Page 109 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO SEE

is a lot of ocean before you reach landfall in New
Zealand or southern Chile for instance, where you AURORAE ON OTHER PLANETS
have a smaller chance of seeing a display.
The word aurora comes from the Roman Aurorae aren’t only confi ned to Earth. In addition, activity has also
They have been seen on many other been seen around some of Jupiter’s
goddess of dawn. In mythology she fl ies around in
Solar System worlds too. Jupiter and largest moons, such as Europa and
the mornings announcing the arrival of the Sun.
Saturn both have magnetic fi elds Io. Aurorae have also been seen
Knowing the word’s origin enables us to appreciate much stronger than Earth’s, and so it on Uranus and Neptune, and to a
the names given to these phenomena. At the North isn’t surprising to fi nd some amazing much lesser extent around Mars,
Pole is the Aurora Borealis, which translates as aurorae around their north and south where there are only small regions
the Northern Dawn, while its counterpart at the magnetic poles. of magnetic fi eld.
South Pole is referred to as the Aurora Australis,
or Southern Dawn.
Thankfully, aurorae are not always confi ned
to high latitudes, and this is where the activity of
the Sun comes into play. An active Sun has more
sunspots, but it also sends more particles streaming
out, which leads to more auroral activity on Earth
in terms of frequency and magnitude. The most
massive Sun-particle storms can even cause aurorae
to be seen at the equator. While these are extremely
rare, you can certainly see a couple of aurorae each
year from Scotland and northern England.
Unfortunately, the Sun won’t get noticeably more
active for another two or three years, but there’s
still bound to be the odd display visible from the Hubble reveals a strong auroral Jupiter exhibits aurorae in these
north of the UK. So make sure you keep an eye out display at Saturn’s poles images from Hubble
for signs of the wonderful aurora.

WHEN EARTH MEETS THE SUN

The magnetosphere is the area of infl uence we rely on our magnetic fi eld to defl ect similar to the wake behind a rock in a
that Earth’s magnetic fi eld has in space. It the particles that constantly fl ow through fl owing river; this is called the magnetotail.
protects life on our planet from all sorts of our part of space. When the solar wind is particularly
radiation that would otherwise penetrate The shape of the magnetosphere is a strong, Earth’s magnetic fi eld gets
into the atmosphere and reach the ground. result of how the magnetic infl uences of the overloaded, and the extra particles follow
The Sun releases vast amounts of charged Earth and Sun interact. On the Sun side it the magnetic fi eld lines that cascade down
particles called the solar wind that fl y in all is pushed towards Earth and the boundary into the atmosphere towards the North
directions through space. Being so close to is known as the bow shock. On Earth’s far and South Poles, giving rise to the
the Sun, Earth gets a fair blast of this and so side, the magnetosphere trails, looking marvellous aurorae.

Magnetosphere boundary
WHAT CAUSES THE AURORA?
Bow shock
Electrons
hit air
molecules
200km
SOLAR Molecules
Sun WIND get excited JACK FINCH/SCIENCE PHOTO LIBRARY, JPL/NASA/STSCI X 2, ILLUSTRATIONS BY PAUL WOOTTON

80km
Molecules give
Magnetosphere off coloured
light to lose
energy
Aurorae Magnetotail
Diagram not to scale

Charged particles from the Sun are channelled to the poles by Earth’s magnetosphere Solar wind electrons hit atmospheric particles

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