Tracking the Sun’s history
Raijin is helping researchers to track variations in the Sun’s activity over recent millennia.
Dr Andrew Smith at the Australian Nuclear Science and Technology Organisation (ANSTO) has been making trips to Antarctica for almost two decades, bringing back ice cores for analysis.
He is searching for beryllium-10, a radioactive atom that holds the secret to the story of our Sun’s recent history.
“Beryllium-10 is produced when cosmic rays smash into the Earth’s atmosphere and collide with the nuclei of atmospheric gases,” explains Dr Smith.
“Beryllium-10 is then rained or snowed out of the atmosphere down to the Earth’s surface, where it is deposited in layers of Antarctic ice.”
Not all of the cosmic rays bombarding Earth make it to our atmosphere; many are deflected by the Sun’s magnetic field as they travel through the Solar System. This affects how much beryllium-10 reaches the ground.
“When the Sun is more active, its magnetic field is stronger so there is a greater degree of deflection of the cosmic rays,” explains Dr Smith.
“That means the production rate of beryllium-10 decreases. And so, in principle, by measuring the concentration of beryllium-10 in successive layers of the Earth’s polar ice you can reconstruct the activity of the Sun in past times.”
Understanding patterns of variation in the Sun’s intensity is critical to understanding future climate change, says Dr Smith, whose expeditions are funded by the Australian Antarctic Division.
Of course, nothing is ever as simple as it seems. There are a whole range of factors, such as the Earth’s dipolar magnetic field, that affect the levels of beryllium-10 found on Earth’s surface.
Dr Smith’s colleague, Dr Ulla Heikkilä, has been using Raijin to model some of these processes.
“There is a seasonal variation in beryllium-10 at the site at which we collect our ice cores,” says Dr Smith. “We have a maximum in summer and a minimum in winter.
“If you didn’t know any better you might interpret that increased concentration as being a decrease in solar activity, but it’s not.
“The modelling on Raijin revealed that it is actually caused by a change in the weather systems which allows stratospheric air, which has more beryllium-10 in it than tropospheric air, to descend down to the site.”