If you live within 20 degrees latitude of Earth's magnetic north pole, seeing the northern lights (or aurora) is a common occurrence. For those of us that live in Rochester, NY, a glimpse of the aurora was a treat this past Monday night. So what are the northern lights and how do they occur? Well, the aurora on Monday was the result of a coronal mass ejection (CME) hitting Earth's magnetic field. A CME is a large outburst of charged particles that is suddenly released from the sun. A quick search for CME on YouTube will get you lots of nice videos. When the CME hit Earth's magnetic field, the particles were directed to the north and south magnetic poles, essentially "grounding" this stream of charged particles. The colorful lights in the sky occur due to an interaction of the electrons and energy in the CME and the oxygen and nitrogen in the atmosphere.
Chemical elements can be in one of many states. In the ground state, they have the required number of electrons orbiting their nucleus and are stable and happy. Elements can also be in an excited state where the electrons are all there, but they sit in what chemists call higher energy levels. Basically the atom gained some energy and the electrons are holding onto that energy, making the atom slightly less stable. Another thing that can happen is that an atom gains so much extra energy that an electron gets "kicked out" and lost completely from the atom. This is called ionization. When a CME full of energy and electrons hits Earth, it can bump atoms into higher energy states, ionize atoms, and also give back electrons to ionized atoms making them more stable. When an atom goes from being excited back to the ground state, or when it regains an electron, it releases excess energy in the form of light. Different elements release different amounts of energy due to their physical properties, and therefore emit different colors of light. Oxygen often emits green light, while nitrogen emits blue or red light, depending on whether it's regaining an electron or falling to the ground state. This emission of light by the elements in our atmosphere due to interactions with particles in the CME is what causes the northern lights! Below are some images taken by members of the Rochester Academy of Sciences Astronomy Chapter of the aurora visible from upstate NY.