Very High Frequency Radiation Emitted by Negative Narrow Bipolar Events Occurred Over Malacca Strait

In this paper, Very-High Frequency (VHF) radiation pulses associated with 11 negative Narrow Bipolar Events (NBEs) produced by a tropical storm over Malacca Strait are examined. The lightning data were recorded from a measurement station (ST) which consisted of a fast antenna (FA) and three VHF sensors (two 5 m perpendicular baselines interferometer). The average rise time (RT), average zero-crossing time (ZCT), average pulse duration (PD), and range of peak currents of the negative NBEs were 1.4 ± 0.4 μs, 2.7 ± 1.0 μs, 12.0 ± 6.9 μs, and -10 to -64 kA, respectively. The key finding is that all VHF radiation pulses have been found to precede the negative NBEs with an average lead time of 0.7 ± 0.3 μs. An interferometer map for one negative NBE (labelled as NBE10) detected at 35.7 km from ST has shown a characteristic of mixed propagation direction of fast streamers. The first VHF radiation source was detected at 12.4 ± 0.4 km above sea level. The total length and estimated velocity of the main propagation of the VHF radiation sources were 2.2 ± 0.7 km and between 1.4×108 and 2.8×108 ms-1, respectively. Moreover, based on the Himawari satellite image, the maximum extent of the cloud top height was estimated to be around 20.9 km over sea level (over Malacca Strait). All the VHF radiation sources associated with NBE10 were suggested to be detected above the main negative charge region (6 km altitude that corresponds to –10°C). Thus, it could be suggested that NBE10 was initiated most likely in the environment of the ice crystals alone, based on the first altitude of the VHF radiation source and maximum extent of cloud top height.