Spectral Characteristics of the ISUAL Blue Luminous Events



Jung-Kuang Chou1, Rue-Ron Hsu1, Han-Tzong Su1, Cheng-Ling Kuo2, Afred Bing-Chih Chen3, Shu-Chun Chang1, Yen-Jung Wu1, Kang-Ming Peng1, Harald U. Frey4, Yukihiro Takahashi5, and Lou-Chuang Lee6


1 Department of Physics, National Cheng Kung University, Tainan, Taiwan

2 Institute of Space Science, National Central University, Jhongli, Taiwan

3 Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan

4 Space Sciences Laboratory, University of California, Berkeley, California, USA

5 Department of Cosmosciences, Hokkaido University, Sapporo, Japan

6 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan



ISUAL has recorded many mystic blue luminous events (BLEs) that emit clear middle ultraviolet to blue emissions (230−450 nm) but contain dim red emissions (653−754 nm). Most BLEs appear to be dot-like on the ISUAL images, and a few BLEs develop further into blue jets (BJs) or type II gigantic jets (GJs). The associated sferics of the BLEs in the extremely low frequency to the very low frequency (ELF/VLF) band and the low frequency (LF) band exhibit similar features to those of the VLF/LF waveform for the narrow bipolar events (NBEs). NBEs are the distinct intra-cloud lightning discharges, and they emit little visible light in comparison to normal lightning. The ISUAL BLE is conjectured to be the accompanied light emissions of the NBE-like event. The electrical breakdown of undisturbed air and an assumed hot channel are believed to occur at the same time for a NBE. The optical emissions from the conventional streamers and the lighting spectrum as the emissions from a hot channel are examined. By means of computing the peak photon flux ratios for the BLE, we have examined the spectral characteristics of the ISUAL BLEs. The results indicate that the emissions of a NBE-like event mostly from the non-thermal air plasma produced by the locally-enhanced electric field, rather than those from the hot air plasma in local thermodynamic equilibrium (LTE).