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Apr 23, 2019

Reverse Magnetism

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By Allen Martis, USA [ Published Date: October 30, 2004 ]

In our day to day lives, we as humans make use of directions (North and South) for navigational purpose.  This direction is actually derived from the polarity of our Earths Magnetic field.  Also a number of Earth's creatures, including some birds, turtles, and bees, rely on Earth's magnetic field to navigate.

On Earth one needs a sensitive needle of a compass to detect magnetic forces, and out in space they are usually much, much weaker. But beyond the dense atmosphere, such forces have a much bigger role, and a region exists around the Earth where they dominate the environment.  This force is called the Earths Magnetic Field and it acts as a protective shield against cosmic radiation.

Before we delve into the subject matter, let us try to define magnetism.  Magnetism is a phenomenon by which materials exert an attractive or repulsive force on other materials.  Now what is magnetic reversal?  A magnetic reversal, in simple terms, occurs when the force between the two poles (North and South) reverses direction. 

Earth’s magnetic field is fading.  This weakening was first observed by a group of scientists in the 19th century who were performing studies on the Earths magnetic field.  If the trend continues, the field may collapse altogether and then reverse.  Compasses would point south instead of north. Without our planet's magnetic field, Earth would be subjected to increased levels of cosmic radiation.  Scientists are keeping an eye on our planet's weakening magnetic field as they try to learn more about how Earth's geodynamo works. The geodynamo is the mechanism that creates our planet's magnetic field, maintains it, and causes it to reverse.

According to Earth's geologic record, our planet's magnetic field flips, on average, about once every 200,000 years. The time between reversals varies widely, however. The last time Earth's magnetic field flipped was about 780,000 years ago. The magnetic field of the Earth has actually switched its direction many many times during Earth's history. Although this is not completely understood, the leading theory of how it works is that Earth's magnetic field is caused by the motion of the liquid outer core.

The churning of the liquid in the outer core acts as a giant electromagnet, moving electrical charges around, in what is known as the "geomagnetic dynamo." The rotation of the solid inner core also contributes to the magnetic field. When a certain combination of inner and outer core motion occurs, the Earth's magnetic field will quickly reverse.

For example, lava that solidified 30,000 years ago shows that the magnetic field was in the opposite direction at that time. Evidence from the geologic record shows that this reversal could take less than 1000 or even less than 100 years. The way these reversals throughout geologic history were discovered was by looking at the seafloor. New ocean floor is created along the mid-ocean ridges. When lava along these ridges cools, its minerals harden in line with the Earth's magnetic field. This causes the seafloor to have magnetic "stripes," which can be measured and mapped.

When this reversal happens, it could knock out power grids, scramble the communications systems on spacecraft, temporarily widen atmospheric ozone holes, and generate more aurora (a spectrum of beautiful colors seen with the naked eye around the North and South poles) activity, not to mention unobstructed solar flares from the Sun.  The cosmic radiation alone could result in millions more dying from cancer every year.

So although the magnetic reversals may not occur during our lifetime, our children's children's children are inevitably going to face it. 

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praveen kumar, India :
Dear Mr. Allen Martis,

Recently I have gone through your article, "Reverse Magnetism". Its very interesting and informative.

I beg your attention on the following point.

1.Our Earth's magnetic field is generated from the motions in the outer core. In general the direction of lines of force is from North to South. Will the direction of these magnetic flux, depend on the direction of motion of the liquid in outer core?if so, the motion in the outer core is very complex, then how the flux has North-South direction? The motion of both outer & inner core changes such that, it will lead to reversal. Is this the thing you have explained in the 6th para of your article?

2. Do we need magnetic field to be present in order to produce magnetic field?(Relate to the case of our Earth).

I request you, a detailed explanation, how certain combination of inner and outer core motion leads to reversal.

I await your reply.
Thank you very much in advance.

Regards
Praveen Kumar.
praveenkv@iitb.ac.in
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