(Files in red–history)
9. Magnetic trapping
9H. Poincaré, 1896
10. Trapped Motion
10H. Einstein, 1910
10a. Particle Drift
11. Explorers 1/3
11a. Geiger Counter
12. Rad. Belts
12H. Argus 1958
12a. Inner Belt
12b. Outer Belt
13. Fast Particles
14. Synch. Orbit
Space physics can be weird. In regions of magnetic fields, the relation between electric fields and currents is very different from its form in everyday technology.
Ohm's law tells that electric fields drive electric currents, from high voltage to low voltage. In a conductor such as a wire, electrons move from (-) to (+), while ions (if they are free to move), are pushed in the opposite direction, (+) to (-). In space, on the other hand, the the entire plasma is moved sideways, perpendicular to both magnetic and electric field lines. No steady electric current results from the electric field, and both ions and electrons advance in the same direction.
On the other hand, electric currents often flow in space without any voltage driving them. No electric field is involved--the magnetic field is doing it all, when it has the appropriate structure.
This strange behavior is explained below. No math is used, but the arguments are a bit complex--skip this part, if you want. If you decide to continue, go slow: it only takes a short time to read this web page, but much longer to understand it. Make sure to assimilate each part of the argument before going to the next one.
Electric DriftThe drawing shown here explains what happens when electric and magnetic fields act together on ions and electrons. Consult it in each stage of the discussion.
1. Why electric fields parallel to magnetic field lines are rare in space
It will be assumed in what follows that the direction of the electric force ("direction of the electric field") is always perpendicular to the local direction of magnetic field lines.
There exists a reason. In space, ions and electrons spiral around their guiding magnetic field lines, but at the same time they can also slide along those lines, like beads threaded on a wire.
If the electric force had some part in that direction (a "vector component"), those ions and electrons, as they advance along their guiding field lines, would also be accelerated by it, and gain speed. However... gaining speed also means gaining energy. Because energy in nature is conserved, whatever the particle gains, weakens the accelerating part of the electric field, and unless fresh energy is constantly supplied, that part does not last long.
Without such fresh energy (the usual case), the electric force along the field line quickly drops to zero. When that happens, the same voltage exists at all points along a magnetic field line, leaving no voltage differences that might drive currents in that direction. The remaining electric field is then perpendicular to the magnetic field lines, as in the drawing here.
An exception to this rule is discussed in section #28, dealing with the origin of the aurora. There energy is being supplied and the electric force does have a component in the same direction as the magnetic field line.
A negative electron, marked e-, is pushed towards the bottom, in the -y direction You can imagine (if you wish) a positive charge somewhere below the drawing, and a negative charge somewhere above it, creating that force--repelling or attracting the proton or electron.
Barium Clouds and Solar Wind
Such an "electric drift" takes place in the barium cloud (section #8 whose figure is repeated here). The green cloud of neutral barium stays still, while any electric field present makes the purple cloud, consisting of ions and electrons, drift away from it (see illustration). Of course, since ions and electrons remain free to slide along magnetic field lines, the ion cloud also expands slowly in that direction (or rather, in two opposite directions--up and down the field lines).
Where can such electric fields come from? Probably from far out in space. As noted earlier on this web page, a magnetic field line tends to have the same voltage everywhere along its length. If an electric field is created anywhere on that line, its voltage will be transmitted to the rest of it, and with it, the electric field is also transmitted. Thus an electric field created far in space can spread to the end region of the line, where the line comes down into the atmosphere, and where the transmitted field causes barium clouds to drift.
One example is the solar wind, a steady flow of plasma spreading out from the solar corona, the hot upper atmosphere of the Sun, which is too hot for the Sun's gravity to retain it (see section #18). The solar wind spreads radially outwards, while the interplanetary magnetic field lines which accompany it are expanding spirals around the Sun (section #18a).
The radial motion of solar wind ions and electrons must cut across those spirals. How do those particles avoid being forced into tiny spirals around those lines? By an electric field! The flow of the solar wind is driven by powerful energy sources, which make its motion take precedence, which it does by creating the appropriate electric field.
Suppose as before that magnetic field lines are perpendicular to the drawing, and that the same (x, y) axes are used as before. Only now (drawing on the left) no electric field exists, and instead the strength of the magnetic force changes with distance in the y direction--it is much greater at the top of the drawing than at the bottom.
As before both ions and electrons circle around magnetic field lines, as drawn (we ignore the sliding motion). However, the size of the circle also depends on the strength of the magnetic force--the stronger the force, the smaller the radius of the circle. (In the limit where the magnetic force drops to zero, the particles move in straight lines--same as circles of infinite radius!)
Because the way the strength of the force changes, the orbits, again, are no longer circles but flat spirals (see drawing), curving more sharply at the top of their motion.
The result as before, is again a crablike sideways "drift." This time, however, protons and electrons drift in opposite directions. Protons move to the left, electrons to the right, and both motions contribute a right-to-left electric current.
The ring current described in section #9 is of this type. The figure from that section, reproduced here, looks down on the equatorial plane of the Earth, from the north. All field lines point upwards, as in the previous drawing, and the strength of the magnetic field increases inwards, towards the Earth. The drift is therefore in the 3rd perpendicular direction, which carries the particles around Earth--electrons counterclockwise, protons clockwise, and the current flows clockwise too. The earlier drawing illustrating magnetic drifts may be viewed (qualitatively) as a magnified blow-up of the situation at the bottom of the ring current drawing.