When there are northern lights, or auroras, why do they occur in both hemispheres...and not just one?
It all begins with charged particles from solar outbursts, such as flares. Some of those particles collide with the earth's magnetic shield, called the magnetosphere.Those move like iron filings toward the ends of a bar magnet (remember that high school science experiment). In this case, the ends of the 'bar magnet' happen to be the north and south poles. When those charged particles collide with the molecules of gas near the poles, the molecules glow. Different types of gas molecules glow in different colors.