For thousands of years, mariners have used
the Earth's magnetic field as a compass to find their way
to safe harbor. The Earth's field looks very much like the
magnetic field of a common bar magnet.
A Closed Circuit
Every square meter of the Earth
is pierced by a line of magnetic force, which loops from
deep inside the Earth, and far into space, only to return
back in a great closed circuit thousands of kilometers away.
If there were no Sun or solar wind, the Earth's magnetic
field would extend far beyond the orbit of the moon and
millions of kilometers into interplanetary space, in the
same shape as a bar magnet field outlined by iron filings.
In reality, the action of the solar wind changes this picture
rather dramatically.
On the daytime side, the field is pushed
in by the solar wind pressure, and on the nighttime side,
it is invisibly stretched out like a comet's tail. Scientists
call the region near the Earth where its field controls
the motions of electrically charged particles the magnetosphere.
As the Earth rotates, and as the solar wind and coronal
mass ejections buffet it from the outside, the magnetosphere
trembles and can become stormy. When these rapid, though
subtle, changes happen, compass bearings can become unreliable
by up to several degrees at the Earth's surface. In space,
even more dramatic changes can happen.
A Sun-Earth System
When the solar wind and the magnetosphere
are taken together they operate like a powerful, but invisible
system. Depending on the magnetization of the particles in
the solar wind the field can be southward or northward attracting
or repelling the Earth's field. On the daytime side of the
Earth, these fields connect together and cause a transfer
of particles and magnetic energy into the Earth’s magnetosphere
from the solar wind. These changes in the Earth's particles
and fields cause severe ‘magnetic storms’ which
can be easily seen at ground level with sensitive magnetic
field detectors called magnetometers.
The magnetotail resembles a comet’s
tail and is stretched by the solar wind into a vast cylinder
of magnetism nearly one million kilometers long. Changes in
the solar wind and in the magnetosphere can also cause the
magnetotail region to change in complex ways.
Rubber Bands and Rings
Magnetic fields in the magnetotail
can snap like rubber bands and reconnect themselves, and the
particles tied to field will flow down these field lines and
plunge deep into the interior of the magnetosphere cavity
producing auroras.
Some of these particles can take up temporary
residence in an equatorial zone called the ring current. In
this vast, invisible river nearly 40,000 kilometers wide,
positively-charged particles flow westwards and negatively-charged
particles flow eastwards like two trains on opposite tracks.
In fact, the flows are so dilute that they actually occupy
the same space. Other particles from the magnetotail ride
the field lines deep into the Earth's atmosphere carrying
the currents which power the beautiful aurora.
Find
out more about the Sun-Earth Connection at the Sun-Earth
Connection Education Forum Web site.
Text adapted from the
Sun-Earth Connection Tutorial courtesy of NASA, written by
Dr. Sten Odenwald. Images and videos courtesy of NASA.
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