Hi Briana,

That's a great question! The aurora borealis (northern lights) and the aurora australis (southern lights) are both caused by complicated interactions between the solar wind, the Earth's magnetic field, and the Earth's atmosphere.

You may be wondering how there can be "wind" in space because there is no air. Well, the Sun is constantly sending streams of ionized gas, or plasma, out into space at speeds of about 400 km/s or 864,000 miles/hour. This plasma is what scientists call the solar wind. The solar wind also carries the Sun's magnetic field out into interplanetary space. When the solar wind and its interplanetary magnetic field encounter the magnetic field of the Earth, or another magnetized planet, a big shock wave, called a bow shock, forms ahead of the obstacle presented by the planet. This bow shock is a lot like the bow wave that forms in front of a fast moving boat on the water. Inside the bow shock, is a magnetic cavity scientists call the magnetosphere. On the sunward side, the magnetic field of the Earth (or other magnetized planet) is compressed by the solar wind flow. On the night side, the magnetic field of the Earth gets drawn out into a long tail by the solar wind flow - a bit like the wake that forms behind a fast moving boat on the water.

A lot of science textbooks say that the aurora are produced when the solar wind directly enters the Earth's atmosphere near the geomagnetic poles. This is not true! What really happens is a lot more complicated. The orientation of the interplanetary magnetic field in the solar wind is constantly fluctuating. When the orientation of the interplanetary magnetic field is in the same direction as the Earth's magnetic field, we say the magnetosphere is "closed." To help you understand this idea, think about what happens when you try to push the north poles of two bar magnets together - they repel! When the orientation of the interplanetary magnetic field is opposite to the Earth's magnetic field, we say the magnetosphere is "open" and the Earth's magnetic field can connect to the interplanetary magnetic field. This allows solar wind energy and particles to enter the magnetosphere. Where the Earth's magnetic field becomes connected to the interplanetary magnetic field, the Earth's field lines get dragged back into the magnetotail by the solar wind. The deposits a lot of energy into the magnetotail. Eventually, the magnetotail will become unstable and release the stored solar wind energy towards the Earth. When this happens, huge electrical currents that flow down into the Earth's atmosphere are generated, and electrons are accelerated towards the atmosphere. When the electrons hit the atmosphere, they cause oxygen in the ionosphere to emit the green light we see as the northern or southern lights.

In order for other planets to have aurora similar to the Earth's, they need two things - a magnetic field and an atmosphere. Mercury actually has a strong magnetic field, but it doesn't really have an atmosphere. Jupiter, Saturn, Uranus and Neptune all have strong magnetic fields, thick atmospheres, and very large magnetospheres. However, the magnetospheres and the atmospheres of Jupiter, Saturn, Uranus, and Neptune are a bit different from the Earth's. For example, Jupiter's magnetosphere has a weird interaction with Jupiter's moon Io, so some of the auroral activity seen on Jupiter is linked to this moon.

For more about Jupiter's aurora, see: http://hubblesite.org/new...ve/releases/1996/32/

For more about Saturn's aurora, see: http://hubblesite.org/new...ve/releases/1998/05/

There may even be something similar to aurora on Mars, but this is a new and somewhat controversal idea. Even if it turns out there really are aurora on Mars, they are very different from the northern lights on Earth or the aurora on Jupiter, Saturn, Neptune, and Uranus. The reason is that Mars does not have a strong, global magnetic field, only localized areas that are weakly magnetized. For more about the Martian aurora: http://sci.esa.int/scienc....cfm?fobjectid=37523

Kris