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I have looked around a bit, but haven't found a simple answer. At an altitude between 100-1000 km,
Is the magnetic field B still around $\sim 10^{-5}\ T$? Is there a simple equation that would provide an quick order of magnitude estimation?

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You could (at a first approximation) consider the Earth's magnetic field as a dipole at the centre of the earth, and proceed with your estimations from there. But don't expect things to be spectacularly accurate. –  Kitchi Apr 27 '13 at 2:52
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You could try this. Can't vouch for its accuracy but it looks pretty official. –  Michael Brown Apr 27 '13 at 3:14
    
Thanks for the link. It doesn't matter if it's entirely accurate, it certainly answers the order of magnitude question. It remains the same. –  Christopher Akritidis Apr 27 '13 at 4:45
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up vote 4 down vote accepted

For future reference, if you even have only the most basic understanding of the physics behind it, you can guess that the fastest the mag field would drop off would be order $1\over r^2$. And you can guess that it originates from the center of Earth. Since we're sitting pretty at ~6371km, adding another 100-1000 km would not even decrease it by half, let alone an order of magnitude.

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You're right, I could have figured out that much, assuming I knew that there were no other effects to consider. –  Christopher Akritidis Apr 29 '13 at 19:09
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