Thursday, February 04, 2010

Is the Ozone Keeping Out Cosmic Rays?

A paper that appeared recently on (a site where people can post physics papers that have not been reviewed or edited by any journals) and the Technology Review blog, says that the volume of cosmic rays that reach particle detectors on Earth matches up with temperature fluctuations in the ozone layer. Is there a direct correlation?

Well - lets back up.

The researcher who put the paper on the arXiv is with the IceCube experiment, which is a neutrino detector buried under the ice in Antarctica. IceCube is looking for neutrinos - subatomic particles that barely interact with regular matter. But occasionally, very very occasionally, they do interact. So scientists have built VERY sensitive detectors to catch those rare few.

Those neutrino detectors are so sensitive that they can't help but catch all the other particles coming at them (particles that do interact with regular matter). Namely, cosmic rays (which are mostly mesons) come hurtling through space (they are believed to be the remnants of supernova explosions) and reach our atmosphere, where they may either pass through the atmosphere and reach the ground, or collide with atomic nuclei in the air. If they collide, they produce a huge shower of particles called muons. Some of these muons can travel through the ice and create static in the detectors, making it difficult to pick out those nuggets of neutrino gold.

So, neutrino experiments try to put up shields between their sensitive detectors and the parade of disruptive particles coming from the skies. The Main Injector Neutrino Oscillation Search, MINOS, buried itself in an old mine in Minnesota. (They have a pretty great visitors tour if you ever get to visit. You get to ride a rickety, open, mine elevator down 2,341 feet.) IceCube put itself under a layer of ice. The layers of Earth and ice help to block out cosmic rays and muons, making room for neutrino detection. But many of those pesky particles still get through, and the scientists at MINOS and IceCube are constantly working to better understand cosmic rays so they can filtered the noise out of their data.

For quite some time, scientists at MINOS recognized that there seemed to be seasonal fluctuations in the rate of muons striking the detector. No one had done a real study of it, but it seemed clear that during the colder months, muon rates goes up, and in the warmer months it goes down. Recently, a graduate student with MINOS did his PhD work on this subject, and showed that the suspicions were correct. Cosmic rays have more collisions with air particles, and produce more muons, when it's cold.

Cosmic rays come barrelling toward the Earth at energies higher than the most powerful particle accelerators on Earth. When the air up there is colder, it becomes dense, and the air molecules get bunched together. The cosmic rays then have a higher chance of hitting a nucleus and creating a shower of muons. And vice versa - when the air gets hot, it expands and the particles spread out, making room for cosmic rays to pass through. (Thanks physics!). So more muons hit the underground detectors in the winter.

But here's the issue with the paper from IceCube: the cosmic rays that come pummeling through our atmosphere will collide with any nucleus. Hydrogen, helium, oxygen, nitrogen - they have no preference. The "ozone layer" is a portion of the stratosphere that is defined by it's chemistry - it has high levels of ozone, or 3 oxygen atoms bound together. So it doesn't quite make sense that an area defined by chemistry would have an effect on the cosmic ray rates, which are not affected by chemistry. (The ozone does absorb a great deal of ultraviolet light - perhaps this is related?)

The IceCube researcher who posted the paper compared IceCube's data showing seasonal fluctuations in the volume of cosmic rays, with atmospheric data from the National Oceanic and Atmospheric Administration, NOAA, reporting the temperature of the ozone. She found that the two seemed to match up. I have no idea if you could say the same thing about other portions of the atmosphere. This could be something big, or it could end up being an example of how correlation does not equal causation. This was not an in depth analysis like the one that the MINOS graduate student did, so we'll have to wait for that. I'm guessing the researcher will do a more in depth analysis as some point, but for now what we should be looking for is another reason why the cosmic ray rate might be affected by this particular portion of the atmosphere. And that, I will leave to the professionals.

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