Why do the Northern Lights vary in colour?

Answer

The question What causes the Northern and Southern Lights? has already been answered on the Icelandic Web of Science. In the answer provided by Aðalbjörn Þórólfsson and Ögmundur Jónsson there is an explanation about what happens when particles from solar wind enter the earth's magnetic field:

The charged particles that enter the earth's magnetic field move at great velocity along spiral tracks around the magnetic field lines between the magnetic poles. So the Protons and electrons flow towards the magnetic poles and as they approach, the particles collide with the atmosphere, usually at altitudes between 100 and 250 km. Energy in the electrons and protons stimulate molecules and atoms in the atmosphere which emit energy in the form of the visible light we call Northern or Southern Lights depending on the pole they are seen at. The colours we usually see are green and red - purple, caused on the one hand by charged oxygen molecules and on the other by charged nitrogen.

When it is said that the solar wind stimulates molecules and atoms in the earth's atmosphere it means that the electrons in these particles, or the particles themselves, take additional energy. It is said that they have moved up to a specific energy level, with more energy than at the ground energy level, which they were at at the beginning. The difference between the two energy levels depends on the element in question, whether its atoms are charged (ionised) and whether its molecules have split into atoms. The energy levels are the same for all atoms that have the same attributes as described above. Viðar Guðmundsson explains this in his answer to the question Can you compare the Universe to an atom? as follows:

It is said that electrons can only have bound energy while they are part of an atom [or it could be a molecule] on the other hand. This means that the energy can only have specific values that can be specified in a particular manner, but the energy can not have intermediate values.

Here it is often said that the energy is quantified. After atoms and molecules have been excited to a given energy level in this manner, the particles reduce to the ground energy level and emit light or other electromagnetic radiation with a given wave length and frequency. The frequency is equivalent to energy according to the following equation

E = h f

where E is the energy difference between ground and maximum energy levels, and f is the frequency and h is Planck's constant, which is one of nature's basic constants.

Many of the colours of the Northern Lights are created by the so-called forbidden changes, that is the changes in energy level in molecules and atoms in the outermost layers of the earth's atmosphere. This means that the changes do not take place in the most usual manner and thus take much longer than usual. This does not occur under normal conditions near the earth's surface, where the gas is dense and collisions between molecules so frequent that forbidden changes do not have time to occur. But scientists have succeeded in emulating the spectrum of the Northern Lights in laboratories.

In addition to the fact that the energy level of molecules and atoms depends on their element and whether they are ionised, it also depends on the level of the particles that excite, in this instance first and foremost electrons in the solar wind, whether a specific energy level is reached or not. This subsequently decides whether the colours that correspond to the energy level appear in the radiation from the gas.

This is how the rare red Northern Lights are generated from oxygen at great heights, over 200 km. Oxygen at 100 km produces bright yellow and green lights that are the brightest and most common colours in the northern Lights. Blue and purple colours come from ionised nitrogen molecules, while uncharged nitrogen gives a red colour. The purple colour at the lower band and the wavy edge of the Northern Lights also comes from nitrogen molecules.

Translated by Paul Richardson.

Sources:

• Website associated with the University of Alaska (Poker Flat Research Range í Fairbanks).
• Web page about the colours in the Northern Lights
• Herzberg, Gerhard, 1944. Atomic Spectra and Atomic Structure. New York: Dover. The image is also from this source.