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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.

REFERENCES

http://www.nature.com/nphys/journal/v4/n4/full/nphys897.html

user1048419
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Any process that is slow compared to the bounce- and gyro- timescales but comparable to the drift timescale can conserve the first two invariants but breaks the third invariant (given by the total magnetic flux enclosed by the trapped particle due to its gradient-curvature drift motion across field lines around the magnetized body). Such processes (in Earth's magnetosphere) include the arrival of an interplanetary shock, storm-time compression of the magnetosphere, and also sub-storm dipolarization events on the night-side. MHD waves generated by these processes can also interact with particles on the drift timescale, causing drift-resonances and radial transport (or diffusion in the case of multiple waves).

Alex
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