Understanding Physics Topics is essential for solving complex problems in many fields, including engineering and medicine.
What is the Concept of Geomagnetism? What are the Elements of the Earth’s Magnetic Field?
A freely suspended magnet always sets itself at rest along the north-south direction. If disturbed, it will again come to the pre-vious position after a few oscillations. This phenomenon is observed anywhere on the earth. Since only a magnetic field can influence a magnet, we can infer that a magnetic field exists throughout the earth, i.e., the earth behaves as a magnet. The following two phenomena can be mentioned in support of this concept:
i) If a soft iron rod be kept at a place on the surface of the earth facing north-south direction, after a long period of time (say, six months), a feeble magnetism is found to have been developed in the rod.
ii) If a closed conductor is moved in a magnetic field, an electric current is induced in that conductor (see the chapter ‘Electromagnetic Induction’). Similarly, if a closed conduc-tor is moved at any place on the earth’s surface, a feeble electric current is seen to be induced in that conductor.
The earth is a huge magnet: To explain the cause of the above-mentioned phenomena, scientists came to the conclusion that the earth behaves as a huge magnet. In 1600, physicist Wil-liam Gilbert suggested this theory for the first time. Later on, Gil-bert performed a simple experiment. He shaped a lodestone into a sphere and demonstrated that small magnets placed at different positions on the sphere behave exactly as they do on earth.
Locations of the earth’s magnetic poles: Like an ordinary magnet, the earth’s magnet also has two poles. The north pole of earth’s magnet is situated at Ellef Ringnes Island in Canada. The latitude and longitude of this region are 78.3°Af and 104° W. This region is at an approximate distance of 1300 km from the geographical north pole. The south pole of the earth’s magnet is situated in sea near the sea-shore of Wilkes Land belonging to Antarctica. The latitude and longitude of this region are 65 °S and 139 °E. This region is at a distance of about 2550 km from the geographical south pole.
These poles are not stationary. The north pole shifts towards north-west by 15 km every year (The positions of the geomag-netic north pole at different times are shown in Fig.). The
shifting of the south pole is nearly the same. At the present times, the magnetic axis of the earth is inclined at an angle of 11.5° with its geographical axis and distance of earth’s magnetic axis from the centre of earth is about 1120 km .
Nature of earth’s magnetic poles:
The pole of a magnetic needle that points towards the north, is called the north seeking pole or simply north pole; and the pole which points towards the south, is called the south seeking pole or simply south pole. The magnetic poles of earth at the north and at the south are called magnetic north pole and magnetic south pole. Now, the north and south poles of a magnetic needle direct themselves towards the manetic north and south poles of the earth. It means that, the north pole of earth behaves as the south pole of an ordinary magnet and the south pole of earth as the north pole of an ordinary magnet. Sometimes the north pole of earth’s magnet is called blue pole and its south pole is called red pole.
Geomagnetic Field
Magnetic field of earth extends up to a great height above its surface. Practically, the influence of this field is found up to a height of 105 kilometre (approx.) above earth’s surface.
There was no clear concept about the source of geomagnetic field for a long time. If we imagine a bar magnet kept inclined at an angle of 11.5° with the geographical axis at the centre of earth, it is possible to describe the alignment of the geomagnetic field more or less correctly. For a hypothetical bar magnet, the nature of the lines of force is shown in Fig.
Sources of geomagnetic field: Geomagnetic field has three different sources. From these three sources the following three magnetic fields are produced.
1. Main field: The source of this magnetic field is the outer core of earth which is made of liquid iron. Electric current in this part produces the main field. Main field develops 97% to 99% of the geomagnetic field.
2. Crustal field: The source of this magnetic field is the earth’s crust. Some part of earth’s crust made of hard rock becomes magnetised due to the presence of main field. This magnetised part creates the crustal field. 1% to 2% of the geo-magnetic field is due to this crustal field.
3. External field: The source of this magnetic field is the ionosphere present in the atmosphere of earth. This part, made up of ions, produces the external field under the influence of solar wind. 1% to 2% of the geomagnetic field is due to this external field.
Direction of geomagnetic field: Magnetic lines of force of the geomagnetic field is naturally from the N -pole to the S-pole. But, actually, the geomagnetic N-pole is in the south and the S-pole in the north [Fig.]. As a result, to show the direction of the geomagnetic field at a place, the arrow sign on each line of force should be from the south to the north direction.
Elements of Earth’s Magnetism
To know the magnitude and direction of the geomagnetic field at any place on earth, the quantities required are called the ele-ments of earth’s magnetism. There are three elements of earth’s magnetism. These are:
- angle of dip,
- angle of declination and
- horizontal component of geomagnetic intensity.
Dip Or angle of dip: The angle made by the intensity of earth’s magnetic field with the horizontal at any place on the earth is called dip or angle of dip at that place.
If a bar magnet is suspended from its centre of gravity by a thread, the magnet at rest, does not remain horizontal but inclines a little [Fig.].
This means that the magnetic axis sets itself along the direction of the intensity of geomagnetic field at that place. The angle (θ) made by the magnetic axis with the horizontal straight line drawn on the magnetic meridian is called dip or angle of dip at that place. So, if the angle of dip of a place is known, we can determine the direction of the intensity of geo-magnetic field at that place.
Positive and negative dips: The inclination of a magnet is different at different places on earth, i.e., the values of dip are different at different places. If the north pole of a magnet leans downwards at a place, the value of dip is taken as positive; but if the south pole of the magnet leans downwards, the dip at that place is taken as negative. Positive and negative dips are denoted by the symbols N and S, respectively. For any place at the northern hemisphere of earth, the angle of dip becomes positive, but for places at the southern hemisphere, this angle of dip is negative.
‘The angle of dip at Kolkata is 31° N’, means that if a magnet is suspended from its centre of gravity at Kolkata, the north pole of the magnet leans downwards and the magnetic axis of the magnet inclines at an angle of 31° with the horizontal plane.
At the two magnetic poles of earth, angles of dip are 90° each, i.e., at these two places, a freely suspended magnet remains ver-tical. At the magnetic equator, angle of dip is 0°, i.e., a freely sus-pended magnet remains horizontal at the magnetic equator.
Declination or angle of declination: The geographical poles and the magnetic poles of earth are not located at the same places. So if a magnetic needle, capable of rotating in the horizontal plane freely, is kept at a place, we will observe that, at rest, the magnetic axis of that needle makes a definite angle with the geographical north-south horizontal line at that place. At different places on earth’s surface, the value of this angle is dif-ferent.
The vertical plane passing through the magnetic axis of a freely suspended magnetic needle at any place is called the magnetic meridian at that place. Again, the vertical plane at a place containing the geographical north and south poles of earth, is called the geographical meridian at that place.
Definition: The angle between the magnetic meridian and the geographical meridian at a place is called the angle of dec-lination at that place.
It is usually denoted by the symbol δ [Fig.]. If the north pole of the magnetic needle turns towards the east or the west with respect to the geographical meridian, the angle of declination is called δ°E or δ° W, respectively. For example, ‘angle ofdeclina-tion of Delhi is 2 °E means that at Delhi, the north pole of a magnetic needle capable of rotating freely on a horizontal plane moves away from the geographic meridian towards east through 2°.
Naturally, the angle of declination at different places on the earth are different. At a place, where magnetic meridian and geographical meridian coincide, the angle of declination becomes zero.
Horizontal component of geomagnetic intensity:
The direction of the geomagnetic intensity at a place does not lie on the horizontal plane usually, but it inclines at a definite angle with the horizontal plane. This definite angle is called the angle of dip. Since magnetic intensity is a vector quantity, the geomagnetic intensity can be resolved into a horizontal component and a vertical component. It should be clear that these two components lie on the magnetic meridian.
In Fig., ABCD is the geographical meridian and GBCJ is the magnetic meridian. At the point B, the magnitude and direction of the geomag-netic intensity I can be represented by BR. The magnitudes and directions of vertical component V and the horizon-tal component H of I can be expressed by BM and BN, respectively.
Let the angle of dip be θ and the angle of declination be δ.
∴ V = Isinθ and H = I cosθ
or, \(\frac{V}{H}\) = \(\frac{I \sin \theta}{I \cos \theta}\) = tanθ
∴ V = Htanθ
Again, V2 + H2 = I2sin2θ + I2cos2θ = I2
The value of the horizontal component H of geomagnetic intensity is not the same throughout earth’s surface. At the magnetic equator, θ = 0° and hence H = I; this is the maximum value of H. Again, at the magnetic poles, θ = 90° and hence H = 0; this is the minimum value of H. Note that, at a place where the angle of dip is 45°, the values of horizontal Fig. and vertical components are equal.
The elements of earth’s magnetism for northern hemisphere are shown in Fig. 2.31. In case of southern hemisphere, the elements are shown in Fig.
‘Horizontal component of earth’s magnetic field at Kolkata is 0.37 Oe’ means that, the magnitude of the horizontal compo-nent of the geomagnetic intensity, i.e., the force acting on a unit pole along the magnetic meridian at that place is 0.37 dyn.
In most cases we require the horizontal component of the geomagnetic intensity, vertical component is of less importance. This is because the magnetic needles we use in the laboratory, can rotate in the horizontal plane but not in the vertical plane. As a result, only the horizontal component acts on the needle to create deflection in it, but the vertical component has no effect on it.
Table-3
Values of the geomagnetic elements at some places
Mariner’s Compass
Mariner’s compass, as the name suggests, is used for ascertain-ing the direction in the sea when the sun, the pole star or other stars are not visible.
The construction of a compass in based on the directive prop-erty of a magnet.
Working principle: Noting the position of the crown mark on the compass disc, navigators determine the direction. The crown mark on the compass disc indicates the magnetic north. To determine the geographical north at a place, the angle of dec-lination at the place should be collected from the magnetic maps. If the value of that declination is δ° W, it should be understood that the crown mark lies at an angle of δ°, west of geographical north. In this way from the position of the crown mark on the compass-disc, navigators can determine directions [Fig.],
Magnetic Maps
Values of the elements of earth’s magnetism are different at dif-ferent places on earth. But the value of any one of the elements at different places on earth’s surface may be the same. The places on earth’s surface possessing’the same value of a particu-lar element are along a line. Thus different lines are drawn for different values of that element. The geographical map of earth containing such lines are called magnetic maps. For three different elements of earth’s magnetism, three different magnetic maps are obtained.
The values of earth’s magnetic elements at a particular place change gradually with time. Hence new magnetic maps should be drawn at different times. Maps of this kind are essential to the navigators and also for searching minerals under earth’s crust. Three different kinds of magnetic maps are shown in Fig’s.
Isogonic lines:
Places on earth’s surface, having equal declination are joined by lines, called isogonic lines. In Fig., the lines shown are iso-gonic lines drawn in 2000 AD. The lines of zero declination are called agonic lines.
Isoclinic lines:
The lines obtained by joining the places on earth’s surface hav-ing equal dip, are called isoclinic lines. In Fig., the lines shown are isoclinic lines drawn in 2000 AD. The line of zero angle of dip corresponds to the magnetic equator, and it is called
an aclinic line.
Isodynamic lines:
Places on earth’s surface having equal horizontal components of geomagnetic field intensity are joined by lines, called isody-namic lines. The lines shown in Fig. are isodynamic lines drawn in 2000 AD.
Variations of the Elements of Geomagnetism
The values of the elements of geomagnetism are different at dif-ferent places on the earth. Again, at a particular place the values of these elements do not remain the same always; they vary with time. This change is periodic, i.e., after a definite period of time, the elements return to their initial values. This kind of variation is known as periodic or regular variation.
Besides this, the elements of geomagnetism suffer another kind of variation. This kind of variation is known as a magnetic storm.
Regular variation: Regular variation are of three kinds—
- daily variation,
- annual variation and
- secular varia-tion.
Daily variation; The elements of geomagnetism undergo a kind of slow variation daily. At two different specific times on a day, any element of geomagnetism attains maximum and mini-mum values.
Annual variation: The elements of geomagnetism also undergo a kind of very slow variation. On two specific months in a year, the value of any element becomes maximum and mini-mum. For example, at London, the declination is found to have maximum value in February and a minimum value in August every year. Variations of declination in the northern and south-ern hemispheres are just the opposite.
Secular variation: A kind of secular variation is observed in the elements of geomagnetism. The rate of this kind of variation is very slow; and its time period is approximately 960 years. Perhaps this variation is due to the rotation of the earth’s mag-netic north and south poles around its geographical south and north poles, respectively.
Magnetic Storm: Sometimes sudden huge changes in the elements of geomagnetism are seen throughout a large region on earth’s surface. This phenomenon is known as a magnetic storm. This kind of variation cannot be predicted. Although after a while, the values of the elements return to the normal state. Magnetic storms usually occur due to earthquakes, volcanic eruptions, aurora borealis, appearance of large sunspot, etc. During a magnetic storm, radio communication, television, telephone systems, etc. are disturbed greatly.