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![]() (1) Circling around magnetic field lines. Electrons and ions, of the energies commonly encountered in the magnetosphere, tend to circle around magnetic field lines. Like the motion of planets around the Sun, this motion too can sustain itself with no energy input, and can therefore (in principle) persist for a long time. Opposite charges circle in opposite directions; around a field line pointing towards the viewer, ions circle clockwise, electrons counterclockwise. |
Because a particle is repelled as it moves into a region of stronger field, its advance along its guiding field line slows down. Its sliding velocity finally drops to zero and then reverses, causing the particle to bounce back or "mirror."
Without this sort of "mirroring," ions and electrons would not be trapped in the Earth's magnetosphere, but would instead follow their guiding field lines into the atmosphere, where they would be absorbed and become lost. What usually happens instead, is that every time a trapped particle approaches Earth, it is reflected back. It is thus confined to the more distant section of the field line. The magnetic field produced by the ring current contributes (rather slightly) to the magnetic field observed at the surface of the Earth. There are however times when the population of trapped particles is greatly reinforced. The ring current then becomes stronger and its magnetic effect at Earth may grow 10-fold or more: that is known as a magnetic storm. The reinforcing particles are generally of moderate energy, but can be quite numerous. As discussed in a later section such events can interfere with the operation of communication satellites and cause other problems. If trapped orbits are so stable that their particles cannot be easily lost, those particles should not be able to easily enter them, either. How then can the radiation belt and ring current arise? The inner radiation belt, discovered by Explorers 1 and 3, turns out to be a slowly accumulating by-product of cosmic radiation, as explained in a later section. The ring current however (its energetic part is often called the "outer radiation belt"), owes its existence to magnetic storms, which can replenish it in the matter of hours. The process by which that happens is still incompletely understood, but it involves electric forces, combining with magnetic ones. Electric fields are able to push trapped particles earthward, and unlike the purely magnetic motion described earlier, an electric field can also energize them. The ultimate source of the energy and electric field must be the solar wind, and theories exist to explain how they are transmitted. Some are mentioned in further sections, but many details are still unclear and controversial.
Further Reading:A proper understanding of the motion of ions and magnetic fields requires a fair amount of mathematics. Texts on plasma physics generally cover the main principles, though some aspects important to the trapping of particles in the Earth's field may not be included.For a non-mathematical overview of some of the physical principles involved, especially the notion of adiabatic invariants, click here.
Questions from Users: *** Radius of particle gyration *** Deflection of a beam of Electrons in the Earth's Field |
Next Stop: #9H. Trapped Radiation--History
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: education("at" symbol)phy6.org
Co-author: Dr. Mauricio Peredo
Spanish translation by J. Méndez