GS14: Elements of the Earth's Magnetic Field.
Updated: Jul 27, 2020
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The strength of the Earth’s Magnetic field varies depending upon the location i.e. latitude and longitude. It is found to be of the order of 0.00001 T or 0.01 Gauss (1000 Gauss= 1 Tesla).
The earth’s magnetic field is explained vividly by the Dynamo effect. This effect explains that due to the electrical currents arising from the convective motions of the metallic fluids (consisting mostly of iron and nickel) in the outer core of the Earth. The magnetic field lines around the Earth are analogously manifested by the bar magnet. The axis of the magnetic dipole does not coincide with the axis of rotation of the Earth but is presently tilted by the approx. 11.3 degrees with respect to the later. The magnetic poles on the surface of the Earth are located where these magnetic lines are ejected from or enter into the Earth.
The location of the North Magnetic Pole is at a latitude of 79.74 degrees N and a longitude of 71.8 degrees W, somewhere in North Canada.
The location of the South Magnetic Pole is at 79.74 degrees S, 108.22 degrees E, somewhere in the Antarctic.
The MAGNETIC north pole has a denotation to the south end of the assumed magnetic bar positioned in the core. The name, however, is linked to the end of the compass pointing towards that direction. So, don’t be confused! Also, the magnetic equator is not coinciding with the geographic equator, it has some offsets. Can you tell the offset in the comment box? (The location is the key!)
Magnetic declination and dip.
Before we discuss the nub of this article, we shall learn a few basic definitions:
Longitude: The geographic coordinates which describe the East-West position of a point on the Earth’s surface is a longitude of that point. It can be present in any celestial body and is a circular section running along the N-S direction. It is an angular measurement, expressed in degrees, and measured with reference to the Prime Meridian, passing through Greenwich, England. The Prime Meridian is a 0 degrees longitude, to the east of which we have a positive longitude and to the west of which we have a negative longitude.
Latitude: The geographic coordinates that specify the North-South position of a point on the Earth’s surface is known as the latitude of that point. All types of celestial bodies can have latitude as well. They are present as circular planes running along the E-W direction. They too are measured in degrees with respect to the angle made with the equatorial plane.
Geographic meridian: It is the vertical plane containing the longitude circle and the axis of rotation of the Earth.
Magnetic Meridian: It is a vertical circular plane which passes through the imaginary line joining the magnetic north and magnetic south poles.
Are you thinking, why are these sections circular? Yes, it is because if you cut a thin section of a sphere then you will get a circular disc. However, you have to just consider the perimeter of a disc. Therefore, we get a circular section.
The Intensity of the geomagnetic field is strongest at the magnetic poles, where magnetic lines of force are oriented vertically.
It is a wrong belief that a magnetic compass points to the north magnetic pole of the Earth. If you thought the same, today you will learn, why?
A compass needle consists of a magnetic needle that floats on a pivotal point. When the compass is held level, it points along the direction of the horizontal component of the field at the location. Thus, the compass needle would stay along the magnetic meridian of the place. However, it has a little offset from the geographic meridian. It is tilted with respect to the geographic axis of the Earth, the magnetic meridian makes an angle with the geographic meridian at a point. This angle between the true geographic north and the magnetic north of the compass needle is known as magnetic declination.
The declination is greater at higher latitudes and smaller near the equator. In India, it is 0 degrees 41’ E at Delhi and 0 degrees 58’ W at Mumbai.
Another quantity of interest in today's article is the inclination or angle of dip. If a magnetic needle is perfectly balanced about a horizontal axis so that it can swing in a plane of the magnetic meridian, the needle would make an angle with the horizontal. This angle is known as the angle of dip. This dip is an angle that the TOTAL magnetic field of the earth makes with the surface of the Earth.
What happens to the magnetic compass at the poles?
You would have heard that the magnetic compass behaves erratically when it is taken to the poles. There is a very simple explanation of this complex-looking problem. At the poles, the magnetic field lines are converging or diverging vertically so that the horizontal component is negligible. If the needle is only capable of moving in a horizontal plane, it can point arbitrarily in any direction. Therefore, it becomes useless. What one needs in such a case is called a dip needle which is a compass enabling a movement of a pin in a vertical direction as well. The needle of the compass then shows the angle which the magnetic field makes with the vertical. At the magnetic poles, such a needle will point straight down.
The element of the Earth’s magnetic field:
There are three elements of the Earth’s magnetic field, viz. The declination D, the angle of dip or inclination I, and the horizontal component of the Earth’s field H. There exists a relation between the three. If we assume the vertical component of the field as V and the total magnetic field as B, we have:
tan (I)= Z/H
The minimum value of B occurs in Southern Brazil, South Atlantic, which is approx. 25 micro Tesla.
For a better understanding, let us see how the magnetic compass would behave when it is 45 degree North and South, and near the latitude.
When a magnetic compass is read near the 45 degree North, with compass needle pointed at North magnetic pole with 45 degree inclination into the Earth.
At 45 degree south latitude, the compass needle still points towards the North magnetic pole with needle tilted at 45 degree into the air.
Near the equator, the compass needle lies horizontal to the Earth's surface and the head of the needle (also known as Crown) points in the North direction.