Analyses of the ISUAL Dancing Sprites and Secondary Sprites

Aketch ON, Lee L, Chou J, Huang S, Chang S, Wu Y, et al. "Analyses of the ISUAL Dancing Sprites and Secondary Sprites." American Geophysical Union, Fall Meeting 2013. 2013.


From July 2004 to May 2012, about 1,700 sprites were recorded by ISUAL (Imager of Sprites and Upper Atmospheric Lightning). Most of them were singly occurring sprites that were not followed by other sprites; while about 7% of them were multi-sprites, which typically start with a sprite and then followed by another sprite that showed a spatial displacement relative to the preceding sprites. Almost all of these events show horizontal shifts between the preceding sprites and the follow-up ones, which previously have been called the dancing sprites. In contrast to the majority cases of dancing sprites with horizontal displacements, three follow-up sprites were found to exhibit a vertical displacement relative to the preceding sprites, which are termed as the secondary sprites in in this report. These three secondary sprites exhibit similar occurring sequences and characteristics; with the preceding clustering sprite spanning the altitudes of ~60-85 km, and then 30 ms or more later, a secondary sprite appears at ~40-65 km altitudes and seems to be connected to the dimming channels of the preceding sprite. From analyzing the spectral and the ULF data, possible generating mechanisms for dancing sprites and secondary sprites are proposed in this report. Several researches [Lyons, 1994; Lyons, 1996; Lu et al., 2012] had indicated that the successive sprite production in the dancing sprites aligned with the lateral leader propagation direction of lightning. We consider that the successively occurring dancing sprites and the secondary sprites are related to the extending leaders of the cloud-to-ground lightning, which are often followed by a continuing current or even a second stroke. The dancing sprites may be induced by the subsequent leaders in the cloud extending mainly in the horizontal direction, while the secondary sprites may be triggered by the leaders extending primarily in the vertical direction. In addition, a numerical quasi-electrostatic (QE) field model is developed with the aim to validate the occurring scenario of the secondary sprites. Based on the information inferred from the associate ULF data of a secondary sprite, salient parameters, including the charge, the charge height, and the discharging time constant, are estimated and used in the QE model calculations. Through performing QE modeling with the ULF inferred parameters, we find that the electric field in the region below the preceding sprites could be enhanced by the continuing current.

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