Friday, 30 November 2012

Cosmic Rays reveal event in earth'magnetic field




The most recent event of this kind, known as the Laschamp excursion, took place 41 000 years ago. Evidence for the event was uncovered by the researchers in sediment cores collected off the coasts of Portugal and Papua New Guinea. In the samples, they found an excess of beryllium-10, an isotope produced solely by collisions between particles of cosmic origin and atoms of nitrogen and oxygen. The beryllium-10 (10Be) produced in the atmosphere then falls to the Earth's surface where it is incorporated into ice and sediments. In sedimentary beds dating from the age of the Laschamp excursion, the researchers found up to twice as much 10Be as normal, evidence of the intense cosmic ray bombardment that the Earth underwent for several thousand years.
Traditionally, the presence of various iron oxides, especially magnetite, in volcanic lavas, sediments and ancient pottery provides information on the history of the magnetic field by indicating its direction and strength at the time when these materials solidified.

This so-called paleomagnetic approach does not always allow global variations in the magnetic field to be quantified accurately. The researchers combined this method with the measurement of beryllium-10 concentrations in the same sedimentary records. This enabled them to demonstrate that peak concentrations of this isotope are synchronous and have the same dynamics and amplitude in Atlantic and Pacific sediments as in the previously analyzed Greenland ice cores. The method based on beryllium-10, which has been developed over the past 10 years at CEREGE, therefore makes it possible to obtain a continuous reconstruction of variations in the strength of the Earth's global magnetic field. It is also known that over the past 3000 years the magnetic field has lost 30% of its strength. This trend suggests that in the coming centuries, the Earth might undergo an excursion similar to the one that took place 41 000 years ago. Since high energy cosmic rays can cause mutations and cell damage, such an event would have a significant impact on biodiversity, and in particular on humans. This is why the researchers are seeking to find out the precise rates of the magnetic field's reversal and excursion sequences, in order to identify potential regularities in its behavior and thus shed light on the cause of these phenomena, which originate in the Earth's core. - PHYSORG.