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Imagine, what would have happened, if there had not been earth’s magnetic field. Ships could not have sailed through the oceans, airplanes could not have reached their destinations, birds could not have migrated to new lands and sea animals could not have navigated through long distances. If there had not been geomagnetism, harmful high-energy radiations/protons from the sun could have reached the earth to destroy life. Earth's magnetosphere would have turned off and eventually the solar wind could have blown the atmosphere away.
A day without earth’s magnetic field would be a day of extinction, perhaps mass extinction for many species.
Magnetic Navigation By Birds, Animals and Sea Creatures:
One of the first concrete signs that animals can tap into the magnetic field was observed, as in many great discoveries in science, by chance. It was the fall of 1957, and Hans Fromme, a researcher at the Frankfurt Zoological Institute in Germany, noticed that several European robins he kept in a cage were becoming restless and were fluttering up into the southwestern part of the cage. There was nothing unusual about it, it was known that migrating birds in cages become edgy at that time of year, and European robins in Germany migrate southwestwards to Spain to over winter. What made it striking was that the birds were in a shuttered room. They could see neither visual landmarks, nor their fellow, non-captive robins, nor the sun or stars, which were known to serve them as navigational aids. Clearly they were acting on something invisible, and Fromme deduced it must be the Earth's magnetic field.
Careful tests with homing pigeons and other birds displaying the ability to judge direction show that the birds are affected by changing magnetic fields. If birds are released at places where the earth's magnetic field is anomalously strong, their homing ability is entirely disrupted. Magnetic storms do the same.
Numerous experiments undertaken have shown that many living things avail themselves of the magnetic field. Organisms as diverse as hamsters, salamanders, sparrows, rainbow trout, spiny lobsters and bacteria all do it, everything from fruit flies to frogs.
How do we know organisms have this ability? There are some standard methods to test for it. Small coils placed near the birds' heads to create unnatural magnetic fields do disturb the ability of pigeons to find home. In another experiment, 24 blind mole rats were trained to reach a goal box at the end of a complex labyrinth. When all had mastered the task, half the rats were let do it again under the natural field and half under a reversed field. The latter rats' performance fell far short of that achieved by their magnetically unmanipulated fellows.
A step further than the blind mole rat, other animals use the magnetic field like we do the Global Positioning System, to determine their location on the surface of the Earth and using that to negotiate unseen pathways during migration. Kenneth and Catherine Lohmann of the University of North Carolina at Chapel Hill and their team have shown through many experiments that during their 8,000-mile migration around the Atlantic Ocean, young loggerhead sea turtles can detect not only the field's intensity but also its inclination. The turtles use these two pieces of information, which vary at every point on the planet's surface, as navigational markers that help them advance along their migratory route
Geomagnetic Dynamo- A Driving Force for Earth’s Magnetic Field:
Albert Einstein described the problem of the origin of the Earth's magnetic field as being one of the five most important unsolved problems in physics. However it is commonly believed that fluid dynamos in the Earth’s mantle produce the Earth’s magnetic field. We know, from our elementary knowledge of Physics that an electric current passing through a metal wire produces a magnetic field around that wire. Likewise, a wire passing through a magnetic field creates an electric current within the wire. This is the basic principle that allows electric motors and generators to operate.
To understand what's happening, we have to take a trip ... to the center of the Earth where the magnetic field is produced. At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it "the inner core." It's really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2° of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as "the outer core."
Earth's magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes like water in a pan on a hot stove. The outer core also has "hurricanes"--whirlpools powered by the Coriolis forces of Earth's rotation. These complex motions generate our planet's magnetism and the process called the dynamo effect.
Earth’s Inconstant Magnetic Field:
Scientists have long known that the magnetic pole moves. James Ross located the pole for the first time in 1831 after an exhausting arctic journey during which his ship got stuck in the ice for four years. No one returned until the next century. In 1904, Roald Amundsen found the pole again and discovered that it had moved--at least 50 km since the days of Ross.
There is a good deal of evidence that the Earth's magnetic field does, and has, reversed many times in the Earth's history, but it is very difficult to use this information to directly answer the question of "Why”. Part of the answer to this question lies in the equations of dynamos. For each solution that yields a magnetic field of normal polarity there exists another solution that yields a field of reversed polarity. This argument only allows for the possibility of two separate polarities however, and has not explained the reason that the magnetic poles actually do reverse.
But then, Professor Gary Glatzmaier and Paul Roberts at University of California created a supercomputer model of Earth's interior using the equations of magnetohydrodynamics (MHD), a branch of physics dealing with conducting fluids and magnetic fields. Their software heats the inner core, stirs the metallic ocean above it, and then calculates the resulting magnetic field. They run their code for hundreds of thousands of simulated years and watch what happens. What they see mimics the real Earth. The magnetic field waxes and wanes, poles drift and, occasionally, flip. They learnt that change is normal. The source of the field, the outer core, is itself seething, swirling, turbulent. Professor Glatzmaier noticed that the changes detected on our planet's surface are a sign of that inner chaos.
Therefore, those scientists in the know—palaeomagnetists, to be exact—expect that perhaps sometime in the future, compass needle will start pointing south rather than north. This is not alarming at all. These reversals happen on average only about once every 250,000 years, and they take hundreds of years to complete so species have time to accommodate to the change.
Magnetosphere:
The greatest blessing of geomagnetism is the magnetosphere, the region above the ionosphere in which the magnetic field of the earth has a dominant control over the motions of gas and fast charged particles. In spite of its name, the magnetosphere is quite non-spherical. The boundary of the magnetosphere, “magnetopause”, is roughly bullet shaped. Magnetopause behaves roughly like a droplet of liquid exposed to supersonic flow. The magnetopause will ripple, flap, and sometimes droplets will break off. It is the location where the outward magnetic pressure of the Earth's magnetic field is counterbalanced by the solar wind, a fast outflow of hot plasma from the sun in all directions. Most of the solar particles are deflected to either side of the magnetopause, much like water is deflected before the bow of a ship. However, some particles become trapped within the Earth's magnetic field and form radiation belts. An additional feature is a collision-free bow shock which forms in the solar wind. Bow shock is the boundary at which the solar wind abruptly drops as a result of its approach to the magnetopause. The particles making up the solar wind follow spiral paths along magnetic field lines. The velocity of each particle as it gyrates around a field line can be treated similarly to a thermal velocity in an ordinary gas, and in an ordinary gas, the mean thermal velocity is roughly the speed of sound. At the bow shock, the bulk forward velocity of the wind drops below the speed at which the particles are corkscrewing. We say that the bulk velocity of the fluid,in this case, the solar wind, drops from "supersonic" to "subsonic".
This means a severe weakening or disappearing of magnetic field would lay us open to the harmful radiations from the sun.
Measuring the Earth’s Magnetic Field:
Measurements of the Earth's magnetic field are continually made around the world at magnetic observatories and data is collected from various oceanographic, land and even satellites surveys. There are two obvious reasons for doing this.
In the study of the Earth's history, very few physical quantities have left any record that can be measured today. The study of seismology and the Earth's gravity field have revealed many secrets about the Earth's interior, but there is no way to examine that how these measurements may have, or are, changing with time. Measurements that can be examined through time are extremely important to make inferences about what the Earth may have been like in the past. Therefore scientists run magnetic surveys to attempt to model what the structure is like beneath the ground. When palaeomagnetists measure the magnetic field, they actually measure a combination of the Earth's background magnetic field and variations that are being caused by near-surface changes in the magnetic properties of the underlying rocks. Many rock types contain sufficient magnetic minerals to produce significant magnetic anomalies.
Magnetic surveying can thus be used for a broad range of applications, from small-scale engineering or archaeological surveys, to large-scale surveys to investigate regional geological structure.
By Muhammad Azeem, (M.Phil) Nanotechnology Lab., Physics Department, Govt. College University Lahore, Pakistan.
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