Monthly Archives: September 2004

The Magnetic Reversal


   Curious Friends, in today’s VERITAS we will explore the  earth’s magnetic field and some strange phenomenon related to it. This article is inspited by something that my friend Amit Chopra told me.

        The exact cause of the earth’s magnetic field is still a mystery to man. The magnetic field of earth resembles the field of a giant bar magnet buried in the centre of the earth. But this could not be the reason because the core of the earth is very hot ( more than 1000 degrees

celsius) and at this temperature  any permanent magnetism is lost. The most likely cause of earth’s magnetism is that molten charged lava is constantly flowing in the core. These electric currents cause earth’s magnetism. This theory is called the dynamo theory of earth’s magnetism.

        The field of earth’s magnetism is huge. It extends several thousands of kilometres into space. The magnetic poles of the earth are slightly tilted with respect to the axis of earth’s rotation. But these poles are not static. They keep changing their locations independently of each other and are not always at opposite sides of the earth.

Nobody has a good theory to explain why the poles keep changing their locations.

        Every once in thousands of years something very dramatic happens!

The earth’s magnetic field reverses its direction! The north pole becomes south pole and vice versa!!! The average time between reversals is about 250000 years. But this time is not always exact. It varies from thousands to millions of years. The actual reversal takes about 1000 years. This also is not always the same. It may take as little as 100 years for the poles to reverse . And there is evidence that atleast once in earth ‘s history the polarity of the magnetic field remained the same for 30 million years. The most recent confirmed flip occurred about 780,000 years ago.

        How do scientists know that earth was once magnetized in the opposite direction? Rocks that solidified at a particular date always contain a record of the magnetic field at that date. Rocks collected below deep oceans and lakes show a definite magnetic field effect opposite to what we have today. When these rocks solidified by sedimentation thousands of years ago their crystal structure oriented itself to the magnetism of that time.

      Scientists believe that the earth’s magnetic field weakens over time after each reversal and after a long time it shuts off. it then appears again. It may appear again with the same polarity( this is called geomegnetic excursion) or it may appear with the opposite polarity ( reversal). Measurments carried by scientists over the last 150 years have shown a constant decrease in the magnetic field strength. The magnetic field has decreased by about 10 % since 1845. So in about 1000 years or so the magnetic field will shut off for some time to reappear again with either the same or the opposite polarity.

        No body knows why the earth’s magnegtic field changes direction. Several scientists are working on this but nobody knows for sure.

        If the magnetic field reverses how will it affect us. As I mentioned earlier the magnetic field of earth extends several thousands of kilometres into space. It protects us from the harmful effects of charged particles emitted by the sun. These charged particles interact with the magnetic field and are deflected around the earth. There are belts around the earth where the magnetic field has trapped a lot of charged particles. These are called Van Allen Belts. At the north and south poles the interaction of these cherged particles with the magnetic field produces brilliant radiation: the northern lights( aurora boleris) and the southern lights ( aurora australis).

   Curious Friends, sometimes Science too becomes a victim of fashion. We see today that every young Physicist wants to just do research in particle physics or field theory. But there is mystery everywhere. Our Earth’s magnetic field is as mysterious as anything else. One should not follow the fashions. One should follow nature and nature alone:


                ….. First follow nature and your judgement frame

                By her just standard, which is still the same

                Unerring NATURE , still divinely bright

                One clear, unchanged and universal light

                Life, force and beauty must to all impart

                AT once the source, and end, and test of Art

                Art from that fund each just supply provides

                Work without show and without pomp presides:…..


                                                        -Alexander Pope





  Go, wondrous creature! mount where Science guides:                 

  Go, measure earth, weigh air, and state the tides:                 

  Instruct the planets in what orbs to run,                           

  Correct old time and regulate the Sun;