Northern lights - Aurora borealis

The northern lights occur when solar winds grow in energy, especially during powerful eruptions emitting the so-called solar flares.

The solar wind is a stream of charged particles and when these particles hit the earth’s magnetic field, northern or southern lights appear at the poles, aurora borealis and aurora australis.

The fiercer the solar wind, the more powerful the northern lights. They occur at altitudes of 90 to 300 kilometres, when the intensity of the solar winds in the form of charged particles is deflected into the Earth’s magnetosphere, forming a belt around the earth’s magnetic poles. During strong solar activity, fierce solar storms for instance, the northern belt widens and the northern lights can be seen further south.

The process itself consists of energy-rich protons and electrons in the solar storms smash into atmospheric molecules, increasing their energy states for a moment.

The molecules and atoms then release excess energy or recapture displaced electrons which decay to their ground state. It is this emission process that creates the aurora borealis - and different wavelengths create the different colours.

The colours of the aurora depend on which gas is being excited by the electrons and on how much energy is being exchanged. Oxygen emits either a greenish-yellow light (the most familiar colour of the aurora) or a red light; nitrogen generally gives off a blue or purple light.

Blue, red and purple are also colours that are seen less frequently and tend to appear when solar activity is high. In all cases, the colours are caused by particles colliding with our atmosphere.

• Date of issue: 28.02.2022
• Value: 19,00 and 43,00 DKK
• Number: FO 975-976
• Stamp, size: 40,0 x 30,0 mm
• Photos: Thomas Vikre & Árni Øregaard
• Printing method: offset
• Printer: Cartor Security Printing, France
• Postal use: Letters and maxi-letters inland, 0-100 g