How many planets are there in the solar system? If your answer was nine then you need to check again. Tiny Pluto, orbiting the Sun at a distance of more than thirty times that of the Earth was recently removed from the official list of planets by the International Astronomical Union. Each of the eight remaining official planets is a unique world, influenced by the materials they are made from, and their distance from the Sun. The Sun is at the centre of the solar system and is the vital source of light and heat for the Earth and all the planets. As well as light, the Sun is constantly shedding particles from its upper layers, which stream away in all directions. This is the solar wind and although it is very low density (you could take all the particles from a volume the size of the moon and fit them inside a soft drink bottle) it has very important effects on the planetary environments.
The goddess of dawn
Consider our favourite planet, the Earth, which has its own magnetic field. The magnetic field links the north and south poles and extends into space to form a protective bubble around the planet. While you can't see the magnetic field, or the solar wind surrounding it, when they interact the result is one of the most spectacular natural displays. This is the aurora, otherwise known as the northern and southern lights. The aurora are brilliant displays of light in the sky, which can be observed at night from high-latitude regions. Although the aurora are named after an ancient word for dawn, because of the way they lit up the night sky, they are not caused by sunlight, or by other ancient theories which included vast fires, reflections from the armour of warriors, and messages from the gods. The aurora are generated when particles from the solar wind become trapped on magnetic field lines and crash into the upper atmosphere, exciting the atmospheric gases and causing them to emit light. The most common aurora seen from the ground is green and is emitted from oxygen in the atmosphere. Sometimes red and purple colours can be seen too, depending on the energy of the particles, and the gas that emits the light. The aurora can take the form of narrow ribbons or curtains of light, or a diffuse green cloud which fills most of the sky. They can change very quickly, sometimes flickering or waving across the sky, and disappearing in the time it takes to try to capture them in a photo.
The aurora generally take the form of ovals around the magnetic poles - one in the north and one in the south. Spacecraft orbiting the Earth have been able to image the ovals from above under different solar wind conditions. In this way, the aurora act like a giant old-fashioned TV screen, with electrons crashing into the atmosphere to emit light and giving a global view of what is happening in the space environment, and the Earth's interaction with the solar wind. The brightest displays of aurora are seen during magnetic storms, in which more particles are ejected from the Sun and become captured by the Earth's magnetic field. Bursts of particles are accelerated into the Earth's atmosphere over a larger range of latitudes. Under the most extreme conditions the aurora can be seen far from the pole - as far as northern Africa in 1870.