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
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
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.
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.