Aurora, Amazing Northern Lights

However, ohmic resistance is not the only obstacle to current flow in this circuit. The convergence of magnetic field lines near Earth creates a "mirror effect" that turns back most of the down-flowing electrons (where currents flow upwards), inhibiting current-carrying capacity. To overcome this, part of the available voltage appears along the field line ("parallel to the field"), helping electrons overcome that obstacle by widening the bundle of trajectories reaching Earth; a similar "parallel potential" is used in "tandem mirror" plasma containment devices. A feature of such voltage is that it is concentrated near Earth (potential proportional to field intensity; Persson, 1963), and indeed, as deduced by Evans (1974) and confirmed by satellites, most auroral acceleration occurs below 10,000 km. Another indicator of parallel electric fields along field lines are beams of upwards flowing O+ ions observed on auroral field lines.
Some O+ ions ("conics") also seem accelerated in different ways by plasma processes associated with the aurora. These ions are accelerated by plasma waves, in directions mainly perpendicular to the field lines. They therefore start at their own "mirror points" and can travel only upwards. As they do so, the "mirror effect" transforms their directions of motion, from perpendicular to the line to lying on a cone around it, which gradually narrows down.
In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation (AKR, discovered in 1972). Ionospheric absorption makes AKR observable from space only.
These "parallel potentials" accelerate electrons to auroral energies and seem to be a major source of aurora. Other mechanisms have also been proposed, in particular, Alfvén waves, wave modes involving the magnetic field first noted by Hannes Alfvén (1942), which have been observed in the lab and in space. The question is however whether these waves might just be a different way of looking at the above process, because this approach does not point out a different energy source, and many plasma bulk phenomena can also be described in terms of Alfvén waves.