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ACE
Advanced Composition Explorer |
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Launched on August 25th, 1997 to study energetic particles from both the Sun and outside our solar system. Among other measurements, this spacecraft determines the speed of the solar wind and also measures the strength and direction of the magnetic field in the solar wind. When the direction of the magnetic field measured by ACE is "negative," i.e., pointing down relative to the Earth's North pole, increasing amounts of solar wind energy are transferred into the Earth's magnetosphere, the immediate space around the Earth protected by the geomagnetic field.