The Viking low frequency wave experiment, with unique two point measurements of plasma density fluctuations and measurements of potential fluctuations, provided detailed diagnostics of small scale, large amplitude solitary structures in the auroral acceleration region. The structures have a spatial scale of the order of 100 m, density depletions up to 50%, negative potentials up to about 5V, and propagate upwards along the magnetic field lines with velocities from about 5 to more than 50 km s−1. Some structures have a net potential drop of up to a few volts, in general directed upwards. The structures are characterized as solitary waves (no net potential drop) and weak double layers (a net potential drop comparable to kTe/e), or holes in the thermal ion (and electron) phase space distribution. Their occurrence is correlated with that of beams of upward flowing energetic ions, dominated by protons, and electrostatic ion cyclotron waves. The field-aligned currents seem to be relatively weak. A great number of structures with a net potential may account for a large scale potential drop along the geomagnetic field lines and the observed acceleration of auroral particles up to kilovolt energies.
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