A NEW-TYPE OF ION INJECTION EVENT OBSERVED BY VIKING

A new type of low energy (< 200 eV) plasma injection event is found ne ar local noon several degrees equatorward of the cusp. Its V-shaped en ergy-pitch angle pattern is very similar to that of impulsive or trans ient magnetosheath plasma injection events reported in the past. Thus, the source region of these injected particles is not a persistent one and that it is likely of magnetosheath origin. However, there are thr ee facts which distinguish this newly found phenomenon from the other impulsive or transient magnetosheath plasma injection events associate d with solar wind pressure pulse or southward/fluctuating interplaneta ry magnetic field conditions. First, the magnetic field and the electr ic field observations do not show such a strong electromagnetic coupli ng between the ionosphere and the source region. Second, a high energy (> 300 eV) population is absent, making the ion energy-time dispersio n very weak. Third, this event does not seem to be related to either a ny solar wind density increase or southward/disturbed interplanetary m agnetic field conditions. These observations suggest that this is a mo re stagnant phenomenon, and we propose to call this a ''Stagnant Plasm a Injection'' event.