GEOMAGNETISM

Geomagnetism is the science devoted to the knowledge of the Earth magnetic force.

The study of geomagnetism is one of the oldest of the geophysical sciences. Since before the publication of William Gilbert's De Magnete in 1600, people have tried to unravel the mysteries of Earth's magnetic field.

 

  

 

 GEOMAGNETIC FIELD IS WELL APPROXIMATED BY A DIPOLE

Actually it is known that the Earth's magnetic field is closely approximated bythe field of a dipole positioned near the centre of the Earth. A dipole defines an axis. The two positions where the axis of the dipole that best fits the Earth's field intersect the Earth's surface are called the North and South geomagnetic poles. An imaginary line joining the magnetic poles would be inclined by approximately 11.3° from the planet's axis of rotation.

Often, geomagnetic strenght is measured in gauss and not in Tesla: 1 Tesla=10^{4 gauss.} The strength of the field at the Earth's surface ranges from less than 30 microteslas (0.3 gauss) in an area including most of South America and South Africa to over 60 microteslas (0.6 gauss) around the magnetic poles in northern Canada and south of Australia, and in part of Siberia.

Magnetic fields extend infinitely, though they are weaker further from their source. The Earth's magnetic field, which effectively extends several tens of thousands of kilometres into space, is called the magnetosphere.

 

   MAGNETIC POLES ARE NOT STATIC BUT WANDER

The locations of the magnetic poles are not static but they wander as much as 15 km every year. The Earth's magnetic field continuously changes in strength and position. The two poles wander independently of each other and are not at directly opposite positions on the globe. Currently the magnetic south pole is farther from the geographic south pole than the magnetic north pole is from the geographic north pole.

The figure on the left show the North magnetic pole path from 300 A.D.

 

   MAGNETIC ORIENTATION REVERSED MANY TIMES IN THE PAST

Based upon the study of lava flows of basalt throughout the world, it has been proposed that the Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 250,000 years. The last such event, called the Brunhes-Matuyama reversal, is theorized to have occurred some 780,000 years ago.

Using a magnetic detector (a variant of a compass), scientists have measured the historical direction of the Earth's magnetic field, by studying sequences of relatively iron-rich lava flows. Typically such layers have been found to record the direction of Earth's magnetic field when they cool. They have found that the poles have shifted a number of times throughout the past.

There is no clear theory as to how the geomagnetic reversals might have occurred. Some scientists have produced models for the core of the Earth wherein the magnetic field is only quasi-stable and the poles can spontaneously migrate from one orientation to the other over the course of a few hundred to a few thousand years. Other scientists propose that the geodynamo first turns itself off, either spontaneously or through some external action like a comet impact, and then restarts itself with the magnetic "North" pole pointing either North or South.

(informazioni prese da WIKIPEDIA)

Se vuoi approfondire la conoscenza sulla magnetostatica, clicca su questi link:
	Magnetismo (stub)
	Il campo magnetico della Terra nello spazio: la magnetosfera
	The industry for magnetic confinement: the tokamak
	Paleomagnetismo: la geologia incontra il magnetismo
	L'alternatore

 

---

---