OSB archive
OSB archive

Cosmic rays in MINOS's labyrinth

Pete Wilton

In this week's Geophysical Research Letters Oxford's Scott Osprey and Giles Barr report how data collected from the MINOS experiment in a disused US iron-mine is shedding light on what's happening 20 miles up in the Earth's stratosphere.

The UK's NCAS and STFC highlighted this new work, I asked Scott and Giles about cosmic rays, climate science and mines...

Oxford Science Blog: Why do we need underground detectors to monitor what's happening in the stratosphere?
Scott Osprey: Paradoxically, the further underground detectors are placed, the better we find correlations between cosmic rays (muons) and stratospheric temperature. This counterintuitive result is a consequence of the very high energy cosmic rays being sensitive to changing conditions in the atmosphere.

Giles Barr: At a more technical level, the cosmic rays start off as protons which interact in the atmosphere to make pions, these then decay to make muons which we can detect. The chain is therefore protons -> pions -> muons. While the cosmic rays are pions, they can interact again which causes them to be lost (they split up in to many lower energy particles which we can't distinguish from background). This possibility of them interacting depends on the temperature.

It turns out, as Scott says, that the higher energy ones (which we can detect most easily by going underground) have the best sensitivity to the temperature, because they have come from pions which interact enough.

OSB: What are the challenges of doing research in a disused iron-mine?
GB: Our hosts at Fermilab and the University of Minnesota have built an underground lab which is spacious, brightly lit, dry and has a flat floor. We have fork-lift trucks, internet access, microwaves and desks down there. The mine professionals who run the operation are really good at their jobs.

When we made the detector, everything had to be designed to fit down the mineshaft, so the steel sheets which the detector was made of were hauled down in strips and then welded together underground. There is no-one there overnight, so the detector is made to stay running without humans around: We have made it so that virtually everything is controllable remotely over the internet, so we can get things going again without having to go down the mine. 

OSB: What is the significance of being able to link an increase in cosmic rays to Sudden Stratospheric Warming?
SO: It better highlights the physical link between cosmic ray muons and the atmosphere by identifying them with other known phenomena occurring in the atmosphere. This gives us confidence that we actually understand the underlying physics. For those interested in other aspects, it adds to our armoury of observations, and potentially gives us a new tool to use.

GB: There are many things which affect cosmic rays on their journey through the galaxy, into the solar system and then the atmosphere.  From the cosmic ray physics point of view, this allows us to check some aspects of this process. 

OSB: How much do we still not understand about the overall impact of cosmic rays on climate?
GB: There are some other groups who are studying the effect that cosmic rays might induce cloud formation, that as the cosmic ray whizzes through bits of the atmosphere its ionisation trail could start droplets off which could then form clouds, but we are not looking into that at all. Our research points out that there is an effect in the opposite 'cause>effect' direction, that the state of the atmosphere affects the number of cosmic rays we see.

SO: These results do not say anything about the possible effects of cosmic rays on climate. Actually, this turns the topic on its head: by highlighting the impact of a changing atmosphere on cosmic rays. However, the 'philosophy' may be relevant for those investigating the possible effects of cosmic-rays on climate: by promoting others to examine environmental effects in the cosmic rays of lower energy, thought relevant to these studies.

OSB: How might further research in this area benefit climate researchers?
GB: The basic physics behind these environmental effects has been known for a long time in the particle physics community. However, very few studying climate will know of these. In the early days of cosmic ray research, in the 1940's and 1950's, there was a need for both meteorologists and particle physicists to work closely together. Since then the communities have diverged somewhat, following the subsequent development of large and powerful 'atom-smashers'.

Recently, certain hot-topics have brought aspects of each others work to the other's attention. Some of this has been met with a mixed reception. However, future work, such as from the NCAS/MINOS collaboration, can only prove beneficial in the long run.

Dr Scott Osprey and Dr Giles Barr are based at Oxford's Department of Physics, Dr Osprey is lead scientist of this study for NCAS, Dr Barr is funded by STFC for his work with MINOS.