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Earth’s Invisible Defender – The Magnetic Field January 9, 2012

Posted by peterxu422 in Science.
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Earth’s magnetic field is a tireless guardsmen, protecting the inhabitants of its planet always. People often take it for granted and many do not even know it exists. To appreciate it, let us first understand what a field is. Physicists are very careful and precise about how they define things, so it is a bit difficult to formally define a magnetic field. But I can give you a qualitative understanding, or at least, a mental picture of it.

A field is the area of influence exerted by a force. Let’s take the force of gravity for instance. Back then, scientists knew that the planets orbited the Sun because the Sun exerted a gravitational force on them. It was almost as if the Sun was pulling on these planets. But the idea of an object, as if by magic, reaching out through space and instantaneously pulling on another object was absurd. To resolve this, the concept of a field was created. Imagine you had a source, that emitted this invisible “stuff” until it filled up all space. At different points in space, you would feel a certain amount of force from this source because everything is immersed in this “stuff.” This “stuff” is the field. Take the Sun as our source for example. It is a source of gravity, and the sun emits a gravitational field throughout space. If you stood close to the sun, you would feel its gravitational force (a strong one). If you stood farther away from the sun, you would still feel its gravitational force, but it might feel a lot weaker. This is why a field is considered the area of influence exerted by a force.

Magnetic fields behave similarly, except the source of a magnetic field is charges in motion (i.e. current) rather than a Sun. Therefore, if Earth has a magnetic field, there must be a source of moving charges that is generating it. Where is that source you may ask? It is believed to be coming from the Earth’s core, which is composed of mostly molten iron. The flow of liquid iron creates electric currents in the core and this creates a magnetic field around the Earth, otherwise known as the magnetosphere.

The magnetosphere is invisible to our eyes but it plays a very important role in protecting the earth against solar wind, which is a stream of charged particles (mostly electrons and protons) that is ejected from the surface of the Sun. Only charged particles are affected by magnetic forces when they enter a magnetic field. So when solar wind strikes the Earth, the magnetosphere deflects all of the charged particles away, leaving the Earth’s surface safe. If there were no magnetosphere, solar wind would damage power stations leaving everyone without electricity for months. Food would spoil and people would starve.

While the magnetosphere is associated with this morbid doomsday scenario, it can also be attributed to one of nature’s most incredible spectacles: aurora borealis, or more commonly known as the northern lights. These are the spectacular aurora displays of dancing colorful nights that usually occur at the north and south poles. These lights result when solar wind particles collide with oxygen and nitrogen atoms in the Earth’s atmospheres at high energies. These collisions excite the electrons of the oxygen and nitrogen atoms and cause them to release certain colors of light as the electrons return to their ground states. The following chart shows what colors correspond to which atoms:

Green – oxygen, up to 150 miles in altitude
Red – oxygen, above 150 miles in altitude
Blue – nitrogen, up to 60 miles in altitude
Purple/violet – nitrogen, above 60 miles in altitude

These lights primarily occur at the Earth’s poles because the magnetic field is weaker at the poles than any other part. Solar wind particles tend to collect here whereas they are strongly deflected at other parts of the magnetosphere.

VIDEO: Amazing Northern Lights Time Lapse

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