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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

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  • $\begingroup$ See my answer here. $\endgroup$
    – honeste_vivere
    Commented May 4, 2015 at 11:56
  • $\begingroup$ @honeste_vivere I am afraid, but I don't see anything related to my question in your previous answer. You were mentioning about wave particle interaction in most of the answer. I know that wave particle interaction is important but that is not my interest. Besides, one of the reference I gave says "this anisotropy can excite very low frequency whistler mode chorus waves which resonate with electrons". My question is, how is the anisotropy generated? $\endgroup$
    – user1048419
    Commented May 4, 2015 at 12:18
  • $\begingroup$ I linked that answer because the particles in the radiation belts do not get their energies by simply conserving the first or second adiabatic invariants. The conservation of these invariants is what causes the particles to stay trapped for long periods of time in the radiation belts, but it is not generally considered the source for their energization. $\endgroup$
    – honeste_vivere
    Commented May 4, 2015 at 12:37
  • $\begingroup$ Thank you, @honeste_vivere OK, I understand that adiabatic compression less contributes to the energization. But still I have a question about the origin of the anisotropy. There are several observation of electron and ion anisotropy. So I'll modify the question. $\endgroup$
    – user1048419
    Commented May 4, 2015 at 12:49
  • $\begingroup$ Unless the mirror points move (i.e., in the original idea for Fermi acceleration), the conservation of the 2nd adiabatic invariant should not change the total kinetic energy of the particle. Conservation of the 1st adiabatic invariant conserves kinetic energy as well (assuming no temporally- or spatially-varying processes), which is why I said that conserving these two invariants would not, alone, energize particles. $\endgroup$
    – honeste_vivere
    Commented May 4, 2015 at 15:42

1 Answer 1

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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).

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