Alfvén's theory suggested how this current could be carried. Charged particles, such as ions or electrons, were not only guided by magnetic field lines, but as they slid along such lines (more accurately, spiralled around them), they were also repelled (in a way) from regions of more intense magnetic field.

Motion of a Trapped Particle

    Field lines of the Earth, like those of a bar magnet, stretch from the vicinity of one magnetic pole to that of the other, and the magnetic force on each line is weakest halfway between the line's "footpoints" on Earth, in the part where the line is most distant from Earth. Ions and electrons can be trapped in that weak field, bouncing back and forth, turned back each time they try to slide down into the more intense field which exists closer to Earth.

Regions of the Magnetosphere

    The energy source of the ring current and of all its associated phenomena turned out to be the solar wind--a continuous flow of ions and electrons, spreading out in all directions from the Sun's topmost layer, the million-degree hot corona. Magnetic storms arise from unusually fast flows in the solar wind, especially those connected to explosive events associated with active sunspots. The solar wind compresses the magnetic field lines facing it on the day side of the Earth and confines those lines into a rounded cavity. In the opposite direction, on the night side, the same solar wind stretches field lines into a long "magnetotail" and the cavity then becomes a long cylinder.

    It is this tail where many active phenomena of the magnetosphere--part of our "space weather"--originate. Of special interest is a thick layer known as the plasma sheet, spread in the region of weak magnetic field which is sandwiched between two bundles of magnetic field lines. One bundle, north of the equator, consists of field lines that head into the vicinity of the north magnetic pole, and the other bundle is its mirror image south of the equator, of field lines directed out of the southern polar cap. The plasma sheet is where ring current particles originate, and where "substorms" shove particles earthward and create bright auroral displays.

The Polar Aurora

    In space such currents most easily flow along magnetic field lines, because the particles that carry them (mainly electrons) tend to stay attached to such lines. Some flow from space to Earth, some from Earth to space, and near Earth their circuit is completed by a layer in the high atmosphere that conducts electricity well, the E-layer of the Earth"s ionosphere, located some 125 km above ground. The currents do not reach the ground, because air at lower levels is a very good electric insulator.

Solar Wind       Dynamo

Electric currents, of course, need to flow in a closed circuit, but we are still not sure where or how these circuits close. For some such currents, a popular guess (probably an oversimplification) is that the distant ends of field lines that carry them dip into the solar wind (which can conduct electricity), somewhat like the wires which Faraday dipped into the water below Waterloo bridge (see section on dynamos). This would create a dynamo drawing its energy from the flow of the solar wind.

    The polar aurora is created by electrons moving earthward while carrying this current. They gain speed from the driving voltage and after reaching the atmosphere, smash into atoms of oxygen or nitrogen, causing them to emit light. The typical aurora has an eerie greenish glow, emitted by oxygen.