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September
29, 2009: Planning a trip to Mars? Take plenty of
shielding. According to sensors on NASA's ACE (Advanced Composition
Explorer) spacecraft, galactic cosmic rays have just hit a
Space Age high.
"In
2009, cosmic ray intensities have increased 19% beyond anything
we've seen in the past 50 years," says Richard Mewaldt
of Caltech. "The increase is significant, and it could
mean we need to re-think how much radiation shielding astronauts
take with them on deep-space missions."
Above: Energetic iron nuclei counted by the
Cosmic Ray Isotope Spectrometer on NASA's ACE spacecraft reveal
that cosmic ray levels have jumped 19% above the previous
Space Age high. [larger
image]
The cause of the surge is solar
minimum, a deep lull in solar activity that began around 2007
and continues today. Researchers have long known that cosmic
rays go up when solar activity goes down. Right now solar
activity is as weak as it has been in modern times, setting
the stage for what Mewaldt calls "a perfect storm of
cosmic rays."
"We're
experiencing the deepest solar minimum in nearly a century,"
says Dean Pesnell of the Goddard Space Flight Center, "so
it is no surprise that cosmic rays are at record levels for
the Space Age."
Galactic
cosmic rays come from outside the solar system. They are subatomic
particles--mainly protons but also some heavy nuclei--accelerated
to almost light speed by distant supernova explosions. Cosmic
rays cause "air showers" of secondary particles
when they hit Earth's atmosphere; they pose a health hazard
to astronauts; and a single cosmic ray can disable a satellite
if it hits an unlucky integrated circuit.
The
sun's magnetic field is our first line of defense against
these highly-charged, energetic particles. The entire solar
system from Mercury to Pluto and beyond is surrounded by a
bubble of magnetism called "the heliosphere." It
springs from the sun's inner magnetic dynamo and is inflated
to gargantuan proportions by the solar wind. When a cosmic
ray tries to enter the solar system, it must fight through
the heliosphere's outer layers; and if it makes it inside,
there is a thicket of magnetic fields waiting to scatter and
deflect the intruder.
 Right:
An artist's concept of the heliosphere, a magnetic bubble
that partially protects the solar system from cosmic rays.
[larger
image]
"At
times of low solar activity, this natural shielding is weakened,
and more cosmic rays are able to reach the inner solar system,"
explains Pesnell.
Mewaldt
lists three aspects of the current solar minimum that are
combining to create the perfect storm:
1.
The sun's magnetic field is weak. "There has
been a sharp decline in the sun's interplanetary magnetic
field down to 4 nT (nanoTesla) from typical values of 6 to
8 nT," he says. "This record-low interplanetary
magnetic field undoubtedly contributes to the record-high
cosmic ray fluxes." [data]
2.
The solar wind is flagging. "Measurements by
the Ulysses spacecraft show that solar wind pressure is at
a 50-year low," he continues, "so the magnetic bubble
that protects the solar system is not being inflated as much
as usual." A smaller bubble gives cosmic rays a shorter-shot
into the solar system. Once
a cosmic ray enters the solar system, it must "swim upstream"
against the solar wind. Solar wind speeds have dropped to
very low levels in 2008 and 2009, making it easier than usual
for a cosmic ray to proceed. [data]
3.
The current sheet is flattening. Imagine the sun
wearing a ballerina's skirt as wide as the entire solar system
with an electrical current flowing along its wavy folds. It's
real, and it's called the "heliospheric current sheet,"
a vast transition zone where the polarity of the sun's magnetic
field changes from plus to minus. The current sheet is important
because cosmic rays are guided by its folds. Lately, the current
sheet has been flattening itself out, allowing cosmic rays
more direct access to the inner solar system.
 Right:
The heliospheric current sheet is shaped like a ballerina's
skirt. Image credit: J. R. Jokipii and B. Thomas, Astrophysical
Journal 243, 1115, 1981.
"If
the flattening continues, we could see cosmic ray fluxes jump
all the way to 30% above previous Space Age highs," predicts
Mewaldt. [data]
Earth
is in no great peril. Our planet's atmosphere and magnetic
field provide some defense against the extra cosmic rays.
Indeed, we've experienced much worse in the past. Hundreds
of years ago, cosmic ray fluxes were at least 200% to 300%
higher than anything measured during the Space Age. Researchers
know this because when cosmic rays hit the atmosphere, they
produce an isotope of beryllium, 10Be, which is
preserved in polar ice. By examining ice cores, it is possible
to estimate cosmic ray fluxes more than a thousand years into
the past. Even with the recent surge, cosmic rays today are
much weaker than they have been at times in the past millennium.
[data]
"The space era has so far experienced
a time of relatively low cosmic ray activity," says Mewaldt.
"We may now be returning to levels typical of past centuries."
NASA
spacecraft will continue to monitor the situation as solar
minimum unfolds. Stay tuned for updates.
Author: Dr.
Tony Phillips | Credit: Science@NASA
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