Role of the EMP and the QE field processes in the generation of columniform sprites

Sprites are transient luminous events at the altitude of the mesosphere induced by cloud-to-ground lightning discharges (CGs). Recent observations revealed that sprites consist of two portions: sprite halo which is a diffuse glow occurring at the altitude from 70 to 85 km, and sprite streamers which include fine structures extending downward from the bottom of the sprite halo. It is suggested that the quasi-electrostatic (QE) field induced by CGs is an essential source for producing both the sprite halo and the sprite streamers. Recent electrodynamic models of the sprite halo have reproduced the spatial and temporal evolution of the halo obtained from optical observations. However, the evolution of the sprite streamer region has not be reproduced by computer simulation, and it is suggested that there are other important factors in addition to the QE field for their generation. To clarify the important factors related to the generation of the sprite streamers, we investigated the role of the electromagnetic pulse (EMP) radiated from parent CG. We analyzed sprite image data and ELF sferics data obtained in the wintertime sprite campaign conducted in Japan. In the campaign, we observed 51 columniform sprites of which streamer region is characterized by long vertical columns. We compared the structures of columniform sprites and the characteristics of parent +CGs obtained by Franklin Japan Corporation and Tokyo Electric Power Corporation. It is found that the peak current of +CGs is correlated to the number of sprite columns while the charge moment of +CGs is correlated to the vertical length of sprite columns. From these results, we suggest that +CGs with large peak current values make a precondition for the generation of sprite streamers by radiating strong electromagnetic pulses. On the other hand, +CGs with large charge moment values induce strong QE field above the parent thunderstorm system that can produce sprite columns with long vertical lengths.