oxygen and nitrogen and it’s these elements that give the aurora its
characteristic green, red and sometimes blue colour.
The aurorae occur in rings or rather ovals around the Earth’s
magnetic poles. It’s not often that one has a spectacular auroral
display from Britain or the United States (excluding Alaska!) but it
can happen.
Remember of course that we are dealing with the magnetic poles
and not the rotational ones, the two being somewhat separated.
As the aurorae occur in ovals around the magnetic poles, the poles
themselves are not the best places to see a display.
Northern Norway, for example, is right under the auroral oval
and any dark night would seem very drab without its polar lights.
Northern Scotland sees a good many aurorae per year, whereas in
Sussex, where both authors live at present, they are rare indeed.
Much depends on the Sun’s activity and there is a fairly well-
defined visual solar cycle of 11 years. Every 11 years the Sun is
really active, with a disturbed surface showing dark patches known
as sunspots, and this is a good time to look for aurorae.
Sunspots tend to occur in groups known as sunspot groups or
active regions. These represent regions of intense magnetic energy.
A highly energized active region may harbour enough energy for
flares, the result of which is an enormous outpouring of energy.
A powerful flare can produce a cloud of charged particles that can
be sent out through the Solar System, known as a coronal mass
ejection or CME. If one of these happens to head our way, it can
disturb our magnetic field, creating what’s known as a geomagnetic
storm. This can also cause the auroral ovals to grow in size and
spread further south, producing a bright display that may be seen at
lower latitudes than normal. The Sun is dealt with in more detail in
chapter 4.
Although they are incredibly beautiful, the magnetic
disturbance behind a bright auroral display can actually disrupt
communications and induce currents in long runs of
overhead cables.Zodiacal Light
Aurorae then, due to the Sun, are purely astronomical phenomena.
So is a much more elusive phenomenon, the “zodiacal” light. Under
very good clear conditions, after sunset, a cone of light may be
seen extending up from the horizon, lasting for some time, and
occasionally appearing quite bright. This zodiacal light is due to
the Sun’s influence on particles spread along the main plane of the
planetary system. It does not last for long after sunset or before
sunrise, because the sky has to be completely clear, with the Sun
some way below the horizon. From the latitude of the Canary
Islands, for example, zodiacal light is by no means rare, but it is very
elusive indeed to see from Britain or the northern United States.[17] The Zodiacal Lifght photographed from Tenerife, courtesy of Dr Brian May1617Earth and sky