Van Allen Belts

The populations of energetic particles trapped in Earth's magnetic field have come to be known as radiation belts because the doughnut-shaped regions within which they are confined encircle Earth like huge belts. There are two distinct belts: an inner one whose lower boundary is at an altitude of about 250 mi (402 km) and whose less-well defined outer boundary is at a radial distance of about 10,000 mi (16,100 km) and an outer one which extends outward from 10,000 mi (16,100 km) to over 50,000 mi (80,500 km). Both belts encircle Earth in longitude and have the greatest concentration of trapped particles at its magnetic equatorial plane. The concentration diminishes with increasing latitude north and south of the magnetic equator and falls to nearly zero over the north and south polar caps at latitudes greater than about 67°. Each trapped particle spirals around a magnetic line of force, oscillates between magnetic "mirror" points in northern and southern hemispheres, and drifts slowly in longitude. This defines a doughnut-shaped region.

The principal source of particles in the outer belt is solar wind, a hot ionized gas that flows outward from the Sun through interplanetary space. Some of the electrons, protons, and other ions in the impinging solar wind are injected into Earth's external magnetic field. These electrons, protons, and ions subsequently diffuse inward and are accelerated to greater energies by natural fluctuations of electrical and magnetic fields induced by the varying solar wind. The solar wind also sweeps back the outer portion of Earth's magnetic field to produce a long wake called the magnetotail extending on the night side of Earth to a distance of about 4,000,000 mi (6,440,000 km) far beyond the orbit of the Moon. In the heart of the outer belt, typical intensities of electrons with energies greater than l,000,000 electron volts (1.0 MeV) (the most penetrating component there) are 2,000,000 particles per square inch per second. The numbers of particles of lesser energy and lesser penetration are much greater. The outer belt exhibits marked variations on time scales of hours, weeks, and months.

In the heart of the relatively stable inner belt, typical intensities of protons of energy exceeding 30,000,000 electron volts (30 MeV) are 120,000 particles per square inch per second. The residence times of such protons are many years. Penetrating particles in the inner belt are attributable to neutrons from reactions of cosmic rays in the gas of the upper atmosphere. A small fraction of such (uncharged) neutrons decay into protons, electrons, and neutrons as they move outward. At the points of decay the electrically charged protons and electrons are injected into trapped orbits. A radiation belt of this type would be created around a magnetized planet by cosmic rays even in the absence of the solar wind, though no such example has been found.

The fate of trapped particles is loss into the atmosphere or outward into space. A quasi-equilibrium population of any specified species of trapped particle is achieved when losses are equal to sources.

Radiation belts are part of a more complex system called the magnetosphere which also contains large populations of relatively low energy ionized gas (plasma). This gas plays a central role in the overall physical dynamics of the system.