Charge moment change of lightning discharge associated with terrestrial gamma-ray flash

Terrestrial Gamma-ray Flashes (TGFs) are gamma-ray emission phenomena associated with lightning discharge. TGFs have a duration time of a few msec and an energy range of a few 10 keV-20 MeV. TGFs were firstly discovered by BATSE on CGRO satellite in 1994. RHESSI satellite observed over 170 TGFs per year from 2002 to 2005. The orbit of RHESSI is circular with an inclination of 38 degrees at an altitude of 600 km. So far the characteristics of lightning associated with TGFs have been investigated. However, the relationship between the TGF occurrence and the charge moment change of parent lightning discharge has not been examined in global scale. Tohoku University has been measuring ELF magnetic field in the frequency range of 1-100 Hz at 4 sites located globally, including Onagawa in Japan, Esrange in Sweden and Syowa in Antarctica. From these data, we can estimate the charge moment change, location, and polarity of lightning around the world. We derived the charge moment change of lightning discharge generating TGFs from the ELF waveforms and the timings of TGFs listed by RHESSI observation. 174 TGF events were observed by RHESSI in 2004. We analyzed 141 events whose ELF data at 3 stations are available. We estimated lightning location based on the time of arrival method. If the rise timing of lightning ELF pulse, called ELF sferics, is estimated within +/- 15 msec of the timing of TGFs observed by RHESSI and the estimated location of lightning discharge is within a range of 1500 km from the sub-satellite point, we regard it as parent lightning of TGFs. Detection Limit of the charge moment change with global uniformity was 175 C-km. We analyzed 141 events observed in 2004 and identified 9 TGFs with ELF sferics. We consider the other 132 TGF events are related to smaller lightning discharge whose charge change moment change are less than the detection limit of 175 C-km. The minimum charge moment change in 9 events is 176 C-km, and the maximum is 465 C-km. We derived the occurrence probability of TGFs, dividing TGFs occurrence number by lightning occurrence number for each range of the charge moment change. Relative occurrence probabilities of TGFs in the ranges of 175-350 C-km and 350-525 C-km to that in the range of 0-175 C-km is 14 % and 4 %, respectively. This fact suggests that the occurrence probability of TGFs is higher for lightning discharge with smaller charge moment change.