Sunday, 8 November 2009

Cosmics Rays and Tree Rings, A Mysterious Link

A recent article from the BBC news science pages, reported an interesting mystery. Patterns of tree ring grow, seem to match up with the solar cycle, and more precisely with the amount of cosmic radiation hitting the earths upper atmosphere. The story has spread around the web as a big mystery. Researchers in Scotland took slices of spruce trees, and measured there yearly grow, by the size of the rings in the in the tree slices. They where looking for a link between climate change and tree growth, instead they found tree growth matched the ammount of cosmic rays hitting the Earth's surface. Cosmic ray intesity varies with the Solar Cycle when there's less Solar Wind theres more more cosmic rays and vice versa,. The Study was published in the journal New Phytologist, and the effect was shown to be statistically significant. For more about cosmic ray variation and its recent peak, see the measurements here.

So we have a mystery of how could Cosmic Rays possible aid tree growth. To date no one has made this link, but I think I can provide an explaination using plain ordinary physics. When a cosmic ray hits the atmosphere, it creates a shower of ionizing radiation, each of the secondary particles (with the exception of neutrino and muons, which are to penitrating) are energetic enough to ionize oxygen and nitrogen molecules in the atmosphere, forming nitrogen oxides, these react ready with water forming nitric acid, which will precipitate in dilute form in the rain. Only lightning and cosmic rays can form nitrogen oxide, and lightning is relatively rare, so the amount of available free nitrates in the soil, depends very much on the amount cosmic rays hitting the earth.

Plants of course need nitrogen to grow, the trouble is they can't absorb nitrogen from the atmosphere (except for Legumes (pea, and beans and similar plants)). So for the majority of plants and trees, not feed by human fertilizers, the amount of fertilizing nitrate available to them, is directly proportional the cosmic ray flux.

Lets try and back this qualitative theory with some rough back of the envelope calculations. We need to know how much cosmic ray energy hits each meter squared of soil in a year. Starting from wikipedia's cosmic ray flux graph, we can estimate for the range of energies from 1 GeV to 1000 TeV, a flux of

F =(m^2 sec GeV^-1) 10^25.5 E^-2.5 ( Where E is in GeV)

Integrating over the Energies assuming a base energy of 10 MeV, we get 2*10^22 eV despoited per meter squared per area, or about 3KJ. Lets assume that about 20% of this energy eventially goes into splitting the bonds between N2 nitrogen molecules in the air, the bond strength is 0.59 KJ per Mole, so we might get 2.5 Moles or 46 grams for nitrogen dioxide produces by cosmic rays in a year. This isn't a bad ammount for fertilizing perposes. Compare this is Nitrogen Fixation by Lightning, its estimated that the world wide production of nitrogen dioxide by lightning is 14.4 million tonnes a year, which works out as 28 grams per year per square meter. Once you factor in that temperate region like scotland have much less lightning than tropic regions, we can easierly begin to believe that in the Scotish Highlands where the measurement where taken, much of the fertization comes from cosmic ray nitrogen fixation.

So I've backed up my tree ring theory with some very rough calculations, the true figures are probably fairly different, (especially the very rough cosmic ray flux integration), and the assumption that 20% of cosmic ray energy goes into fixing nitrogen. Still the ball park figures give the idea the OK, and I can claim to have solved the mysterious link between scottish tree rings and cosmic rays.