Isolated breakdown activity in Swedish lightning

• The initial breakdown processes or preliminary breakdown (PB) processes are generally associated with either cloud or cloud-ground lightning flashes, and hence have been studied mainly in connection with those subsequent activities. However, it has been observed in the summer thunderstorms of Sweden that there are breakdown processes that may not culminate into any subsequent activity. As these processes do not lead to any subsequent activity, they have been termed as isolated breakdown activities. Such isolated breakdown activities involve microsecond-scale pulses with both initial polarities. Based on their polarities, breakdown processes are divided into two groups, the negative breakdown pulses that are similar to the breakdown pulses leading to negative return strokes in ground flashes and the positive breakdown pulses similar to the breakdown pulses generally leading to the cloud flashes. The occurrence of the two types of breakdown processes is found to vary from a thunderstorm day to the other. In the present study, the signatures of isolated breakdown activities have been analyzed and are compared with those leading to the subsequent activities observed during the same measurement campaign. The average duration of the isolated breakdown activity associated with the negative initial polarity pulses is found to be 0.98 ms with average number of pulses 8.5. Similarly, the average duration and number of pulses in the isolated breakdown process with positive initial polarity pulses are 6.9ms and 3.9, respectively.

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isolated

breakdown

lightning

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