Beads: Long Lasting Stationary Optical Features in Sprites

Abstract:

Sprites are spectacular transient luminous structures observed in the 50-95 km altitude range above active thunderstorms following large cloud-to-ground lightning strikes. Recordings at 10,000 images per second show impressive streamer activity with bright streamers moving up or down at velocities up to half the speed of light. Not surprisingly, the streamers have attracted much research attention. However, here we will concentrate on sprite beads, which together with sprite glows, are the dominant features in video images (30 or 25 images per second) and in photographs. Sprite glow forms after streamer activity near the sprite onset altitude and beads are small point-like optical features also associated with streamer activity. The reason that the glow and the beads are dominant in longer exposure images is primarily that they do not move much and hence light can integrate over the longer exposure times. In general, streamer activity at the start of the sprite lasts a few 10s of ms, while glow and beads last considerable longer; we have observed beads lasting in excess of 500 ms. High-speed imaging shows the glow and the beads to form along the path of streamers, but they do not appear optically before almost a millisecond after streamer passage. To first order, beads appear to be stationary, but a detailed analysis of high-speed observations often indicates a slow upward or downward motion. The optical emissions decay much slower than in other parts of the sprite indicating the presence of a longer lasting source of energy. The processes in beads leave an imprint, presumably creating a local irregularity, on the atmosphere lasting longer than the optical emissions. This imprint can serve as seed for streamer initiation in a subsequent sprite (re-ignition). Based on our observations the imprint can last up to 1 second. There have been suggestions that beads are formed around local pre-existing inhomogineities, but a recent observation indicate their formation more related to internal dynamics within the streamer channel. A sprite observation in August 2014 shows a streamer channel with beads spaced at regular intervals along the streamer path, which seems to be an unlikely distribution of pre-existing irregularities. A modeling effort based on these ideas is underway.