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The Earth is not a bar magnet
by
Professor Abdellah Kharicha

When looking at the earth's magnetic field lines, one can think that Earth is like a gigantic bar magnet. However, the Earth’s magnetic field lines are not perfectly symmetrical with respect to the Earth’s magnetic axis, unlike the lines in the bar magnet. In addition, while the position of the south pole is relatively stable, the position of the north pole is continuously changing with time. Since 1831, scientists have been meticulously measuring its position. Over the past 200 years, Earth's magnetic field has been weakening and shifting its magnetic north pole -- the one on which a compass point, not to be confused with the geographical north pole.

Since then, Earth's magnetic north pole has been slowly drifting north-northwestward for over 600 miles (1,100 km), with a rate of advance that has increased from around 10 miles (16 km) a year to around 34 miles (55 km) a year. The magnetic fields it produces have been moving steadily northwest ever since, now sitting halfway across the Arctic Ocean, 450 kilometers south of the geographical North Pole, drifting closer to Russia every year by around 55 kilometers. The Earth's northern magnetic pole has been moving around in unpredictable ways, which has fascinated explorers and scientists.

The North Magnetic Pole is the spot on the Earth's northern hemispheres surface where the planet's magnetic field points vertically downwards (in other words, if you let a compass magnetic needle spin in three dimensions, it would point directly downwards). Because the planet's magnetic field is not perfectly symmetrical, the north and south magnetic poles are not antipodal, meaning a straight line drawn from one to the other does not traverse Earth's geometric center. The geomagnetic north pole, an associated dot, is the pole of an ideal, dipole-shaped model of the planet's magnetic field, which is more tightly fitted to the actual Earth’s magnetic field.

The bar magnet is usually made from metals such as iron. Each type of material magnet has a maximum temperature that it can resist, and if it goes above this temperature, it loses magnetic properties altogether. This is known as the Curie temperature, for iron it is 1043 K. The temperature of Earth’s interior is currently estimated to be at 6000C, far higher than the Curie temperature. It is thus clear that unlike a bar magnet, the Earth’s magnetic field cannot not be the result of magnetization of earth’s interior material.

So how does the earth generate its magnetic field?

The answer to these questions lies inside the dynamics of the Earth’s liquid outer and solid inner core. In fact, the origin of the earth’s magnetic field is not completely understood, but is thought to result from electric currents generated from movement of the iron and nickel melts inside Earth's outer core.

Although temperatures in the inner core are now thought to be warmer than those on the Sun's surface, the immense pressures that are so deep within the planet's interior overwhelm temperature effects, preventing iron from being rendered liquid. Earth's inner solid core is surrounded by a liquid outer core, in which a natural convection driven by thermal buoyancy must take place.

Because materials in Earth's outer core are predominantly iron and nickel, this liquid metal movement generates electrical currents when it flows through the weak underlying magnetic field. As iron from the outer core crystallizes onto the surface of the inner core, it changes the density of the outer fluid, driving turbulent and swirling motion that is able sustain the planet's magnetic field. The detailed mechanisms-- on how flow kinetic energy is transformed into electromagnetic energy --are the topic of the Dynamo theories. These theories employ magnetohydrodynamic equations to explore how a fluid motion could continually re-generate the Earth’s magnetic field. According to Dynamo theories, there must be a difference of 1500 degrees C between the core and mantle in order to stimulate the thermal motion of the outer core, which - together with the rotation of Earth – generates a magnetic field. One of the best successes of the Dynamo theory is its ability to explain why the magnetic poles are drifting, and sometimes flipping where north and south poles flip their position. According to geological records, Earth’s magnetic field has undergone numerous reversals of polarity. Since the first measurements in the 1840s, the strength of the earth's magnetic field is almost continuously decreasing. This has led some scientists to think that we may be in the early stages of a magnetic pole reversal.

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