Today magnets can be found in almost every home: refrigerator magnets, compasses, screwdrivers, cow magnets, bar magnets, and hard drives in computers. Long ago, compasses were the only commonly used magnets. Compasses helped guide travelers because the needle on a compass would always point north. People used compasses long before anyone knew that a compass was magnetic, and long before anyone knew about magnetism.
Image to right: Earth's magnetic field is very similar to that of a bar magnet, with a south and a north pole. The red lines represent the invisible magnetic lines of force with which other magnets and charged particles interact.
In the late 1500's, William Gilbert realized that the compass was a tiny magnet and it was interacting with a larger magnetic field in order to point north. In 1600, he published "De Magnete" explaining that "the globe of the earth is magnetic, a magnet," Chapter 17, Book 1.
If you take two bar magnets and play with them for a while, you will realize that each end of the bar magnet will either repel or attract one end of the other bar magnet. Each end of a bar magnet is called a magnetic pole with one end as a north pole and one as a south pole. Like poles (e.g. south-south) repel, and opposite poles (e.g. south-north) attract. Because the bar magnet has two poles, its magnetic field is called a dipole magnetic field. It appears that all magnetic fields have an even number of poles, as scientists have been looking for a single magnetic pole for over 50 years now but have never found one.
Image to right: In 1716, Sir Edmund Halley pointed out that aurora is aligned with Earth's magnetic field. And in 1741, Olof Hiorter noticed that the magnetic field measured on Earth's surface would change when the aurora passed overhead. Somehow the aurora and the magnetosphere are connected.
Relatively close to Earth (out to about 8 Earth Radii or 50,000 km at the Equator), Earth's magnetic field looks approximately like that of a bar magnet, as shown in the figure. There are some strange places on Earth that make the dipole different that in the picture. And if we go further out into space, Earth's magnetic field gets weaker and is affected by charged particles in space. We will talk more about this in the Sun-Earth Connection page. We call Earth's magnetic field and the particles and space within this field, Earth's "magnetosphere."
Why are we talking about magnetic fields? These pages are about the aurora and auroral substorms! Yes, but Earth's magnetosphere plays a very important role in the aurora and in auroral substorms.