http://nanopatentsandinnovations.blogspot.com/2011/11/magnetic-pole-reversal-happens-all.html
"Charged particles leaving the sun are captured by the earth's magnetic field" and follow along field lines towards the earth's poles.
The earths
magnetic field is stronger towards the poles and as
the particles approach the North pole the radius of
the their paths along the field lines become smaller.
"The high concentration of charged particles ionizes the air" it is the collisions resulting from electrons and atoms recombining which cause light to be emitted and the Aurora Borrealis to be seen (Giancoli 2005).