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Cosmic rays hit
space age high

Posted: October 01, 2009

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According to NASA's Advanced Composition Explorer (ACE), our Solar System is awash with a record number of galactic cosmic rays.

ACE orbits the L1 libration point – a point of Earth-Sun gravitational equilibrium about 1.5 million kilometres away – from where it continuously monitors the solar wind and high energy particles accelerating through the Solar System. Cosmic rays are subatomic particles such as protons or heavy nuclei that originate from outside the Solar System, typically in supernova explosions where they are accelerated to speeds close to the speed of light. They can pose a serious health hazard for astronauts, and also disable satellites in Earth orbit.

Energetic iron nuclei counted by the Cosmic Ray Isotope Spectrometer on NASA's Advanced Composition Explorer (ACE) spacecraft reveal that cosmic ray levels have jumped 19 percent above the previous Space Age high. Image: Richard Mewaldt/Caltech.

"In 2009, cosmic ray intensities have increased 19 percent 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."

Scientists believe that the cause of the surge in cosmic ray activity is the Sun's current quiet state typical of solar minimum – it is well known that cosmic ray hits go up when solar activity falls because the Sun's magnetic field, which acts as a shield to these cosmic rays, is weakened at times of low solar activity, allowing more cosmic rays to reach the inner Solar System. The entire Solar System from Mercury to Pluto and beyond is protected by a bubble of solar magnetism called the heliosphere which is generated by the Sun's inner magnetic dynamo and inflated across this vast distance by the solar wind. Right now the Sun is displaying weaker activity than ever before recorded in modern times, setting the stage for what Mewaldt calls "a perfect storm of cosmic rays."

The heliosphere is a magnetic bubble that partially protects the Solar System from cosmic rays. Image: Richard Mewaldt/Caltech.

Three different aspects of the current solar minimum are conspiring to create perfect cosmic ray 'storm' conditions: a reduced solar magnetic field strength, a reduced solar wind pressure, and the flattening of the Sun's current sheet. Indeed, measurements by the Ulysses spacecraft show that solar wind pressure is at a 50-year low, meaning that the heliosphere is not being inflated as much as usual and that there is less resistance for cosmic ray particles travelling in the opposite direction. The current sheet flattening can be imagined by picturing the Sun wearing a ballerina's skirt as wide as the entire Solar System with an electrical current flowing along the folds – this is the heliospheric current sheet. Since cosmic rays are guided by its folds, the flatter configuration allows these deadly particles 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," predicts Mewaldt.

The heliospheric current sheet is shaped like a ballerina's skirt. Image: J. R. Jokipii, University of Arizona.

Fortunately, Earth is nicely protected from space radiation by its atmosphere and magnetic field and the situation doesn't seem so bad when considering cosmic ray fluxes were at least 200 percent higher than they are now hundreds of years ago. "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 the solar minimum unfolds.

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