An artist's concept of the heliosphere, a magnetic bubble that partially protects the solar system from cosmic rays.
Credit: Richard Mewaldt/Caltech
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."
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.
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.