Optically perceptible characteristics of sprites observed in Central Europe in 2007–2009

Abstract

Sprites are luminous optical emissions accompanying electric discharges in the mesosphere. 489 sprite events have been observed with a TV frame rate video system in Central Europe from Sopron (47.68°N, 16.58°E, ∼230m MSL), Hungary between 2007 and 2009. Characteristic sprite forms, i.e., column, wishbone, tree, angel, and carrot have been identified in the set of records. Characteristic morphological properties corresponding to each type are given; earlier definitions and observations as well as the related theoretical considerations are reviewed. Based on the knowledge and experience from high-speed imaging in sprite observations, probable time sequences of streamer propagation directions were associated with the characteristic sprite types. It is suggested that different streamer propagation sequences corresponding to different dynamic processes may result in similar sprite forms. Several occasionally detectable sprite features are noted and described: tendrils, glows, puffs, beads, and spots. Spots are distinguished from the similar beads by their characteristic brightness, size, and location relative to the bright body of the sprite. The events observed in Central Europe have been classified by the number of individual sprites and by the variety of types appearing in them. More than 90% of the recorded sprites were found to occur in clusters rather than alone, and more than half of the sprite clusters contained more than one sprite types. Jellyfish and dancing sprite events are described as being special subsets of sprite clusters. Statistical analysis of the occurrences of morphological types, various sprite features, and event durations indicated that jellyfish sprites and clusters of column sprites with glows and tendrils do not tend to have long optical lifetimes. Sprite events with more morphological types, on the other hand, more likely have extended durations. The maximum of the encountered event duration was lower for events with many sprite elements. Observed rates of glows and puffs may refer to the occupied height range of sprites. The importance of understanding the driving factors behind the development of various sprite types and sprite features is emphasized and some topics are suggested for further investigation.