THE SOLAR SYSTEM
extremely brilliant. They are due to electrified particles
from space, mainly originating in the Sun, which collide
with atoms and molecules in the upper atmosphere and
make them glow. Because the particles are electrically
charged, they tend to cascade down towards the magnetic
poles, so that aurorae are best seen from high latitudes.
They are very common in places such as Alaska, northern
Norway, northern Scotland and Antarctica, but are much
rarer from lower latitudes such as those of southern
England, and from the equator they are hardly ever seen.
Auroral activity is more or less permanent around the
so-called auroral ovals, which are ‘rings’ placed asym-
metrically round the magnetic poles. When there are
violent disturbances in the Sun, producing high-speed
particles, the ovals broaden and expand, producing dis-
plays further from the main regions. Aurorae have been
known for many centuries. The Roman emperor Tiberius,
who reigned from AD14 to 37, once dispatched his fire-
engines to the port of Ostia because a brilliant red aurora
led him to believe that the whole town was ablaze.
The Earth has a strong magnetic field. The region
over which this field is dominant is called the magneto-
sphere; it is shaped rather like a tear-drop, with its tail
pointing away from the Sun. On the sunward side of the
Earth, it extends to about 65,000 kilometres (40,000
miles), but on the night side it spreads out much further.
Inside the magnetosphere there are two zones of
strong radiation; they were detected by the first successful
American satellite, Explorer 1 of February 1958, and are
known as the Van Allen zones, in honour of the scientist
who designed the equipment. There are two main zones,
one with its lower limit at just under 8000 kilometres
(5000 miles) and the other reaching out to 37,000 kilo-
metres (23,000 miles). The inner belt, composed chiefly
of protons, dips down towards the Earth’s surface over
the South Atlantic, because the Earth’s magnetic field is
offset from the axis of rotation, and this ‘South Atlantic
Anomaly’ presents a distinct hazard to sensitive instru-
ments carried in artificial satellites.
It cannot be said that we understand the Earth’s mag-
netic field completely, and there is evidence of periodic
reversals, as well as changes in intensity. At least it is
certain that the field is due to currents in the iron-rich
liquid core. Incidentally, it is worth noting that the Moon
and Venus have no detectable magnetic fields, and that of
Mars is extremely weak, though that of Mercury is
stronger than might have been expected. Magnetically,
Earth is quite unlike the other inner planets.▼ Noctilucent clouds.
These strange, beautiful
clouds can often become
conspicuous; their origin
is uncertain, but they may
be due to water droplets
condensing on meteoritic
particles. This photograph
was taken from Alaska in
January 1993 (A. Watson).B Atl of Univ Phil'03stp 3/4/03 12:24 pm Page 41