Natasha James, of St Peter's College, Palmerston North, asks :-

Where and when is it best to look for aurora?

Dick Dowden, an upper atmospheric physicist retired from Otago University, responded.

Aurora is the light emitted when atoms in the high altitudes of the Earth's atmosphere are struck by the very fast sub-nuclear particles which the Sun emits. That is the basic process but other factors also influence how to work out where and when to look.

An electrically charged particle, such as an electron or a proton, travelling in a magnetic field experiences a magnetic force which is perpendicular to both the direction of travel and to the magnetic field direction. As the charged particles approach the Earth they come under the influence of the Earth's magnetic field. The net result is that the particles spiral down at high latitudes, ie towards the Earth's magnetic poles. So thats the first result. Here in New Zealand we need to look South. (The displays in the Antarctic are magnificent.) And we need to be able to see in the direction of the sky about 100km above the Earth's South Pole. There is less chance of seeing an aurora if you are north of about Christchurch.

The aurora is a diffuse form of light though the bottom edge is often sharp. Hence we should look when the sky is darkest, away from city lights during periods of no Moon and during winter. (This is the same reason why we see stars more easily in winter than in summer.)

The amount of light emitted in the aurora depends on the number of particles emitted by the Sun. For reasons we dont yet understand, this goes in cycles of about 11 years. At the cycle peak, which is about now, the Sun exhibits excessive numbers of Sun spots, which are enormous eruptions on the surface of the Sun, each of which vastly increases the number of emitted particles. These peaks in Sun spot activity also disrupt radio communications and make it dangerous for astronauts and Concorde pilots. Luckily we have four days warning of the particles reaching Earth from the Sun.

We would see aurora on any planet with an atmosphere and a magnetic field, eg Jupiter.