern such a foreign environment – one that can only be studied from
afar – lies at the heart of understanding a range of events that make up
space weather, from giant explosions on the sun to potentially damaging
high energy particles in near-Earth environs.
distinguish between a host of theories developed over the years on
plasma movement in those near-Earth environs, RBSP scientists have
designed a suite of instruments to answer three broad questions. Where
do the extra energy and particles come from? Where do they disappear to,
and what sends them on their way? How do these changes affect the rest
of Earth’s magnetic environment, the magnetosphere? In addition to its
broad range of instruments, the RBSP mission will make use of two
spacecraft in order to better map out the full spatial dimensions of a
particular event and how it changes over time.
want to understand not only the origins of electrified
particles—possibly from the solar wind constantly streaming off the sun;
possibly from an area of Earth’s own outer atmosphere, the
ionosphere—but also what mechanisms gives the particles their extreme
speed and energy.
know examples where a storm of incoming particles from the sun can
cause the two belts to swell so much that they merge and appear to form a
single belt,” says Shri Kanekal, RBSP’s deputy project scientist at
Goddard. “Then there are other examples where a large storm from the sun
didn’t affect the belts at all, and even cases where the belts shrank.
Since the effects can be so different, there is a joke within the
community that ‘If you’ve seen one storm . . . You’ve seen one storm.’
We need to figure out what causes the differences.”
are two broad theories on how the particles get energy: from radial
transport or in situ. In radial transport, particles move perpendicular
to the magnetic fields within the belts from areas of low magnetic
strength far from Earth to areas of high magnetic strength nearer Earth.
The laws of physics dictate that particle energies correlate to the
strength of the magnetic field, increasing as they move towards Earth.
The in situ theory posits that electromagnetic waves buffet the
particles—much like regular pushes on a swing—successively raising their
speed (and energy).
An artist’s rendition of what the two Radiation Belt Storm Probe spacecraft will look like in space. Credit: NASA/Goddard Space Flight Center
for how the particles leave the belts, scientists again agree on two
broad possibilities: particles go up, or they go down. Perhaps they
travel down magnetic field lines toward Earth, out of the belts into the
ionosphere, where they stay part of Earth’s magnetic system with the
potential to return to the belts at some point. Or they are transported
up and out, on a one-way trip to leave the magnetosphere forever and
enter interplanetary space.
reality, the final answers may well be a combination of the basic
possibilities,” says Sibeck. “There may be, and probably are, multiple
processes at multiple scales at multiple locations. So RBSP will perform
very broad measurements and observe numerous attributes of waves and
particles to see how each event influences others.”
distinguish between the wide array of potential theories—not to mention
combinations thereof—the instruments on RBSP will be equipped to
measure a wide spectrum of information. RBSP will measure a host of
different particles, including hydrogen, helium and oxygen, as well as
measure magnetic fields and electric fields throughout the belts, both
of which can guide the movement of these particles.
will also measure a wide range of energies from the coldest particles
in the ionosphere to the most energetic, most dangerous particles.
Information about how the radiation belts swell and shrink will help
improve models of Earth’s magnetosphere as a whole.
from the radiation belts can penetrate into spacecraft and disrupt
electronics, short circuits or upset memory on computers,” says Sibeck.
“The particles are also dangerous to astronauts traveling through the
region. We need models to help predict hazardous events in the belts and
right now we are aren’t very good at that. RBSP will help solve that
the most immediate practical need for studying the radiation belts is
to understand the space weather system near Earth and to protect humans
and precious electronics in space from geomagnetic storms, there is
another reason scientists are interested in this area. It is the closest
place to study the material, plasma, that pervades the entire universe.
Understanding this environment so foreign to our own is crucial to
understanding the make up of every star and galaxy in outer space.
Johns Hopkins University Applied Physics Laboratory (APL) built and
will operate the twin RBSP spacecraft for NASA’s Living With a Star
program, which is managed by Goddard Space Flight Center for NASA’s
Science Mission Directorate.
Source: NASA Goddard Space Flight Center