Aurora

In most atmospheric optics, the light has come from the sun, but sometimes from the moon, or city lights. Auroras are a different story. The lights of Aurora Australis (or Aurora Borealis in the Northern Hemisphere, or Northern Lights) are actually emitted by gas particles in the atmosphere itself.

The sun emits a ‘solar wind’, which largely consists of protons and electrons. Protons have a positive charge, and electrons have a negative charge. During a solar storm, a larger amount of these charged particles are sent towards Earth.

The Earth’s magnetic field interacts with these particles, sending them towards the poles. The charged particles transfer energy to the atoms and molecules in one of the highest parts of our atmosphere – the thermosphere. After the atoms and molecules have absorbed this extra energy, they then radiate it away once more.

The energy is radiated away as visible light, and the colour of the light depends on the atom or molecule. Oxygen atoms are responsible for most of the green seen in aurora displays, and some of the red. Molecular nitrogen is responsible for the blues and purples, and also emits some reds.

Photo taken at Point Dorset, Wellington in July 2016, by Jonathan Usher.