Charge Moment Differences in Winter of Japan and Summer of the United States

In order to estimate the charge moment changes of sprite-inducing lightning discharges that occurred in both summer and winter, we have analyzed ELF magnetic field waveform data in the frequency range of 1-100 Hz obtained at Syowa station (69.0S, 39.6E) in Antarctica and Onagawa observatory (38.4N, 141.5E) in Japan. During the STEPS (Severe Thunderstorm Electrification and Precipitation Study) campaign that was conducted from May 22 to July 15, 2000 in the US Great Plains, many sprite events were observed at Yucca Ridge Field Station (YRFS), Colorado. At Syowa station sprite-related transient Schumann resonances (SRs) were detected by horizontal search coil magnetometers. We selected 241 sprite-related transient SR events occurred on May 26, 27, 30, June 1, 12, 20, 23, July 4, and 19, 2000. The estimation method of charge moments is based on the normal mode expansion model discussed by Huang et al. [1999]. We estimated the charge moments of causative lightning discharges for all transient SR events obtained an average charge moment of 932 C-km. It is found that for 28 % of all transient SR events charge moments are larger than 1000 C-km. In the two winter sprite campaigns for the periods between December 2000 and January 2001 and between December 2001 and February 2002, we detected 12 and 22 sprite events, respectively. We have estimated the charge moments for 32 sprite events occurred on January 23, 29, 30, December 28 and 29, 2001, and February 8, 2002. The average charge moment is estimated to be 509 C-km which is about half of the average charge moment in summer, USA. It is also found that all transient SR events have charge moments less than 1000 C-km with a distribution of 63 % between 250 and 750 C-km and 19 % between 750 and 100 C-km. It is suggested that smaller charge moment changes of winter sprites are related to the lower cloud top of winter thunderclouds. Relationships between the structures of sprite-producing thunderstorms and the charge moments of parent lightning discharges will be discussed.