First detection of summer blue jets and starters over Northern Kanto area of Japan: Lightning activity

摘要

Numerous summer blue jets and blue starters were observed for the first time over the Northern Kanto area of Japan on 15 August 2008. Sixty jets and starters were observed, and emissions continued intermittently for more than 20 min. Very high frequency(VHF)and low-frequency(LF)lightning activity and extremely low frequency(ELF)transients associated with these phenomena were analyzed during the 5 s before and after their appearance. We found the following unique characteristics of lightning accompanied by jets/starters: (1) the interval of emissions was different for jets and starters; (2)the jets were strongly connected with negative cloud-to-ground lightning discharges(-CGs), but starters might have been more affected by intra-cloud discharges(ICs); (3)the number of -CGs and ICs quickly increased just before a jet and showed a significant reduction after the jet; (4)the -CG activities began to decay 0.5 s before a starter, although IC activities were very active 1 s of a starter; (5)the charge moment change within 1 s of the jets and starters, as calculated from ELF transients associated with CGs, were ～-200 and ～-100 Ckm, respectively; (6)the lightning discharges with jets/starters are clustered in a limited area of overshooting storm top. These results indicate that the same thunderstorm produced both a jet and a starter. This led us to hypothesize that the emittance of either a jet or starter is dependent on the amount of accumulated positive charge and the electrical activities of the parent storm.