Nov 3, 2006

Earth's Invisible Shield


The Solar Wind And Earth's Invisible Shield
Earth is not the only contributor to plasmas in space. Most of what is found in the solar system blows continuously from the sun in what is known as the solar wind. It's a scorcher, a fully ionized and electrically conducting plasma heated to about 1 million deg. C (1.8 million deg. F), and blowing past the Earth at more than 1.6 million km/h (about 1 million mph).

Life on Earth would be different - if it was here at all - without the magnetosphere, a region of space formed by Earth's magnetic field. It deflects the solar wind about 10 Earth radii (about 64,000 km [40,000 miles]) upstream from Earth. (Scientists use the Earth's radius - about 6,400 km [4,000 miles] - as a rough yardstick for large scales like this.)

The environments of the other two earthlike planets are vastly different than Earth, possibly because they have virtually no magnetic fields. Mars and Venus are called earthlike because their size and chemistry are close to those of Earth. Yet the two are dry and hostile.

Mars is thought to have lost, over a few million years early in its history, much of its former oceans and atmosphere to space [Scientific American, November 1996]. This loss was caused, at least in part, by the the solar wind blowing directly on Mars' upper atmosphere. Venus still has a dense atmosphere, but most of its water was lost to space, again possibly due to scouring by the solar wind.

Just like the magnets in a compass, the Earth's magnetic (or geomagnetic) field has two poles, north and south, and the field's pull extends far beyond the surface of the Earth. Like a toy magnet with iron filings lined up on a sheet of paper over it, the magnetic field lines extend outward then loop around to connect at the opposite pole. And here is where the solar wind makes its first difference.

The solar wind presses the geomagnetic field inward on the sunny side of the Earth. On the night side, the wind sweeps the field outward to form a long tail hundreds of Earth radii long. The fountains at the poles spew plasmas into the geomagnetic tail where they are heated to more than 10 million deg. C (1.8 million deg. F), and then accelerated back toward the Earth where they reach temperatures 100 times higher (1 billion deg. C!, almost 2 billion deg. F).

That is 1,000 times hotter than the outer atmosphere of the Sun (the corona). These plasma blasts are responsible for geomagnetic storms that damage electrical systems on satellites and on Earth, expand the the upper atmosphere and thus drag more on low-altitude satellites, and interfere with radio communications. They also give us the magnificent auroras.


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November 20, 1996

Authors: Dave Dooling, B.L. Giles
Curator: Bryan Walls
NASA Official: John M. Horack