Earth's Magnetism
The Earth’S Magnetism
Our Earth itself behaves like a giant magnet, producing its own magnetic field. This phenomenon is known as terrestrial magnetism or the Earth's magnetism. The Earth's magnetic field extends far out into space, forming a protective shield called the magnetosphere, which deflects harmful charged particles from the Sun (solar wind).
Source of Earth's Magnetism
The exact mechanism responsible for Earth's magnetism is complex and not fully understood, but the most widely accepted theory is the dynamo effect. This theory proposes that the Earth's magnetic field is generated by the motion of molten iron and nickel in the Earth's outer core. Convective currents in this conducting fluid, combined with the Earth's rotation (Coriolis effect), act like a self-sustaining dynamo, producing electric currents that, in turn, generate the magnetic field.
The Earth's magnetic field is approximately (but not exactly) like that of a bar magnet placed at the center of the Earth, tilted with respect to the Earth's rotational axis.
Geographic Poles vs. Magnetic Poles
It is important to distinguish between the Earth's geographic poles (North and South, defined by the rotational axis) and its magnetic poles.
- Geographic North Pole: The northern end of the Earth's axis of rotation.
- Geographic South Pole: The southern end of the Earth's axis of rotation.
- Magnetic North Pole: The location on Earth's surface where the Earth's magnetic field lines are directed vertically downwards (into the Earth).
- Magnetic South Pole: The location on Earth's surface where the Earth's magnetic field lines are directed vertically upwards (out of the Earth).
Since the Earth acts like a bar magnet with its magnetic field lines emerging from the magnetic South pole and entering the magnetic North pole (outside the Earth), the Earth's geographic North pole is actually located near the Earth's magnetic South pole, and the geographic South pole is located near the Earth's magnetic North pole. This is why the North pole of a compass needle (which is the geographic North pole seeking end, meaning it's a magnetic North pole) points towards the Earth's geographic North pole region.
The Earth's magnetic poles are not fixed; they slowly drift over time. The magnetic poles are also not exactly antipodal (opposite each other on the globe). The imaginary line joining the magnetic North and South poles is called the magnetic axis. This magnetic axis is tilted at an angle of approximately $11.3^\circ$ with respect to the Earth's geographic axis of rotation. The magnetic poles are also not exactly at the surface, but this simple model is a good approximation.
Elements of Earth's Magnetic Field
To completely describe the Earth's magnetic field at any point on its surface, three quantities are needed. These are called the elements of Earth's magnetic field:
- Magnetic Declination ($\theta$ or D)
- Magnetic Dip or Inclination ($\delta$ or I)
- Horizontal Component of Earth's Magnetic Field ($B_H$)
Magnetic Declination And Dip
Let's define the first two elements:
1. Magnetic Declination ($\theta$ or D): This is the angle between the geographic meridian and the magnetic meridian at a place.
- Geographic Meridian: A vertical plane passing through the geographic North and South poles of the Earth. It contains the lines of longitude.
- Magnetic Meridian: A vertical plane passing through the direction of the Earth's magnetic field at a given point. The magnetic field lines are parallel to this plane. A freely suspended magnetic needle, allowed to move only in a horizontal plane, aligns itself along the magnetic meridian.
Magnetic Declination (D) is the angle between geographic North and magnetic North.
Declination is positive if the magnetic North is east of geographic North and negative if it is west. A zero declination line (agonic line) passes through places where the magnetic and geographic meridians coincide. Declination varies from place to place on Earth's surface. A compass needle points towards magnetic North, which is generally different from true geographic North. One must know the local declination to use a compass for true navigation.
2. Magnetic Dip or Inclination ($\delta$ or I): This is the angle between the direction of the Earth's total magnetic field ($\vec{B}_E$) and the horizontal plane at a place. It is the angle by which a freely suspended magnetic needle, balanced at its centre of gravity, dips downwards (towards the Earth) in the magnetic meridian.
- At the geographic poles, the dip angle is approximately $90^\circ$ (the field lines are vertical).
- At the magnetic equator, the dip angle is approximately $0^\circ$ (the field lines are horizontal).
Magnetic Dip angle ($\delta$) is the angle the total magnetic field makes with the horizontal plane.
The dip angle varies from $0^\circ$ at the magnetic equator to $90^\circ$ at the magnetic poles. It is positive in the Northern Hemisphere (field dips downwards into the Earth) and negative in the Southern Hemisphere (field dips upwards out of the Earth).
3. Horizontal Component of Earth's Magnetic Field ($B_H$): The Earth's total magnetic field $\vec{B}_E$ at a point can be resolved into a horizontal component ($B_H$) and a vertical component ($B_V$) in the magnetic meridian plane.
$ B_H = B_E \cos\delta $
$ B_V = B_E \sin\delta $
The magnitude of the total Earth's magnetic field is $B_E = \sqrt{B_H^2 + B_V^2}$.
The horizontal component $B_H$ is the element needed along with declination and dip to fully specify the Earth's field at a location.Variations in Earth's Magnetic Field
The Earth's magnetic field is not constant. It changes over time in several ways:
- Secular Variation: Slow changes in the declination, dip, and total intensity over decades or centuries. The magnetic poles drift and the tilt of the magnetic axis changes.
- Geomagnetic Reversals: Historically, the Earth's magnetic field has completely reversed its polarity numerous times over geological timescales (thousands to millions of years).
- Diurnal Variation: Small daily fluctuations related to the Sun's influence on the ionosphere.
- Magnetic Storms: Sudden, large fluctuations caused by solar activity (like solar flares and coronal mass ejections).
The study of Earth's magnetism (geomagnetism) is important for navigation, geophysical prospecting, and understanding space weather effects on technology (satellites, communication).