Guiding Centre, curvature drift (astro phys)

1. Feb 3, 2009

yakattack

Take a 10 keV ion in the van Allen belts 1000km above earths surface in a dipole magnetic field of 100 Gauss. Estimate the grad-B and curvature drift if the particle is a proton and compare this drift with the gravitational drift.

3. I know all the formulas needed, but do not know what the B field would be. Do i use spherical co-ords? In which case B would equal ( $$\frac{\mu}{4\pi}\frac{2M}{r^{3}}cos\theta$$ , $$\frac{\mu}{4\pi}\frac{2M}{r^{3}}sin\theta$$ , $$0$$ ) where M is the magnetic moment of the earth? Then put R=($$R$$ , $$0$$, $$0$$ )? Or am i on the wrong tracks entirely? Any hints greatly appreciated.