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To see the invisible

The World & I; Washington Vol. 15, Iss. 3,  (Mar 2000): 157-163.

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Headnote

A satellite fitted with "plasma-colored glasses" is poised to reveal the unseen world of dancing electric currents, sheets, loops, and layers that protects planet Earth from the harsh solar wind.

After a blizzard rages for a day or two, we are often treated to a transformed landscape of snowdrifts sculpted by currents in the invisible air ocean. Other fixed reminders of the invisible substance, air, are recorded by sand dunes and wind-shaped bristle cone pines.

Beyond Earth's air ocean, however, lies a much greater invisible ocean, the magnetosphere, a zone of plasma (electrically charged gases) where nothing tangible -no snow or sand or tree or even a cloud-records titanic currents and pulses. Until the advent of the space age, humanity was oblivious to the magnetosphere's existence, except for such vague clues as the flickering northern lights.

Since the launch of Sputnik in 1957, however, dozens of satellites have sent back data on various aspects of the magnetosphere. So now we know that its boundaries and internal structure are constantly changing in response to an ongoing buffeting by invisible pulses of charged particles from the Sun. The magnetosphere's outer boundary is the magnetopause, a tear-shaped bubble marking the boundary between the region of influence of Earth's magnetic field and the region of influence of the Sun's magnetic field.

At Earth's equatorial plane, the magnetopause is located from 10 to a few dozens of Earth radii out. On the inner side of the magnetosphere, the interface with Earth's atmosphere is subtly varied, so that precise boundaries may be difficult to define. At altitudes between 50 and 300 miles, the upper atmosphere is a mixture of very hot, tenuous neutral gases (the thermosphere) and ionized gases referred to as the ionosphere. Farther out, the ionosphere feeds the plasmasphere, a donut-shaped region of...