SOURCE MECHANISMS FOR MORNING AURORAL FEATURES

Examination of Viking UV images of the morning auroral oval at times of high activity has shown three distinct types of auroral feature. Moving spots at the equatorward border of the oval are found to be associated with Ps 6 magnetic disturbances observed from the ground. Discrete, periodically spaced, features at high latitude are also observed. A variable but stationary emission feature is seen near the dawn terminator. The discrete features are manifestations of the Kelvin-Helmholtz instability (KHI) on shear layers created by high speed flow between them, based on quantitative estimates of phase velocity, wave number, and growth rate. The terminator feature may arise from precipitating electrons forming the upward electrical current in part of the large-scale driven system. This part of the driven system is caused by the conductivity gradient at dawn and has been previously detected by other means.