Earth: Magnetosphere
Earth: Magnetosphere
Space PlasmasIntroduction
The overall shape of the geomagnetic field in space, a region called the magnetosphere, is much like that of a bar magnet, however there are important differences due to its interaction with the solar wind: a flow of plasma from the outer corona of the Sun. The geomagnetic field is shaped like a comet, with the Earth at its head. The field facing the Sun on the dayside is compressed inwards by the pressure of the charged particles in the solar wind. A sharp boundary called the bow shock forms about 10-15 Re from the Earth as the solar wind and geomagnetic field reach an approximate pressure balance. The field on the night side of the Earth is stretched out into a long geomagnetic tail that can extend 20-50 Re from the Earth. Above the Polar Regions, converging magnetic field lines from the dayside and nightside regions also form a magnetospheric cusp, within which plasma from the solar wind may sometime enter. Gases from the Earth's upper atmosphere can also be injected into the magnetosphere through the cusp in features that are called polar fountains.
A sketch of the magnetosphere showing the main components. (Courtesy NASA)
A sketch of the magnetosphere showing the main components. The rounded, bullet-like shape represents the bow shock as the magnetosphere confronts solar winds. The area represented in gray, between the magnetosphere and the bow shock, is called the magnetopause. The Earth's magnetosphere extends about 10 Earth radii toward the Sun and perhaps similar distances outward on the flanks The magnetotail is thought to extend as far as 1,000 Earth radii away from the Sun. (Source: Science@NASA)
The geomagnetic field is a complex electromagnetic system of circulating currents and changing magnetic conditions, which are often driven by distant events on the Sun such as solar flares or coronal mass ejections. The conveyor belt for the worst of these influences is the ever-changing solar wind itself. Since the early 1800's when Alexander Von Humbolt first detected them, magnetic storms can rumble through the magnetosphere and be felt by sensitive compasses or magnetic instruments even at ground level. For many days, the ground-level field may be constant, but during unexpected moments, and lasting for hours at a time, it may also flare up into a changing pattern of intensities that can cause compass needles to give incorrect bearings. These magnetic storms can presage other physical changes in the magnetospheric environment, and even follow the rise and fall of spectacular aurora in the Polar Regions. 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.
Solar Wind and the Magnetosphere
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.
The Sun’s magnetic field and releases of plasma directly affect Earth and the rest of the solar system.
The Sun’s magnetic field, and releases of plasma, directly affect Earth and the rest of the solar system. Solar wind shapes the Earth’s magnetosphere and magnetic storms are illustrated here as approaching Earth. The white lines represent the solar wind; the purple line is the bow shock line; and the blue lines surrounding the Earth represent its protective magnetosphere. (Source: SOHO-NASA)
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.
Related EoC Articles
External Links
- The Exploration of the Earth's Magnetosphere by David P. Stern / NASA
- Space Plasma Physics - Research on plasma at NASA's Marshall Space Flight Center
Preview Image
- Magnetospheres (NASA Science)
Citation
Odenwald, Sten, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2008. "Earth: Magnetosphere." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 25, 2007].
<http://www.cosmosportal.org/articles/view/135525/>
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