The particle radiation in the Van Allen Belt consists of charged particles (mostly protons in the inner belt, electrons in the outer belt).
The charged particles move in helical trajectories along the magnetic field lines from pole to pole, being pushed back and forth between poles due to the magnetic field increasing towards the poles (magnetic mirror).
The AP8 flux models in the link give the omnidirectional flux of particles.
I wonder if there is also a significant anisotropic flux component in any of the belts. E.g. an 8 MeV proton in a 30 µT field would move in a radius of about 10km if my calculations are correct. If the 8 MeV of kinetic energy were mostly perpendicular to the magnetic field it would cause a strong anisotropy, with much less radiation parallel to the magnetic field.
Has any significant anisotropy ever been measured? Do models predict any anisotropy at all?