What is the Aurora Borealis? (And Happy Autumnal Equinox!)

Today I hopped onto NatGeo and came across these beautiful photos of northern lights taken in Norway. I immediately realized I had absolutely no idea what the northern lights really are, or how they’re formed. When you think about it, the sun is the only natural cause of light (besides fire anyway), so how do these phantasmal strips of color form, and why do they undulate across an otherwise dark nighttime sky? Since it’s just recently the Autumnal Equinox, let’s celebrate by learning about a little solar activity!