Why do the Northern Lights vary in colour?
Anna Jóna Þórðardóttir
AnswerThe 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 fwhere 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.
Um þessa spurningu
Þorsteinn Vilhjálmsson and Ögmundur Jónsson. „Why do the Northern Lights vary in colour?“. The Icelandic Web of Science 23.12.2005. http://why.is/svar.php?id=5508. (Skoðað 23.3.2017).