Came across this nice video that explains how the Northern light forms, made by Per Byhring and the physics department of the University of Oslo. In it, it is explained how solar storms trigger Northern (and Southern) lights, during daytime as well as nighttime.
From the hydrogen atoms fusing deep inside the sun to the earth's magnetic fields, and everything in between, this video (with great graphics, by the way) explains it briefly, yet thoroughly.

The Aurora Borealis from Per Byhring on Vimeo.