Galactic cosmic rays have just hit a Space Age high, new
data from a NASA spacecraft indicates.
"In 2009, cosmic ray intensities have increased 19
percent beyond anything we've seen in the past 50 years," said 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."
The surge, which poses no threat to Earth, was detected by NASA's
ACE (Advanced Composition Explorer) spacecraft.
The cause of the surge is solar
minimum, a deep lull in the sun's activity that began around 2007 and
continues today. Researchers have long known that cosmic rays go up when solar
activity goes down, because strong solar activity inflates and bolsters a
protective bubble around our entire solar system.
Right now solar activity marked by sunspots, solar flares
and space storms is as weak as it has been in modern times, setting the stage
for what Mewaldt calls "a perfect storm of cosmic rays."
A shower of rays
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, where they can pose a threat to
orbiting satellites a single cosmic ray can disable a satellite if it hits an
unlucky integrated circuit.
Though some have suggested that cosmic rays might be behind
the Earth's current warming climate, research has shown
no firm link between these invading rays and global warming.
Cosmic rays also pose a health hazard to astronauts. Several
reports have outlined
the risks from cosmic radiation that might exist for future missions to
Mars or stints on the moon.
The sun's magnetic field the heliosphere, which surrounds
the entire solar system is our first line of defense against these highly-charged,
energetic particles.
But the current state of solar activity means the solar
system isn't as protected right now.
"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."
Perfect storm
Mewaldt lists three aspects of the current solar minimum
that are combining to create the perfect storm:
- The sun's magnetic field is weak.
- The solar wind is flagging.
- The heliosphere's so-called "current sheet" is
flattening.
"Measurements by the Ulysses spacecraft show that solar
wind pressure is at a 50-year low," Mewaldt said, "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.
The flattening of the magnetic field's current sheet is also
making it easier for cosmic rays to penetrate the solar system's defenses.
To picture the
current sheet, imagine the sun wearing a ballerina's skirt as wide as the
entire solar system with an electrical current flowing along the wavy folds.
That is the "heliospheric current sheet," a vast transition zone where
the polarity, or direction, of the sun's magnetic field changes from plus
(north) to minus (south). The current sheet is important because cosmic rays
tend to be guided by its folds. Lately, the current sheet has been flattening
itself out, allowing cosmic rays more direct access to the inner solar system.
"If the flattening continues as it has in previous
solar minima, we could see cosmic ray fluxes jump all the way to 30 percent
above previous Space Age highs," Mewaldt said.
Before the satellite era, it was not possible to measure
cosmic rays directly, since they don't penetrate to the ground.
No worries
Earth is in no great peril from the extra cosmic rays. The
planet's atmosphere and magnetic field combine to form a formidable shield
against space radiation, protecting life on the surface.
In fact, humans have weathered storms much worse than this.
Hundreds of years ago, cosmic ray fluxes were at least 200 percent higher than
they are now. 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.
"The space era has so far experienced a time of
relatively low cosmic ray activity," Mewaldt said. "We may now be
returning to levels typical of past centuries."
NASA spacecraft will continue to monitor the situation as
solar minimum unfolds, the agency said.
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