Northern Lights: Colors
The beautiful, varied colors of the aurora paint huge patches in the sky, but these are really the result of processes at the atomic level. As high energy particles from the Sun are accelerated by the Earth's magnetic field, they smash into atoms in our atmosphere and cause them to glow.
The periodic table shows all the known elements. An element is a substance that cannot be broken down by any known chemical process. For example, water is not an element, because it can be broken down into hydrogen and oxygen. You can see hydrogen and oxygen on the periodic table.
An atom is the smallest unit of matter than still retains the properties of that element. Atoms of every element are made of protons (positive charge), neutrons (no charge) and electrons (negative charge). Protons and neutrons make up the nucleus, and electrons orbit in energy levels outside. The thing that makes elements different is the different numbers of protons it contains. For example, hydrogen contains 1 proton, while oxygen contains 8.
Nitrogen and Oxygen atoms play a key role in Aurora. In the images at left(courtesy: http://academic.brooklyn.cuny.edu), you can see that Nitrogen has atomic number 7, so it has 7 protons. Oxygen has atomic number 8, so it has 8 protons.
Energy Levels: Most atoms are neutral. This means they have just as many electrons as protons, so the negative charges cancel the positive charges. The electrons can be found in a variety of energy levels.
Ground state: Usually, the electrons stay in the lowest energy, which is called the ground state of the atom.
Excited state: Sometimes, if the atom is hit by another particle or a photon of light, the electrons gain a little energy and jump to the next energy level. This is called an excited state. The atom does not lose the electron, so it remains neutral, it just has a little extra energy.
Emission: Since the electron prefers a lower energy, it’s just a matter of time until it falls back down and releases the extra energy it had, usually in the form of light. Some transitions take longer than others. Collisions with other atoms can give the excited electrons a constant supply of energy, so they won't fall back to the ground state at all, and they won't emit light.
Neon lights work this way. The neon gas in the tubes gets energized by the electricity when you plug it in, and then the neon atoms de-excite and release the energy in the form of the light you see.
Atoms can have several energy levels (labled 1, 2, 3, 4, etc.), and each energy level can have "shells" of different structure (labeled s, p, d and f). Electrons can move among all of these levels. For example, an electron can go from the ground state to the third energy level, and then maybe fall to the second energy level or back to the ground state. Since electrons can only move between distinct energy levels, the atom emits a very distinct energy of light, with a distinct wavelength and frequency. Steps in energy can be different amounts, so depending on which levels the electrons fall between, they will emit different energies of light. If the energy of the light is in the visible range, we will be able to see it as a beautiful color.
In addition to the complicated energy structure of an atom of a single element, each element has its own variety of energy levels. So during de-excitation (when the electrons fall down to a lower energy state) an oxygen atom will usually emit a different wavelength of light than, say, a hydrogen atom. The variety of energies of light that an atom emits when its electrons fall to lower energy levels is called its emission spectrum. Each element has its own characterisitc emission spectrum.
Hydrogen is the simplest element (only 1 proton and 1 electron), and it has one of the simplest emission spectra. While all of these transitions create light, only the Balmer series of transitions produces wavelengths in the visible region of the spectrum.
So Hydrogen's spectrum in the visible region looks like this. We can see that the Balmer transitions give violet (wavelength 410nm), blue (wavelength 434nm), green (wavelength 486nm) and red (wavelength 656) light.
Image courtesy http://www.thestudentroom.co.uk
This website from the University of Colorado does a great job explaining excitation of atoms and emission spectra.
The air we breath is made mostly of nitrogen and oxygen, even though our bodies use mostly just oxygen. So during a solar storm, it is mostly nitrogen and oxygen atoms that get excited and emit the colors of light we see.
Looking at the spectra of oxygen and nitrogen shows why the typical colors of aurora are green and red. Each color has a specific altitude range. Oxygen transitions take less than a second to emit green light and up to three minutes to make red light. So in the upper atmosphere where oxygen is more abundant and the air is so thin, collisions are rare and the red transition has enough time to occur. At altitudes lower than 200 km there isn't enough time between collisions for the red transition, leaving only the green, most common, transition. Nitrogen in the low atmosphere adds red fringes near the bottom of the pattern, where there are so many collisions that even oxygen transitions stop occurring. Some occasional nitrogen transitions emit blue light, and lighter gases high in the atmosphere like hydrogen and helium can contribute blue and purple, but these colors are very difficult to distinguish in the dark night.
Because green is the most common color in the Northern Lights, your aurora detectors have a light filter to that is most sensitive to green light. Place the detector so that it faces the northern skies; if there is aurora activity, the detector will pick up the green light and send signals to notify you.
In this photo you can see the dominant green, with some yellowish-red fringes near the bottom.
Again, green is dominant...
Here the top part of the Aurora is a bright red.
It seems that a light blue is showing up in this aurora.
This one appears to even have some purple...