Auroral infrasonic wave-generation mechanism

Abstract

The morphology of auroral infrasonic wave (AIW) substorms, as determined from infrasonic observations at Inuvik (68.4°N), College (64.9°N), and Palmer (60.8°N) along a magnetic meridian through Alaska, shows that AIW are never observed propagating in a poleward direction, even though auroral activity frequently occurred south of the stations. AIW have been shown to be infrasonic bow waves generated by supersonic westward, equatorward, or eastward motions of auroral electrojet arcs. All AIW are observed from azimuths that vary from parallel to the auroral oval to transverse to the oval in an equatorward direction depending on the local magnetic time. Specific examples of auroral poleward expansions that cross the Inuvik zenith have been studied. It was found that even highly supersonic poleward motions of arcs with strong electrojets do not produce infrasonic bow wages. When a reversal in direction of the poleward expansion occurs and the arcs subsequently move equatorward, strong AIW are observed from the arcs. This asymmetry in the occurrence of AIW with respect to direction of motion of an arc is interpreted as an intrinsic asymmetry in the generation mechanism within the auroral arcs and not as a propagation effect. It is postulated that the basic acoustic pulse within the electrojet arcs is caused by collisions with the neutral gas of positive ions that are driven by electrodynamic drift in the E region of the auroral arc. Thus Lorentz force is the coupling mechanism between the electrojet current carriers and the neutral gas. An ionization-collision process occurs within the auroral arcs for those arcs that are moving supersonically in a direction parallel to the direction of the neutral ionization drift. The resulting increase in ion density for such arcs reduces the time constant for ion drag, and thus the Lorentz-force coupling is effective in producing an infrasonic shock wave. If the supersonic translation of the primary auroral electron sheet has a component of motion parallel to the electrodynamic drift of the positive ions, an auroral infrasonic shock wave will be produced in the E-region ionosphere and propagate to the ground as a modified shock or bow wave. If, on the other hand, the auroral-arc motion is antiparallel to the drift of the positive ions, no AIW will be produced.