Oldest Measurement of Earth's Magnetic Field Reveals History of Battle Between Sun and Earth for our Atmosphere

Geophysicists at the University of Rochester announce in the 5 March 2010 issue of Science that the Earth's magnetic field was approximately half as strong as it is today and that this weakness, coupled with a strong wind of energetic particles from a young Sun, likely stripped water from Earth's early atmosphere.

5 March 2010 Science -  Geodynamo, Solar Wind, and Magnetopause 3.4 to 3.45 Billion Years Ago  - link will require subscription to Science

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Science publication:
American Museum of Natural History Science Bulletins - March 25, 2010
Ancient quartz crystals recently discovered in South Africa by researchers from the University of Rochester have revealed new information about the history of Earth's magnetic field. By studying microscopic metals within the 3.5-billion-year-old crystals, researchers determined that Earth must have already possessed a weak magnetic field at the time the crystals formed. The young magnetic field would have provided the planet protection from damaging solar emissions, fostering a more hospitable environment where life could eventually exist.

Early Earth's Force Field - Hayden Planetarium, American Museum of Natural History