Computer simulations on the initiation and morphological difference of Japan winter and summer sprites

Two‐dimensional (axisymmetric) computer simulations (electromagnetic code) have been performed to study the initiation and morphological difference (summer, carrots, and winter, columns) of sprites for simulating Japan summer and winter sprites. By changing the physical parameters of a parent lightning, we have found the following findings by looking at the spatial‐temporal distribution of reduced electric field, etc. (1) There are three important factors (the height where the charge is removed ( ds ), charge transfer ( Q ), and lightning current risetime ( τ )) in the initiation of sprites and their morphological differences. (2) For the initiation of sprites, the charge transfer ( Q ) should exceed a certain value for possible charge heights (giving us a threshold of charge moment change ( Qds ) of the order of ∼120–200 C·km), with a combination of small risetime of lightning current waveform. (3) Further, the height for positive charge is much higher than that for negative charge in a typical lightning configuration, which is the essential factor in determining the morphological difference of sprites in summer and winter. Positive charges for summer are located at a much higher altitude than those in winter in Japan, which might result in carrot‐type in summer and columnar‐type in winter even for the same positive polarity. (4) A combinational effect of ( Ids ) and ( Qds ) is important for having sprites. Finally, the present computer results on the initiation of sprites for Japanese lightning have been compared extensively to the well‐documented properties of summer continental sprites and future subjects to study have been suggested.