#28.     The Acceleration of Auroral Electrons

The "ring of fire" seen by satellite cameras around the polar caps (picture below) is probably created by electrons leaking from the tail's plasma sheet, whose magnetic field lines form a somewhat imperfect trap.



    Observers on the ground, however, don't see that"diffuse aurora," which is dim and lacks sharp outlines. The bright arcs of the "discrete aurora" which they do see--ribbons in the sky, often shimmering, folding, and constantly changing--have a different origin. Satellites flying through the electon beams which create such arcs see evidence that their electrons have actually been accelerated. It is as if a giant "electron gun" existed somewhere up in space, like the one found in TV picture tubes, and was shooting electrons down field lines.

Trapped O⁺ ions

    The S3-3 satellite was not in a particularly high orbit, and the O⁺ ions were typically observed at altitudes of the order of one Earth radius (about 6000 km or 4000 miles). If the auroral "electron gun" was below that altitude, it was indeed surprisingly close to Earth, not in the distant magnetotail where many scientists were looking for it.

Voltage drops along magnetic field lines

    Not so when the current flows upwards--and since all currents flow in closed circuit, if somewhere the current flows downwards, somewhere else it must flow upwards. In that case electrons move down, from space towards the Earth. That is the direction in which field lines converge and the magnetic field grows stronger; and as was discussed in the explanation of particle trapping, electrons tend to be reflected back from such regions. That produces an extra resistance to the flow of current.

    Unlike the ring current, carried by plasma which is just "coasting" through space, electric currents which flow into and out of the ionosphere require a driving voltage and a continuous input of energy. After all, part of their circuit lies in the ionosphere, which (like copper wire and seawater) resists the flow of electricity and will not allow a current to flow unless the two above requirements are met.

    The region of converging field lines further hampers the upflowing currents, by the "mirror force", and nature's way of overcoming this is to allocate part of the driving voltage--typically, 5-15,000 volts--to help drive the current through the "bottleneck" of its circuit. That is the voltage which accelerates auroral electrons (and in the process, also some O⁺ ions), and it was shown in the early 1960s by Hannes Alfvén, Swedish Nobel prize winner, and by Alfvén's associate Hans Persson, that such voltages were expected to be concentrated in the near-Earth parts of magnetic field lines.

    That, however, may not be the entire story. Other acceleration processes are also at work, as evidenced by oxygen ions which seem to have been accelerated not along field lines but perpendicular to them, boosting the energy with which they circle their guiding lines. A variety of plasma waves associated with the aurora may be involved here.

Update--December 1998