The green arch of light is typical of moderate to weak auroras. If the geomagnetic disturbance causing the aurora is strong, red or blue emissions may also be seen.

Cause of aurora

Aurora Borealis (the northern lights) and Australis (the southern lights) occur when charged particles from the solar wind enter Earth's atmosphere and ionize air molecules. Since Earth has a magnetic field, and charged moving particles are deflected by magnetic fields, most of the particles will not enter the atmosphere but are deflected into outher space.

The charged particles in the solar wind originate from the sun, and auroras are more likely if the solar wind is strong and its particle density is high. This happens mostly during so-called CMEs (coronal mass ejections) from the sun. CMEs happen when magnetic fields near sunspots collapse, injecting enormous amounts of energy into the solar atmosphere (chromosphere) and outer corona. The explosion hurls a large amount of charged particles from this solar corona into outer space, and sometimes towards Earth.

The sun has periods of increased activity regularly spaced at 11-year intervals, during which CMEs (and therefore aurora) are much more likely. auroras are then not only more likely but may also be seen towards more equatorial latitudes rather than just near the arctic regions. The last active period was in 2000-2001 and will be followed by a minimum around 2006.

Observing Aurora

Unless you live in arctic regions, auroras will probably be quite rare at your latitudes. Aurora over Australia, central Europe, the USA and South America are not very common. You will need to watch current aurora forecasts or be part of an observer's network and receive warnings of aurora in order not to miss an event.

The weakest kind of aurora shows as a green band towards the northern horizon (if you live at mid-northern latitudes). If aurora becomes more active, you may also see red, pink or maybe blue streamers extending up. The curtains of light will change rapidly, over the course of seconds, and a display can flare up unexpectedly. When you are observing aurora and nothing happens over a long time, don't assume it's over!

The different colors are caused by different gases in the atmosphere. Green and red are from excited oxygen, but red can also occur at lower altitudes (around 100km) from nitrogen. Nitrogen also can cause pinkish auroras. Blue auroras are rare and are emitted by hydrogen atoms.

The major geomagnetic storms triggered by CMEs cause widespread aurora that can be visible very far toward the equator, such as these auroras over the Netherlands on April 7, 2000, that were visible as far south as Italy.

Photography

To photograph, use a relatively fast film such as 400 or 800 ISO, or if you use a digital camera, set the sensitivity to this. Wide-angle lenses will capture more of the Aurora, which may extend across much of the sky, so I recommend you to use wide-angle lenses.

Use a large aperture such as f/2.8 or f/4, but don't use the lens' widest aperture, to avoid vignetting (the edges/corners of the frame becoming dark). Typical exposure times are several seconds, such as 5 or 10 seconds, but you should bracket your exposures. Exposure times much longer than that are not good because the aurora will be blurred (it moves quickly).

If you use a digital camera, you can check your photos immediately and experiment with different settings as you photograph. I highly recommend using a digital camera, preferably SLR.