Spiky ion acoustic waves in collisionless auroral plasma

Abstract

A theory of spiky electric fields in ‘inverted V’ precipitation regions is formulated and compared with recent spacecraft observations of solitary waves and double layers. The theory predicts that the electric fields propagate along the magnetic field as perturbed ion acoustic solitons that intensify by exchanging momentum with reflected particles. The solitons have minimum scale lengths of about 100 m and maximum electric potential and field amplitudes of 1–10 V and 1–10 mV/m. They propagate at the local ion acoustic speed (Cs ∼ 10–100 km/s), Doppler‐shifted by the drift speed of upward flowing cold ions. Both rarefactive and compressive solitons, with negative and positive electric potentials, respectively, are possible. Upward propagating compressive modes intensify when the upward flow of ionospheric ions exceeds roughly 10 Cs. The rarefactive mode intensifies when the upward field‐aligned current at 1 RE exceeds 0.1–1 μA/m² or when the upward cold ion drift is supersonic. The kinematic and dynamic properties of rarefactive solitons are consistent with the recent observations, although further analysis, both theoretical and experimental, are required for an unambiguous interpretation. The evolution of localized ion acoustic modes into small amplitude double layers is also discussed.