Characteristics of Japanese winter sprites and their parent lightning as estimated by VHF lightning and ELF transients

Sprites are newly discovered optical emissions in the mesosphere over large thunderstorms. This paper is the observational summary of winter sprites in the Hokuriku area of Japan and their parent lightning in the winter of 2004/2005, by using the coordinated optical and electromagnetic (VHF and ELF) measurements in Japan. As the results of optical observations at two stations, we have found that this campaign has yielded a variety of sprite shapes; V-angle shaped structures have been often observed (25%) in addition to columnar structures familiar for us. All of the sprite events are found to be associated with ++CG lightning, as seen from the macroscopic information by ELF data at Moshiri. However, examining the microscopic properties of parent lightning as seen from the VHF SAFIR lightning detection network, has suggested very complicated characteristics of parent lightning discharges inducing sprites, as compared with the ELF data. One half of the sprite events are also found to be associated with ++CG by the SAFIR observation, but another half has yielded rather new results as compared with earlier results. Four events are definitely associated with --CG and the remaining three events, inter-cloud flashes. The overall picture for Japanese winter sprites and their parent lightning discharges, is significantly different from that for the summer-time, continental sprites. This is indicative of complexity of winter lightning in the Hokuriku area of Japan and this would provide new information on the sprite generation mechanism.