Cosmic ray detector reveals sudden atmospheric warming
23 January 2009, by Tom Marshall
Scientists have found an unexpected use for data from particle physics experiments half a mile down a disused US mineshaft - detecting abrupt warming in the upper atmosphere.
Two scientists work on the neutrino detector of the MINOS experiment.
The experiments were part of a multi-million dollar project called MINOS, which aimed to investigate subatomic particles called neutrinos. But as it turns out, the information they provide on cosmic rays can be equally useful for atmospheric scientists, and may eventually help improve weather prediction and climate models.
Researchers from the UK's National Centre for Atmospheric Science and the Science and Technology Facilities Council have discovered that background noise in the MINOS experiment can be mined for information about a phenomenon called sudden stratospheric warming, which causes temperatures more than 30 kilometres up in the atmosphere to rocket, sometimes by as much as 40°C. Their work appears in Geophysical Research Letters.
Sudden stratospheric warming in 2002 - the only footage yet recorded of this happening over the Antarctic.
These episodes of sudden warming happen on average every other year. They are notoriously hard to predict, though scientists know they almost always occur in the northern hemisphere during winter. They are known to affect weather conditions in the troposphere far below, but sensing them has always been difficult.
A Fermilab scientist with one of the two 'horns' that focus the particle beam for the MINOS neutrino experiment.
The research was born out of discussions between atmospheric scientist Dr Scott Osprey and particle physicist Dr Giles Barr, both at the University of Oxford. Existing research suggested that cosmic ray detectors could sense changes in atmospheric temperature, and Osprey and Barr looked for other datasets to let them probe this link.
Examining the MINOS data, they noted sudden jumps in incoming cosmic rays, and hence in atmospheric temperature, during some winters. These lasted just a few days; the researchers realised they had found a new way to detect sudden stratospheric warming.
'Up until now we have relied on weather balloons and satellite data to provide information about these major weather events,' says Osprey, who led the research at NCAS. 'Now we can potentially use records of cosmic-ray data dating back 50 years to give us a pretty accurate idea of what was happening to the temperature in the stratosphere over this time. Looking forward, data being collected by other large underground detectors around the world can also be used to study this phenomenon.'
Information in the noise
Cosmic rays are high-energy particles that stream into Earth's atmosphere from space. One type of particle, the meson, is unstable and quickly decays into another, the muon.
When the atmosphere heats up, it expands, and this means fewer mesons are destroyed by hitting air molecules. This in turn means more mesons are available to decay naturally into muons, which are less likely to interact with the atmosphere and so more likely to make it to Earth and be picked up by the MINOS detector.
'It's fun sitting half a mile underground doing particle physics,' says Barr. 'It's even better to know that from down there, we can also measure a part of the atmosphere that is otherwise quite tricky to measure,' he adds.
Osprey says the project shows the value of collaboration between different scientific disciplines. 'These environmental effects have been well documented by physicists interested in cosmic rays,' he says. 'But there hasn't been the cross-pollination with the atmospheric science community, because people haven't been speaking to each other. If they had, we could have known about this connection 50 years ago.'
MINOS includes 130 scientists from 28 institutions in several nations; it is managed by the US Department of Energy's Fermi National Accelerator Laboratory. It uses two sensitive detectors, one at the Fermi lab, where the neutrinos are produced, and the other deep underground in a disused iron mine at Soudan in northern Minnesota, more than 700 kilometres away.
Ironically, the 6,000-tonne detector was placed here with the intention of screening out most of the noise from cosmic rays to ease observation of the neutrinos arriving at the end of their underground journey. But the detector is so sensitive that it can still pick up muons, which form the main experiment's background noise.
Osprey says that there is now an opportunity to build a cosmic ray detector that is better-suited to monitoring atmospheric conditions. This would be on or near the Earth's surface, to increase the number of muons picked up, and would be optimised for atmospheric work rather than physics applications.
He adds that if this area of research is to prove fruitful in improving understanding of the effects of sudden stratospheric warming on weather lower in the atmosphere, scientists will need to develop a way of measuring temperature variations in small, well-defined areas, rather than over a broad band of the atmosphere as is the case with the MINOS data.
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