Radiation and the Van Allen Belt

The Particles in the Van Allen Belt

Server Sky arrays in the initial m288 orbit are in the "slot" between the inner and outer Van Allen belts. The radiation is somewhat lower, but still very high. The main difference between the belts is the mechanisms creating them.

The inner belt is mostly created by C.R.A.N.D. - Cosmic Ray Associated Neutron Decay. Cosmic rays striking the upper atmosphere split atoms and generate neutrons with energies above 30MeV. Unbound neutrons have a half-life of 10 minutes and move at more than 10% of the speed of light. The neutrons undergo beta decay and split into an electron, a proton, and an electron antineutrino. 10 minutes at 0.1c is about 20 million kilometers, about 50 times the diameter of the moon's orbit, depositing the particles mostly in that huge volume. The density of these deposited particles falls off as 1/R2, so the highest density of these decays occur near the Earth. The electron gets most of the 2MeV beta decay energy, but the proton retains most of the original momentum and energy of the neutron, and have about the same average energy and direction as the original neutron. The electrons are much less massive, so the 2MeV makes them move faster, approaching the speed of light. If the neutron decay occurs in the Earth's magnetic field, and most of the momentum is perpendicular to, rather than parallel to, the magnetic field lines, then the proton spirals around the field lines and gets captured in the Van Allen belt. The likelihood of any neutron decaying within the Van Allen Belt is small - but that small probability is larger in the inner belt, closer to the source.

The outer belt is mostly created by protons and electrons captured from the solar wind, but does include some helium and larger nuclei. These particles are less likely to penetrate deeper into the magnetic field, so those solar-sourced particles that do get captured are in the outer belt.

Once captured by the magnetic field in either belt, charged particles gyrate around the lines of magnetic field from the earth. Most have some velocity parallel to the field lines, and travel up and down the field lines as they circle helically around them. The field strength increases as it gets closer to the earth, and this causes the particles to lose perpendicular velocity, then bounce back the other way, at the "mirror point". If the ratio of the parallel velocity to the perpendicular velocity is high enough, the particles will collide with the upper atmosphere before they reach their mirror point, and are taken out of circulation. So only a tiny fraction of source particles become captured electrons and protons, and only a small fraction of those do not collide with the atmosphere.

Of the few particles that do get captured, and "survive infancy", most remain in the belts for a long time, some for thousands of years, until they scatter off other particles and increase their ratio of parallel to perpendicular velocity, then drop into the atmosphere. So the generation rates are small, and the recombination rates are small also.

The total mass of particles in the entire Van Allen belt is on the order of 10 grams. However, the particles are moving very fast and store a lot of energy. Any object in orbit will be hit by only a tiny fraction of th