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This study is based on 5.5 years of continuous observation of sprites from Sagamihara, Japan. Up to February 2022, we detected 537 sprites and found that the most significant number of sprites were observed during the winter (303 sprites); on the other hand, there were only 46 sprites in summer. The hourly distribution of the number of observed sprites peaked at midnight JST (15:00 and 16:00 UTC). To understand the seasonal and the hourly distribution of sprites, we estimate the number of sprites considering the energy and the polarity of lightning, the temporal changes of surrounding environments of sprites, and the conditions for generating sprites. We found that the energy of lightning, the monthly ratio of a positive cloud-to-ground discharge, and the hourly change in the electron number density are essential factors to match the observed sprite distributions.

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Sprites are an atmospheric event that until now has been studied with a limited number of events as data points. We investigated the global characteristics to further understand the interactions between sprites and thunderclouds that coincide with them. We analyzed the frequency distribution, morphology, location, and altitude of 525 sprites observed by the cameras located at Sagamihara, Japan, from December 2016 to March 2021. The majority of sprites were observed during winter. The column type sprites commonly occurred during winter. In contrast, sprites with complex morphologies are frequently observed in summer. Using the triangulation of simultaneously observed sprites at multiple locations, the average height of sprites was determined to be 81.6 km, and one instance occurred where the sprites horizontally spread in a range of tens of kilometers. For sprites of triangulation, precipitation maps by radar observations were made, and the location of lightning was investigated. We also investigated the correlation of the sprites to the gamma-ray emissions associated to thunderclouds.