Explanation of the Northern Lights
Have you ever wondered why the Northern Lights exist? Several physical phenomena work together to produce this beautiful natural light show. These phenomena include, magnetism, ionization, and conservation of energy.
The magnetism mentioned is the Earth’s magnetic field. This is the same field that makes a standard magnetic compass point north! This field surrounds the Earth at all elevations from below the ground to high up in the atmosphere and even into space. It is caused by the Earth’s iron core rotating as the Earth spins on its axis. All magnetic fields have a direction associated with them. The direction points from the south magnetic pole to the north magnetic pole. Currently the Earth’s magnetic north pole is at its geographic north pole (very near it anyway) and its south magnetic pole at the south geographic pole. This is not always the case, the Earth’s magnetic poles occasionally swap. There is no need to worry about your compass suddenly flipping around though since this is a gradual process. If the Earth’s magnetic field were reversed, you would still be able to see the northern lights as will be shown later.
Ionization is simply the production of charged particles. This occurs naturally when incoming light from the sun collides with atoms and molecules in the atmosphere. The energy the light brings into the collision is used to break apart or ionize the atoms and molecules into charged pieces. Charge is a property of matter. Matter can either be positively charged, negatively charged or neutral (balanced positively and negatively). Matter of any size can be charged, from objects bigger than yourself to smaller than an atom. Electricity is known to be the flow of these charges. As ionization occurs in the atmosphere the charged particles (positive and negative) produced are traveling along with some speed. An interesting interaction happens between the magnetic field of the Earth and the moving charged particles. Actually any charged particle that is moving through a magnetic field, like the one around the Earth, has a force exerted on it. The direction of this force depends on the direction of motion of the particles, the charge of the particles (positive or negative), and the direction of the magnetic field. The interaction between the Earth’s magnetic field and the moving charged particles causes some of the particles to move in a corkscrew pattern towards the Earth’s north magnetic pole.
This migrating of charged particles towards the Earth’s north magnetic pole happens all around the world, at every longitude. Many of these particles reach great speeds by the time they get to the north pole. As these particles approach the north pole the atmosphere becomes more and more crowded with them since all of them from around the world are converging on one spot. At this point particles begin to slam into one another and slow down; however, energy must be conserved. This means that the same amount of energy must be present at the end of a collision or other process as there was at the beginning. The way that energy is conserved as all of these particles rushing towards the north pole slow down is that they give off the extra energy as light. This light is what you see as the Northern lights.
You may still be wondering how the northern lights would still appear even if the magnetic poles were reversed. First try answering this yourself by thinking about the ionization of particles in the atmosphere and the correlation between the direction of force exerted by the magnetic field on charged particles and the type of charge (positive or negative) . . . If you concluded that the force exerted on a positive particle is exactly opposite that exerted on a negative particle you are correct! Because of this the positive ions (ions are the products of ionization) move towards one pole and the negative ions towards the other. So if the magnetic poles were reversed we would see the same effect just with the opposite type of charge.
Other interesting things to think about may include:
-How else do magnetic fields and electricity interact? (Try putting a compass next to a power cord of an appliance, maybe even next to power cords of different appliances that use different amounts of electrical power.)
-Why doesn’t anyone talk about the Southern lights? (This maybe more of a geographical question than a physical one)
-Why do the Northern lights occur only high in the atmosphere?