#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.

Auroral oval from space

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.

Some enterprising scientists (e.g. the Russian-French "Araks" experiment, and "Project Echo" of the University of Minnesota) did in fact mount electron guns on high altitude rockets, and used them to create patches of "artificial aurora" in the high atmosphere. Artificial aurora was also created by electrons released from high-altitude nuclear tests between 1958 and 1962, visible in Hawaii, Samoa and off the Azores Islands, places too close to the equator for natural aurora. Such tests are now banned by international agreement.

Trapped O+ ions

Clear evidence for the acceleration of the natural aurora came from the US Air Force satellite S3-3. A voltage which accelerates negative electrons downwards will also accelerate positive ions upwards. When instruments aboard S3-3 showed positive O+ ions of oxygen shooting upwards in the auroral zone, scientists realized that the "electron gun" (or at least part of it) must have been at such times below the spacecraft. O+ ions are the main ion type in the ionosphere (peaking around 200 km or 120 miles), but since 1971 they had also been observed in the ring current, at much, much higher energies than those in the ionosphere. S3-3 had just discovered the link between these two ion populations.

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

One possible explanation involves the close association between auroral arcs and electric currents which flow along field lines between the ionosphere and distant space (Birkeland currents). Such currents are generally carried by electrons, which being negative, travel in a direction opposite from that of the electric current. When the current flows downward, the electrons move up, drawn from the ionosphere, where they are quite plentiful. Magnetic field lines diverge away from each other in that direction, the field gets weaker with distance and it is quite easy for electons to move out.

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 Alfven, Swedish Nobel prize winner, and by Alfven'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

Recently NASA's FAST satellite (Fast Auroral Snapshot Explorer) uncovered some details of the process. FAST was launched in August 1996 and (as its name implied) was designed to resolve rapid variations as it flew through auroral arcs. Both upward and downward currents were found to accelerate electrons--upward currents shot electrons of several KeV down into the atmosphere (the process discussed above), but downward currents also accelerated electrons, upwards, by several hundereds of electron volts. In addition to "beams" of accelerated ions, both directions also observed "conics", apparently accelerated by wave processes.

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Last updated March 13, 1999