Where can I find information about the variation of the magnetic field over time across the United States?

One way is to have a look at the interactive geomagnetic field model over at NASA/Goddard . This is the so-called Standard Reference Field model used by professionals to predict what the firld strength and orientation is on the earth near the surface.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the
NASA IMAGE/POETRY Education and Public Outreach program.