#9.     Trapped Radiation

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.

Bouncing back and forth between "mirror points."

As such particles circle their guiding field line, the "guiding center" of their rotation generally slides up or down that line, creating a typical spiral pattern. However, a subtle interaction causes the spiraling particle to be repelled from regions of stronger magnetic field, where field lines converge.

 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 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.

Drift around the Earth

In addition to the rapid rotation ("gyration") around field lines and the back-and-forth "bounce" motion, trapped particles also undergo a slow "drift", by which they jump from one field line onto another one nearby, similar to the original one but slightly rotated around the Earth's magnetic axis. Viewed from the north pole, a positive ion will gradually rotate clockwise, a negative electron counter-clockwise.

The ring current

Because positive ions and negative electrons drift in opposite directions (see drawing), that motion will create an electric current that circulates clockwise around the Earth when viewed from north. The current is aptly named the ring current.

Note how different electric currents in space are from those encountered in everyday life! Currents we use at home--for light, to drive machinery or to generate heat--only flow if pushed by an electric pressure or voltage, against the resistance of the circuitry. The flow of such currents is in many ways similar to the flow of water through a pipe--water, too, will only flow if pressure is available, to help it overcome the friction inside the pipe. The flow of both water and electricity require a constant input of energy: once we click the switch and disconnect a house current from its source of voltage, it stops practically instantly.

The flow of the ring current, and of many other currents in space, is quite different. It needs neither a driving voltage nor an energy input, but persists as long as its ions and electrons are trapped in the magnetic field. Many aspects of such "collision-free plasmas" are quite unlike what one might expect, and are often hard to reproduce in the laboratory: that is why the magnetosphere is probably our best "natural laboratory" for studying the processes of distant space.

Magnetic storms

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.

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.

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 futher 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.

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Last updated March 13, 1999