GS13: Why are the Northern lights wavy in nature?
Updated: Jun 19, 2020
Hello reader! I believe, you enjoyed our previous articles related to the magnetic field around the Earth. Following topics, we have already discussed and you can re-read them by checking the links below:
In this article, we will discuss one of the most interesting corroborations of the Magnetic field around the Earth. In the early days of humanity, our ancestors, who were residing in Canada, Alaska, Iceland, Norway, Finland, Greenland, and New Zealand, were amazed by the beautiful lights in the sky. They were perplexed, Why are these lights wavy and not propagating linearly?
Places where Aurora Australis and Aurora Borealis is observed are: Canada, Alaska, Iceland, Norway, Finland, Greenland, and New Zealand
(Check the link from National Geographic)
With more studies and researches, we know have an answer to this question, which is answered by a very basic principle of physics- Lorentz force.
To answer this question, I want to take you back in time when you were in class 12th or SSC. Most of you might be aware that a moving charged particle experiences a force in the magnetic field which is capable of changing the course of its movement. This force is known as Lorentz Force, named after H. A. Lorentz based on the experiments conducted by Amperes and other legends in the field of magnetism.
This is a very simple law, very simple. It states that a point charge q moving with a velocity v and located at a distance a (position vector, remember there's direction too!) at a given time t in the presence of both the magnetic field B(a) experiences a force due to the field as:
There is one more case when the magnetic force will be zero. That’s for you to answer.
Now, coming back to our original topic, a splendid display of colors is seen in the sky and construed as a dancing color which is fascinating and equally puzzling. As the magnetic field lines bent along the cusp allow some of the charged particles to enter the Earth’s atmosphere, they enter along the field lines. Since the velocity of the charged particles along the field is not affected i.e. the component of velocity parallel to the field, as the F=0, but there is a component of force acting mutually perpendicular to both, the magnetic field line and the velocity vector, the component of velocity perpendicular to the field.
This makes the charge perform helical motion which is interpreted as a dancing charge particle. Even if the field lines are bent, the helically moving particle is trapped and guided to move around the field line (as shown in the above illustrations). Since the Lorentz force is normal to the velocity vector, the field does no work on the charged particle (why?) and the velocity remains the same.
IF you have forgotten about the magnetic cusp, check the article here!
Well, it is fascinating to observe the charge particles dancing but even more exciting to find the valid theory of why it happens? Science is all about finding the answers to the unknown.
When these particles interact with the different gaseous molecules present in the atmosphere, they ionize and excite them, and make them glow like a neon signboard. Excited oxygen atoms emit green light and excited nitrogen atoms emit pink light. This phenomenon of the production of amazing dancing light in the sky near the poles is known as Aurora Borealis (in the northern hemisphere) or Aurora Australis (in the southern hemisphere).