The IceCube neutrino observatory is a kilometre-scale array of photon detectors buried under the ice at the South Pole. When neutrinos pass through the ice, they occasionally bump into atoms creating particles called muons. These muons then generate light as they pass through the ice which is then picked up by the detector allowing scientists to determine the direction of the incoming neutrino.
The trouble is that most of the muons that IceCube sees are not generated by neutrinos at all but by collisions between cosmic rays and atoms in the upper atmosphere. In fact for every muon fathered by a neutrino, IceCube sees a million muons fathered by cosmic rays.
Of course, scientists have all kinds of ways to filter out these atmospheric muons which have always been thought of as unwanted noise. Today, however, Serap Tilav and buddies from the IceCube Collaboration say that this background noise might be more useful than had been thought.
The IceCube team says that the background rate of muons is closely correlated with the temperature of ozone layer above the South Pole. So unknown to all, IceCube has been behaving like a giant atmospheric thermometer.
Here’s how it works. As well as the photon detectors beneath the ice, the detector has an array of detectors on the surface too, looking upwards. The idea is that these detectors can spot the shower of particles called mesons that cosmic rays produce in the atmosphere, allowing scientists to filter out the effects of atmospheric showers on the detectors beneath the ice.
But the way the mesons interact with the atmosphere depends crucially on its density and therefore on its temperature, says the team. During the winter, when the stratosphere is cold and dense, the charged mesons are more likely to interact with atoms in the atmosphere and produce secondary low energy particles which are picked up by the surface detectors. So these detectors pick up more signals when its cold.
On the other hand, during summer when the warm atmosphere expands and becomes less dense, the mesons can travel further and have more time to decay into muons. These muons are picked up by the detectors underneath the ice. So these detectors are busier when its warm.
So the difference between the behaviour of the surface and subsurface detectors turns out to be a highly sensitive measure of the temperature of the atmosphere at the height where cosmic rays interact with it–between 14 and 26 km, that’s roughly the height of the ozone layer.
That could turn out to be extremely useful data. The Antarctic atmosphere is closely monitored by the NOAA Polar Orbiting Environmental Satellites and by the radiosonde balloon launches of the South Pole Meteorology Office, which help to callibrate satellite measurements .
That’s all well and good during the summer but nobody launches radiosondes during the winter which means that NOAA has to use computer models to estimate the temperature. That’s where IceCube could turn out to be extremely useful. If it’s measurements turn out to be reliable, that is.
The team has compared the IceCube data against NOAA’s measurements of atmospheric temperature taken over several years and say there is an extraordinarily high correlation.
For example, historical data shows that IceCube’s forerunner, a machine called AMANDA, was able to see Sudden Stratospheric Warming even which occurred in 2002 when the atmosphere increased in temperature by up to 60K in less than a week.
IceCube’s ability to give hour by hour resolution of temperature measurements could be crucial for understanding future exceptional events like this.
Obviously, more work is needed to determine exactly how much IceCube’s data reveals about the atmosphere and how much researchers can rely on the model used to determine temperature.
But it looks as if the IceCube team have a fascinating future in climate science ahead of them.
Ref:arxiv.org/abs/1001.0776: Atmospheric Variations as observed by IceCube