Narrow positive bipolar radiation from lightning observed in Sri Lanka

• Narrow positive bipolar pulses (NPBPs), whose origin largely remains unknown as yet, have occasionally been noticed in Sri Lanka. These discharges are found to be opposite in polarity to that of negative return strokes, and are found to occur at the beginning and active stage of thunderstorm activities in Sri Lanka. They are emitted from the thunderstorms that produce other activities also and are relatively narrower, bipolar and isolated in nature. They are neither preceded by leader-type pulses nor succeeded by the subsequent activities and hence, could not be associated with any other known activity. Similar radiations have previously been identified and termed as NPBPs. Their features are indeed unique and different from the other known thunderstorm electrical processes. These events have been found to be accompanied by the HF radiations at 5 and 10 MHz, at the trailing part of the pulse. However, such narrow, bipolar and isolated events have not been observed in Sweden, during a lightning measurement campaign at Uppsala, in 2006. So, it is speculated that the meteorological conditions to be responsible for the thunderstorm activity to produce such pulses. Features of such pulses recorded on different thunderstorm days in 2005 and 2006 have been analyzed and presented in this study. The average rise time (10-90%) (Tr), of the pulses was found to be 2.6 [mu]s, the average zero crossing time (Tz) was found to be 5.85 [mu]s, the average duration of slow front (Ts) was found to be 1.86 [mu]s, and the average ratio of amplitude of overshoot to the corresponding peak amplitude (Os/Pa) of these pulses was found to be 0.39.

Nyckelord

Bipolar

Cloud flashes

Electromagnetic field

Lightning

NBPs

Positive bipolar

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TEKNIKVETENSKAP

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