INTRODUCTION: - Earth has its own magnetic field of the order of 10- 5 tesla on its surface. The following points are very important information about geomagetism.
( 1 ) The imaginary magnetic axis of earth makes an angle of 20° with the geographic rotational axis.
( 2 ) The magnetic south pole of earth is in north Canada at latitude 70.5° north and longitude 96° west and the magnetic north pole is at latitude 70.5° south and longitude 84° east.
( 3 ) Magnetic poles are approximately 2000 kilometers away from the geographic poles.
( 4 ) Magnetic and geographic equators intersect each other at longitude 6° west and 174° east.
( 5 ) Thumba near Trivandrum is on the magnetic equator and hence it has been selected for rocket propulsion experiments.
GEO-MAGNETIC ELEMENTS: -
( 1 ) Magnetic Declination: -The geographic meridian of the place is the plane passing through its longitude and a perpendicular drawn on the surface of earth at that place as shown in the figure.
The magnetic meridian at the place is the plane containing the magnetic field line passing through that place and the normal drawn at that place.
The angle between the magnetic meridian and the geographic meridian is called the magnetic
declination of the place. The magnetic needle freely rotating horizontally on a vertical axis at
any place aligns itself to the magnetic North-South direction. The angle between the magnetic needle in this position and the geographic North-South direction gives the magnetic declination at that place. In the figure above, PQRS represents the geographic meridian and P’Q’R’S’ represents the magnetic meridian.
( 2 ) MAGNETIC DIP ANGLE: -
Magnetic field lines are not horizontal at all places on earth except at the magnetic equator. When a magnetic needle is kept at any place such that it freely rotates on a horizontal axis in the plane of magnetic meridian, the angle, f, made by it with the horizontal as sown in the figure is called the magnetic dip angle at that place.
( 3 ) HORIZONTAL AND VERTICAL COMPONENTS OF EARTH'S MAGNETIC FIELD: -The direction of magnetic field, B , at some place is shown in the figure ( front). Along with the magnetic meridian. Bh is the horizontal component and Bv the vertical component of the earth’s magnetic field B. All the above-mentioned parameters of the earth’s magnetic field are shown together in the figure below.
( c ) ORIGIN OF EARTH'S MAGNETIC FIELD: -
The concept that the magnetic field of the earth is due to huge deposits of magnetic ores deep inside it is NOT correct. The core of the earth is extremely hot and contains molten matter which is good conductor of electricity. The ions in this matter rotate due to the rotation of the earth and generate currents. It is believed that the earth’s magnetic field is due to these electric currents though the correct reason for this still under research. This explanation suggest that if any planet containing molten matter in its core rotates with small angular frequency, it would have a weak magnetic field associated with it which is found to be case with the planet Venus. Mon contains no molten matter and hence has no magnetic field. Jupiter rotates with large angular velocity and hence has a very strong magnetic field. Whenever ions move in a molten ore, the current loops generated behave like magnetic dipoles.
( d ) VARIATION IN EARTH'S MAGNETIC FIELD WITH POSITION ON THE SURFACE OF THE EARTH AND HEIGHT FROM THE SURFACE OF EARTH: -
The magnetic field on the surface of the earth is of the order of 10 (exp- 5) T ( tesla ) and it keeps
on decreasing with the height above the surface of the earth. At a height of about 30,000
km, it is of the order of 10 (exp- 6) T. The magnetic field of the galaxy is of the order of 10 (exp- 12) T
Beyond this height, solar winds which correspond to the flow of charged particles originating in the Sun disturb the earth’s magnetic field. These charged particles excite atoms in the atmosphere in this region and ionize them creating show of lights called aurora borealis in the arctic region and aurora australis near the geographic south pole.
( e ) TEMPORAL VARIATIONS IN THE EARTH'S MAGNETIC FIELD: -
The magnetic field of the earth varies. The magnetic declination of London had changed by 35° in 240 years. The magnetic south pole which is in the arctic region of Canada is shifting in northwesterly direction at a rate of 10 km / year for reason not known so far. The earth’s magnetic field reverses in about a million years. This is known from the study of volcanic eruptions at the bottom of oceans. Studying the age of solidified lava from such eruptions gives a clue to the reversal of the earth’s magnetic field.
( 1 ) The imaginary magnetic axis of earth makes an angle of 20° with the geographic rotational axis.
( 2 ) The magnetic south pole of earth is in north Canada at latitude 70.5° north and longitude 96° west and the magnetic north pole is at latitude 70.5° south and longitude 84° east.
( 3 ) Magnetic poles are approximately 2000 kilometers away from the geographic poles.
( 4 ) Magnetic and geographic equators intersect each other at longitude 6° west and 174° east.
( 5 ) Thumba near Trivandrum is on the magnetic equator and hence it has been selected for rocket propulsion experiments.
GEO-MAGNETIC ELEMENTS: -
( 1 ) Magnetic Declination: -The geographic meridian of the place is the plane passing through its longitude and a perpendicular drawn on the surface of earth at that place as shown in the figure.
The magnetic meridian at the place is the plane containing the magnetic field line passing through that place and the normal drawn at that place.The angle between the magnetic meridian and the geographic meridian is called the magnetic
declination of the place. The magnetic needle freely rotating horizontally on a vertical axis at
any place aligns itself to the magnetic North-South direction. The angle between the magnetic needle in this position and the geographic North-South direction gives the magnetic declination at that place. In the figure above, PQRS represents the geographic meridian and P’Q’R’S’ represents the magnetic meridian.
( 2 ) MAGNETIC DIP ANGLE: -
Magnetic field lines are not horizontal at all places on earth except at the magnetic equator. When a magnetic needle is kept at any place such that it freely rotates on a horizontal axis in the plane of magnetic meridian, the angle, f, made by it with the horizontal as sown in the figure is called the magnetic dip angle at that place.( 3 ) HORIZONTAL AND VERTICAL COMPONENTS OF EARTH'S MAGNETIC FIELD: -The direction of magnetic field, B , at some place is shown in the figure ( front). Along with the magnetic meridian. Bh is the horizontal component and Bv the vertical component of the earth’s magnetic field B. All the above-mentioned parameters of the earth’s magnetic field are shown together in the figure below. |
The concept that the magnetic field of the earth is due to huge deposits of magnetic ores deep inside it is NOT correct. The core of the earth is extremely hot and contains molten matter which is good conductor of electricity. The ions in this matter rotate due to the rotation of the earth and generate currents. It is believed that the earth’s magnetic field is due to these electric currents though the correct reason for this still under research. This explanation suggest that if any planet containing molten matter in its core rotates with small angular frequency, it would have a weak magnetic field associated with it which is found to be case with the planet Venus. Mon contains no molten matter and hence has no magnetic field. Jupiter rotates with large angular velocity and hence has a very strong magnetic field. Whenever ions move in a molten ore, the current loops generated behave like magnetic dipoles.
( d ) VARIATION IN EARTH'S MAGNETIC FIELD WITH POSITION ON THE SURFACE OF THE EARTH AND HEIGHT FROM THE SURFACE OF EARTH: -
The magnetic field on the surface of the earth is of the order of 10 (exp- 5) T ( tesla ) and it keeps
on decreasing with the height above the surface of the earth. At a height of about 30,000
km, it is of the order of 10 (exp- 6) T. The magnetic field of the galaxy is of the order of 10 (exp- 12) T
Beyond this height, solar winds which correspond to the flow of charged particles originating in the Sun disturb the earth’s magnetic field. These charged particles excite atoms in the atmosphere in this region and ionize them creating show of lights called aurora borealis in the arctic region and aurora australis near the geographic south pole.
( e ) TEMPORAL VARIATIONS IN THE EARTH'S MAGNETIC FIELD: -
The magnetic field of the earth varies. The magnetic declination of London had changed by 35° in 240 years. The magnetic south pole which is in the arctic region of Canada is shifting in northwesterly direction at a rate of 10 km / year for reason not known so far. The earth’s magnetic field reverses in about a million years. This is known from the study of volcanic eruptions at the bottom of oceans. Studying the age of solidified lava from such eruptions gives a clue to the reversal of the earth’s magnetic field.
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