The other, the Japanese Geotail satellite, was stationed farther away to study that tail.

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Understanding exactly what the Geotail does (something to do with measuring the solar wind's interaction with the Earth's magnetosphere) may require a background in heliophysics.

WSJ: The Charge of the Satellite Brigade | Geek Chic

In 1996 (it took several years to analyse the data) Polar saw some plasma sneak inside the magnetosphere, and Geotail saw the rogue plasma being bounced back toward earth itself.

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