The Magnetosphere

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.

Every square foot 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 miles 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. The axis of the field is tilted by about 11 degrees to the axis of rotation of the Earth. No one knows why, but these kinds of tilts are found among the magnetic fields of some of the other planets, too.

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.

When the solar wind and the magnetosphere are taken together as a system, they operate like a set of powerful, but invisible, valves that open and close depending on their polarity. When the solar wind's magnetic field is of the south-type polarity, it meets up with the south-type polarity of the Earth's magnetic field. On the daytime side of the Earth, these fields reconnect, causing a transfer of particles and magnetic energy into the Earth’s magnetosphere from the solar wind. Severe ‘magnetic storms’ are triggered, and these can be easily seen even at ground level with sensitive magnetic field detectors called magnetometers.

Changes in the solar wind and in the magnetosphere can also cause the magnetotail region to change in complex ways. 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. Magnetic fields in the magnetotail can snap like rubber bands and reconnect themselves, but this time the particles flow down these field lines and plunge deep into the interior of the magnetosphere cavity.

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 and create beautiful aurora.


Teacher Notes:

How is the magnetosphere unique within the Sun-Earth system?

In the middle years, students identify the parts of things and how one part connects and affects another. This leads to the analysis of parts, subsystems and interactions. A system is a collection of things that have some influence on one another. Any part of a system may be considered a system. This is referred to as a sub-system; it has its own interactions.

• (K-2) Most things are made of parts

• (3-5) In something that consists of many parts, the parts usually influence one another.

• (6-8) Thinking about things as systems means looking for how every part relates to others. The output from one part of a system (which include material, energy or information) can become the input to other parts. Such feedback can serve to control what goes on in the system as a whole.

• (9-12) Understanding how things work and designing solutions to problems of almost any kind can be facilitated by systems analysis. In defining a system, it is important to specify its boundaries and subsystems, indicate its relation to other systems, and identify what its input and its output are expected to be.


Science Nuggets:

2000...SAMPEX detects long term changes in auroral electron precipitation following the 11 year solar cycle.

1998...ACE and Polar spacecraft trace the flow of solar wind particles deep into the magnetosphere using iron atoms as a marker of solar wind penetration.

1998...IMP-8 and Geotail observe plasma cloud ejection in the magnetotail region.

1995...Geotail uncovers evidence for magnetic reconnection processes in the Earth's magnetotail.

1997...Polar observations of X-ray aurora show that the triggering electrons come from the magnetotail region.