Ken Tapping, May 29, 2007

Space is a hostile place; it is loaded with radiation, including gamma rays, x-rays and ultra-violet radiation, all of which are dangerous to us and the other life forms with which we share the Earth. Then there are showers and streams of high-energy particles from the Sun, such as protons and atoms stripped of many of their electrons. These can also be deadly. Luckily, our atmosphere stops most of the radiation, and the Earth's magnetic field stops or deflects the particles. An atmosphere and a magnetic field are good things for a planet to have, and are important elements in our welfare. It is therefore a bit worrying that there is evidence that our planet's magnetic field occasionally reverses, and during the transition it vanishes altogether.

The Earth's magnetic field is generated by flows of molten iron and nickel in the Earth's core, by some sort of dynamo process. We have instruments that can measure it precisely, and at the moment we are detecting a decrease of about 5% a century. To put this in context we need to know what has been going on in the long term, over past millions of years. Fortunately, that is possible, because Mother Nature has written it down for us, in the rocks.

Rocks are made up of minerals. Some of them, like certain oxides of iron, respond to magnetic fields. When the rock is molten, in the form of magma or lava, the mineral particles are free to line up with the Earth's magnetic field. When the rock hardens, those particles are stuck in place, recording both the intensity and direction of the magnetic field at the time the rock solidified. If we know the age of a rock, which we can find out from the decay of radioactive elements in the rock, we can determine what the Earth's magnetic field was up to that long ago. What we need is a nice, long, continuous time series of rocks. Luckily we have one, in the rocks on the bed of the Atlantic Ocean.

The Atlantic Ocean is getting a few centimetres wider each year. Molten rock is emerging from the Earth's mantle at the Mid-Atlantic Ridge, providing new seabed, and as it solidifies, records the magnetic field. As we move away from the Mid-Atlantic Ridge, we encounter progressively older rocks, forming a continuous record many millions of years long. It turns out that on the average, the Earth's magnetic field reverses every 500,000 years or so. The rocks tell us the last reversal happened about 780,000 years ago, making the next reversal overdue. At the point of transition the Earth's magnetic field vanishes, along with our protection. Strange changes in the magnetic field in the South Atlantic could indicate the transition has started. If this is true, what can we expect?

The undeflected high-energy protons will fry the electronics in satellites, and endanger astronauts in Earth orbit. There will be increased radiation hazards during high-altitude flight, and higher levels of radiation on the ground. Our numerous and very hi-tech species has never been faced with such a hazard before. It will give us a challenge. One of the intriguing things that will happen is that compasses will point south instead of north!

Mercury lies low in the sunset twilight. Venus and Saturn are quite close in the western sky during the evening. Jupiter rises around midnight, and Mars shortly before dawn. The Moon will be Full on May 31.

---

Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca