According to the reference below, the plasma in a planetary radiation belt increases its temperature anisotropy through radial diffusion; temperature perpendicular to the background magnetic field increases faster than that parallel to the magnetic field.
The question is why perpendicular energization is faster than parallel one?
As far as I know from textbooks, the betatron acceleration, due to the conservation of the first adiabatic invariant, increases the particle's kinetic energy perpendicular to the background magnetic field. On the other hand, the Fermi acceleration, due to the conservation of the second adiabatic invariant, increases the parallel kinetic energy.
If the competition of the two mechanisms is important to the anisotropy, why is the betatron stronger than the Fermi? Any reference mentioning about this will be helpful.