Magnetosphere - Radiation Belts

Radiation Belts

When the first scientific satellites were launched in the first half of 1958—Explorers 1 and 3 by the US, Sputnik 3 by the Soviet Union—they observed an intense (and unexpected) radiation belt around Earth, held by its magnetic field. "My God, space is radioactive!" exclaimed one of Van Allen's colleagues, when the meaning of those observations was realized. That was the "inner radiation belt" of protons with energies in the range 10-100 MeV (megaelectronvolts), attributed later to "albedo neutron decay," a secondary effect of the interaction of cosmic radiation with the upper atmosphere. It is centered on field lines crossing the equator about 1.5 R_E from the Earth's center.

Later a population of trapped ions and electrons was observed on field lines crossing the equator at 2.5–8 R_E. The high-energy part of that population (about 1 MeV) became known as the "outer radiation belt", but its bulk is at lower energies (peak about 65 keV) and is identified as the ring current plasma.

The trapping of charged particles in a magnetic field can be quite stable. This is particularly true in the inner belt, because the build-up of trapped protons from albedo neutrons is quite slow, requiring years to reach observed intensities. In July 1962, the United States tested a thermonuclear weapon high over the South Pacific at around 400 km in the upper atmosphere, in this region, creating an artificial belt of high-energy electrons, and some of them were still around 4–5 years later (such tests are now banned by treaty).

The outer belt and ring current are less persistent, because charge-exchange collisions with atoms of the geocorona (see above) tends to remove their particles. That suggests the existence of an effective source mechanism, continually supplying this region with fresh plasma. It turns out that the magnetic barrier can be broken down by electric forces, as discussed in Magnetic Storms and Plasma Flows (MSPF). If plasma is pushed hard enough, it generates electric fields which allow it to move in response to the push, often (not always) deforming the magnetic field in the process.