Auroral intensifications in the evening sector observed by the Viking ultra violet imager

The Ultra Violet Imager on the Swedish Viking spacecraft frequently detects localized intensifications in the evening sector that expand poleward at about 0.4 km/s and eastward at up to 7.8 km/s, stopping abruptly at midnight. The events occur in a previously quiet auroral oval and seem to be associated with the formation of discrete aurora. If westward travelling surges are associated with these intensifications, they must be too small in size to be detected by the imager. The events are rather frequent in time, and comparatively small in extent and duration on a global scale; however they would appear as substantial breakups when viewed overhead at a ground‐based station. Further study is needed to determine just how these events fit into the framework of currently existing substorm knowledge, and how they differ from the larger scale events observed by the imager.