We investigate conditions under which a diffusion equation is a valid description of the transport across magnetic field lines of low energy magnetospheric plasma. In this paper we consider the case where the guiding center drift UE = cE¿B/B2, and in particular its fluctuating component ΔuE, are comparable in size to the particle thermal speed. We conclude that a two dimensional diffusion equation is valid in the &agr;, &bgr; space of the Euler potential provided that the curl of the total electric field component parallel to B vanishes [∇¿(E⋅eˆ1eˆ1)≡0> and provided the fluctuations have a short enough correlation time that guiding center drifts other than uE remain negligible. Ions in Jupiter's Io plasma torus are thus described by a diffusion equation of this correlation time is no more than a few rotation periods of the planet.