Earth’s Magnetic Field

The Magnetic Field of the Earth is generated by the motion of molten iron alloys in the Earth’s outer core.  The solid inner core is too hot to hold a permanent magnetic field, but the outer core gives rise to Earth’s magnetic field. The geomagnetic field extends from outer core to where it meets the solar wind. At the surface of Earth, the magnitude of Earth’s magnetic field ranges from 25 to 65 microteslas (0.25 to 0.65 gauss).

How magnetic field protects life on Earth?

The magnetic field deflects most of the charged particles emanating from the Sun in the form of solar winds. If there were no magnetic field, the particles of the solar wind would strip away the ozone layer, which protects the Earth from harmful ultraviolet rays. One of the reasons that there is no atmosphere at Mars is that its magnetic field is turned off which led to the loss of carbon dioxide due to scavenging of ions by the solar wind.

How it is formed?

The Earth’s magnetic field is believed to be caused by electric currents in the liquid outer core, which is composed of highly conductive molten iron. The motion of the fluid is sustained by convection, motion driven by buoyancy. At the core, the pressure is so great that the super hot iron crystallizes into a solid. The higher temperature of the fluid lower down makes it buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is plated to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This is called compositional convection.

The mechanism of formation of Earth’s Magnetic field has not yet been understood fully. The basic physics of electromagnetism can be used to somewhat explain the phenomena. Iron, whether liquid or solid, conducts electricity; when we move a flowing electric current, we generate a magnetic field at a right angle to the electric current direction (Ampère’s  law) . The molten outer core of our planet releases heat by convection, which then displaces the flowing electrical currents. This generates the magnetic field that is oriented around the axis of rotation of the Earth, mainly due to the rotational effects on the moving fluid.  However, it has not been explained how the charges, necessary for creation of electric field originate, which in turn give rise to the magnetic field.

This convection caused by heat radiating from the core, along with the rotation of the Earth (Coriolis force), causes the liquid iron to move in a rotational pattern. It is believed that these rotational forces in the liquid iron layer lead to weak magnetic forces around the axis of spin.  The role of the Coriolis Effect is that it causes overall planetary rotation, and tends to organize the flow into rolls aligned along the north-south polar axis.

Reversal of the fields

Based on data from ancient and new rocks, it has been observed that Earth’s north and south magnetic fields have reversed polarity many times. This is because the polarity of the Earth’s magnetic field is recorded in sedimentary rocks and igneous rocks. The switching from north to south (an individual reversal event) seems to take around a couple thousand years to complete; once the reversal takes place, periods of stability seem to average about 200,000 years. No body has been able to explain why the poles reverse, but theories range from the changes in lower mantle temperatures to the imbalance of landmasses on our world (most of the continental landmass is in the Northern Hemisphere). The last magnetic reversal was 780,000 years ago, which gives us current northern and southern magnetic poles.  It is believed that geomagnetic field is slowing weakening, so Earth might be heading for a long-overdue magnetic reversal. Reversals tend to occur when there is a wide divergence between the magnetic poles and their geographic equivalent (as it is now).

Intensity gradient of the Geomagnetic Field

The intensity of the geomagnetic field is greatest near the poles and weaker near the Equator. A map of intensity contours of the geomagnetic field is called an isodynamic chart. Isodynamic chart for the Earth’s magnetic field shows that minimum intensity of the magnetic field is over South America while maximum is over northern Canada, Siberia, and the coast of Antarctica south of Australia.

Magnetic Dip

Magnetic dip or magnetic inclination is the angle made with the horizontal by the compass needle of a vertically held compass. This angle varies at different points on the Earth’s surface. In the northern hemisphere, the field points downwards. It is straight down at the North Magnetic Pole and rotates upwards as the latitude decreases until it is horizontal (0°) at the magnetic equator. It continues to rotate upwards until it is straight up at the South Magnetic Pole. North Magnetic Pole on the surface of Earth’s Northern Hemisphere at which the planet’s magnetic field points vertically downwards. In 2001, it was in Canada, but now, it has moved out of Canada’s territory towards Russia. The south magnetic pole was off the coast of Wilkes Land — a part of Antarctica — about 2750 km from South Pole.

Geomagnetic Equator & Equatorial Electrojet

Contour lines along which the dip measured at the Earth’s surface is equal are referred to as isoclinic lines. The locus of the points having zero dip is called the magnetic equator or aclinic line. In the following graphics, the green line shows the magnetic equator, which runs very close the southern tip of our country. This is the important reason for the establishment of the Vikram Sarabhai Space Centre at Thumba, which is close to Geomagnetic Equator. The reason is that the magnetic equator differs significantly from the geographic equator. Directly above the magnetic equator, at altitudes of around 110 km in the atmosphere, a system of electric currents exists that flows from west to east along the magnetic equator. It is known as Equatorial Electrojet.

The closer we are to the magnetic equator, the better we are placed to study the Equatorial electrojet. In the early 1960s, there were very few places in the world close to the magnetic equator with adequate infrastructure to support research in this field. That is the reason that Thumba was chosen.  Thumba is located in the outskirts of Thiruvananthapuram. Here, Thumba Equatorial Rocket Launching Station (TERLS) was launched in 1963. Eventually, TERLS have given birth to the Vikram Sarabhai Space Centre (VSSC) and to the Indian Space Research Organisation (ISRO).

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