Observation of a bipolar cloud-to-ground flash containing one first positive stroke followed by three negative strokes

Abstract:

We report the high-speed (3,200 fps) observation of a bipolar cloud-to-ground flash with one first positive stroke followed by three subsequent negative strokes that all transferred charge to ground through the same channel. The inception of this flash was barely captured with the high-speed camera, indicating an onset near the cloud base, which allows us to investigate the possible mechanism for its occurrence. The examination of multi-disciplinary data recorded for this event, including high-speed imagery, slow and fast E-field change, broadband (3-300 kHz) magnetic fields, and surface static electric field, indicates that this flash could have initiated from a relatively large positive charge center near the cloud base (which caused a sustained negative surface electric field at the observation site); the preliminary breakdown pulse train has the same polarity as the pulse of leading positive return stroke. The positive leader propagated downward at an average speed of 1×10⁵ m s^-1, culminating a positive stroke with a continuing current of ~70 ms about 120 ms after the flash onset. The ascendance of upward negative leaders into the middle negative cloud charge region, as stimulated by a sequence of K processes that drew negative charge elsewhere at the cloud base, probably resulted in the polarity reversal of charge transfer to ground.