Observation of sprites over the Sea of Japan and conditions for lightning-induced sprites in winter

Sprites are a newly discovered optical emission in the mesosphere over large thunderstorms. We have succeeded in observing sprites for winter lightning in the Hokuriku area (Japan Sea side) of Japan in the winter of 2001/2002. The optical results on three days are compared with the corresponding characteristics of parent (causative) lightning with particular attention to the significant differences between Hokuriku winter lightning and the more widely studied continental lightning. Despite significant differences with Hokuriku winter lightning, we have found nearly the same sprite properties as already observed in continental USA, with one significant difference (simpler shape for Hokuriku winter sprite). We also discuss the criteria for sprite occurrence. Specifically, two similar criteria are found: (1) cloud-to-ground discharges of positive polarity; (2) the presence of a certain threshold in vertical charge moment (200-300 C.km) (roughly consistent with that for continental USA). Mesoscale convective systems are not necessary to store the charge necessary for sprites, but the parent Hokuriku winter clouds are substantially smaller than the minimum scale for sprite occurrence in continental lightning, but they are larger in area than ordinary summer thunderclouds. However, there may exist another condition, such as clustering or the self-organizing effect of thunderclouds, for sprite production.