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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?

Jens
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  • The Van Allen Belt is known from radiation measurements done with some satellites. But these detectors could measure only intensity and not direction of radiation. Detectors for measurement of direction seems to be much larger in size. Are there any satellites with detectors for direction of radiation? – Uwe Jul 28 '18 at 11:25
  • Interesting question! Since the motion perpendicular to the local field direction is circular, then we can say that flux should be isotropic within the plane perpendicular to the field, and if you know number versus velocity (or cyclotron radius) you can derive in-plane flux immediately. For out-of-plane flux (parallel to the field lines), that's going to be a little more involved. – uhoh Jul 28 '18 at 12:23
  • Yes, the particle distributions are often anisotropic in the radiation belts. The anisotropy is energy-dependent as well, where some energies are isotropic for all intents and purposes but others are not. The anisotropies arise from energization mechanisms and losses (e.g., precipitation into the atmosphere or magnetopause shadowing). – honeste_vivere Jan 19 '21 at 23:22

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There are satellite observations of electron anisotropy:

The radiation belt electrons move incredibly quickly, but not toward Earth. Instead, they move in giant loops around Earth. The Van Allen Probes data show that in the direction toward Earth, the most energetic electrons have very little motion at all – just a gentle, slow drift that occurs over the course of months. This is a movement so slow and weak that it can be rebuffed by the scattering caused by the plasmasphere.

The 2018 GRL paper "What Causes Radiation Belt Enhancements: A Survey of the Van Allen Probes Era" (might be paywalled, sorry) shows that the Themis and Van Allen satellites have instruments that can resolve proton and electron directions.

Bob Jacobsen
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