Radiation (van Allen) belts

spaceweb@oulu.fi - last update: 21 December 1998, 1450 UT (RR)

The trapping regions of high-energy charged particles surrounding the Earth are called radiation (or van Allen) belts (Van Allen et al., 1958; Van Allen and Frank, 1959). The inner one, located between about X = 1.1 - 3.3 Re (Earth radii, geocentric) in the equatorial plane, contains primarily protons with energies exceeding 10 MeV. Flux maximum is at about X = 2 Re. (Distances given here are approximate, since the location of particles is energy dependent.) This is a fairly stable population but it is subject to occasional perturbations due to geomagnetic storms, and it varies with 11-year solar cycle. The source of protons in this region is the decay of cosmic ray induced albedo from the atmosphere.

As a result of the offset between the Earth's geographical and magnetic axes, the inner belt reaches a minimum altitude of about 250 km above the Atlantic Ocean off the Brazilian Coast. This South Atlantic Anomaly occupies a region through which low-orbiting satellite frequently pass. Energetic particles in this region can be a source of problems for the satellites and astronauts.

The outer belt contains mainly electrons with energies up to 10 MeV. It is produced by injection and energization events following geomagnetic storms, which makes it much more dynamic than the inner belt (it is also subject to day-night variations). It has an equatorial distance of about 3 - 9 Re, with maximum for electrons above 1 MeV occurring at about X = 4 Re. 'Horns' of the outer belt dip sharply in towards the polar caps.

Recently a new belt has been found within the inner belt. It contains heavy nuclei (mainly oxygen, but also nitrogen and helium, and very little carbon) with energies below 50 MeV/nuc. The source of these particles are the so called "anomalous cosmic rays" of interstellar origin.

The radiation belts are of importance primarily because of the harmfull effects of high energy particle radition for man and electronics:

it degrades satellite components, particularly semiconductor and optical devices
it induces background noise in detectors
it induces errors in digital circuits
it induces electrostatic charge-up in insulators
it is also threat to the astronauts

The investigation of the Earth's radiation environment was one of the main tasks of the CRRES satellite. It has observed, for example, a rapid (1 min) formation of a new radiation belt due to a SSC on 24 March, 1991 (Vampola and Korth, 1992).

References

Vampola, A. K. and A. Korth, Electron drift echoes in the inner magnetosphere, Geophys. Res. Lett., 19, 625-628, 1992.
Van Allen, J. A., G. H. Ludwig, E. C. Ray, and C. E. McIlwain, Observations of high intensity radiation by satellites 1958 Alpha and Gamma, Jet Propul., 28, 588-592, 1958.
Van Allen, J. A., and L. A. Frank, Radiation around the Earth to a radial distance of 107,400 km, Nature, 183, 430, 1959.