Morphology of winter sprites in the Hokuriku area of Japan : Monthly variation and dependence on air temperature

Abstract

Continuous observations of mesospheric optical emissions (sprites) were performed from two optical sites, Shimizu and Chofu during the period of seven months from November 2004 to May 2005, with the target of specific winter sprites in the Hokuriku area of Japan (in the coast of Japan Sea). Two different kinds of analysis have been performed; one is based on the monthly data and the second is the case study. The correlation between the monthly percentage occurrence of different sprite types (columns, carrots and intermediate (including V-shaped sprites, co-existence of columns and carrots)) and monthly variation of the –10oC temperature height has indicated a negative value (r ∼ –0.41) for columns and a high positive value (r ∼ +0.69) for carrots. Further detailed analysis was performed with the height of –10°C at the time of sprite occurrence. It was found that winter sprites occur when the height of –10°C isotherm is located in a range from 1200 m to 3000 m. When the altitude is small, like 1200 – 1400 m, the dominant shape is columnar. When the height of –10°C is increased up to 1800 – 3000 m, a new situation takes place; that is, more spectacular shapes like carrots tend to be frequently observed. This point is also confirmed by analyzing the temperature at a particular height of 850 hPa. Carrots tend to occur above a threshold of temperature of 850 hPa (–7°C). We can conclude that charge height as the consequence charge separation caused by vertical temperature variation plays an essential role in determining the spatial forms of sprites. Finally, we discuss the initiation of sprites and sprite morphology, with a reference to the observed facts in this paper.