What Causes the Northern and Southern Lights

Simply put, these lights are caused by very fast moving electrons striking atoms in the Earth’s upper atmosphere, primarily oxygen and nitrogen atoms which make up most of our atmosphere.  When this happens, it can put these atoms in an excited state.  During the process with which they return to their normal state, they emit this excess energy in the form of visible photons.

So where do these fast moving electrons come from?  Charged particles from the sun’s corona are constantly striking near the Earth and are more or less deflected by the Earth’s magnetic field, which in turn protects life on the Earth from being harmed by these solar winds. When these charged particles encounter the Earth’s magnetic field, they end up traveling along the field lines with some of them ending up interacting with the magnetic field lines, cutting across the field, thus producing a current which can accumulate to upwards of 10 million megawatts!

This current ends up creating a fairly unstable state in the magnetosphere.  Occasionally, some of this current is discharged causing electrons in the magnetosphere to spiral down towards the poles and through the Earth’s upper atmosphere.  As it descends into the atmosphere, it collides with primarily oxygen and nitrogen.  When this happens, the atoms move to high energy orbitals.  This state is fairly unstable for these atoms and they will return fairly quickly to their normal orbitals.  To do so, they must release the excess energy they have stored up from this collision by emitting a photon.  With enough of these atoms going from the high orbital state to the low orbital state, it will produce enough light to be viewable to the naked eye by people standing in appropriate locations on Earth.