A collaborative study by particle physicists and atmospheric researchers has found the first correlations between daily variations in cosmic-ray muons detected deep below ground and large-scale phenomena in the upper atmosphere. The effect suggests that underground muon-detectors could play a valuable role in identifying certain meteorological events and observing long-term trends.
Scott Osprey and colleagues from the UK’s National Centre for Atmospheric Science and Oxford University have worked with members of the Main Injector Neutrino Oscillation Search (MINOS) collaboration in analysing data collected between 2003 and 2007 by the MINOS Far Detector, located 705 m below ground in a disused iron mine at Soudan, in Minnesota. The MINOS experiment intercepts a neutrino beam that goes 725 km from Fermilab to Soudan and studies long-baseline neutrino oscillations; the penetrating muons appear as background noise.
The teams have found a close relationship between the rate of muons detected in MINOS and upper-air temperatures from the European Centre for Medium Range Weather Forecasts. In particular, they discovered strong correlations between the muon rate and the upper-air temperature during short-term events (of around 10 days) in the upper atmosphere, or stratosphere, in winter.
When primary comic rays strike the Earth’s atmosphere they interact, creating pions and kaons. These mesons in turn decay to produce muons – the most energetic of which penetrate deep below the Earth’s surface. The mesons can also interact before they decay, so the number of muons produced depends on the local density of the atmosphere and varies with temperature. An increase in temperature means a decrease in density and, hence, fewer mesons interact and instead decay, increasing the number of muons. Physicists have known of this effect since the Monopole Astrophysics and Cosmic Ray Observatory first observed a seasonal variation in the rate of muons a decade ago (Ambrosio et al. 1997).
Image credit: Fermilab Visual Media Services.
Most of the mesons that give rise to the muons detected in MINOS occur at altitudes of around 15 km in the region known as the tropopause, where there is little variation in temperature. However, the mesons also occur in the mid-stratosphere – at altitudes where temperatures fluctuate, particularly in winter. For the analysis, the team defined an “effective” temperature based on an average temperature over the altitudes where mesons occur, weighted by the calculated distribution of meson production.
The results show a striking relationship between this temperature and the number of muons, with correlated changes occurring over periods of only a few days (Osprey et al. 2009). The data for the Northern Hemisphere winter of 2004–2005 are particularly interesting. The meteorological data indicate the occurrence of a major phenomenon, known as a sudden stratospheric warming, during February. This was linked to break-up of the winter polar vortex, a polar cyclone that brings cooler weather and which extended over the MINOS site in early February. Prior to that, the 2004–2005 winter had seen the lowest recorded temperatures in the polar stratosphere, and ozone concentration in the polar vortex was anomalously low.
The results show that underground muon data contain information that could identify important short-term meteorological events, over and above the already known seasonal effect. This is interesting for atmospheric researchers, as it provides an independent technique to measure such phenomena. Moreover, physicists have cosmic-ray data from experiments dating back 50 years or more, covering periods when upper-air observations from weather balloons were less reliable than today.
Further reading
S Osprey et al. 2009 Geophys. Res. Lett. in press.
