How are the northern lights produced? Although I've never seen it myself, I hear the northern lights are a sight to be seen! I know they're related to the Earth's magnetic field but I don't know much more about them. What is the physical phenomenon that creates the northern lights?
 A: The Solar wind flows past the Earth, interacting with the Earth's magnetic field, the magnetosphere. The boundary between the two, the magnetopause, is buffetted as the Solar wind isn't constant. Blobs of plasma can pass by, the upshot being that the magnetic fields get wound up, tangled if you like.
It has been found that magnetic fields can reconnect, this magnetic reconnection, can release an amount of energy heating the plasma. This is flows along the magnetic fields lines into the polar regions, colliding with the atmosphere and causing the lovely aurore.
It has been a while since I did my degree project on it. My memory may be fading, and I haven't kept up with geophysics for a while.
A: The ionized particles from mainly solar wind are caught and trapped by Earth magnetic field, which behaves like a magnetic bottle. (The region in which ions are trapped is called Van Allen radiation belts.) This trap is weaker in the polar regions, and there the ions are mainly released into the denser parts of atmosphere. There they collide with air particles (mostly N$_2$ and O$_2$) causing their fluorescence seen as the northern lights.
A: Every celestial body huge enough to possess a magnetic field of its own undergoes a phenomenon called the "Magnetic Pole Reversal". For the sun, this occurs once in every 11 years. During a magnetic pole reversal, the north and south poles become exchanged. How and why exactly this occurs is still actively being researched upon, but it is one of the key factors in the occurrence of northern lights. In the sun, different layers rotate at different speeds and this combined with the magnetic pole reversal causes complete chaos. The magnetic field lines that are produced in the core are thrown off balance and eject towards the surface. Once they reach the surface, they turn and twist, forming a helical shape so tight that the temperature increases tremendously. Until this point, the magnetic field was held together by sun's gravity but as the temperature increases gravity can't hold on to it any longer and the chunk of plasma is thrown into space. As plasma consists of charged ions(sun's temperature so hot that electrons strip away from the nucleus), the travel through space and interact with magnetic fields of planets. If the magnetic field of the planet is not strong enough, it cannot withstand the solar wind. Earth's magnetic field is quite strong and since it's weak at the poles, the charged particles deviate towards the poles, where they interact with the earth's atmosphere. Earth's atmosphere contains many molecules of oxygen and nitrogen and these receive energy from the solar wind, exciting them to higher energy levels. The colors of northern lights that we see in the sky are produced when the molecules return to their ground states. 
I have explained the whole phenomenon from the eyes of the plasma on my science blog here: fuelyourcuriosity.me
For those interested in knowing more, do read it. 
