Three Dimensional Analysis of Bipolar Lightning in Winter at the Japan Sea Coast.

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The observation of winter has been continuing using magnetic links, digital recording systems for the current oscillograms, field mills, still cameras and video camera systems for the images of lightning channels at the west coast of Japan. To clarify the structure of winter thundercloud, three slow type antennas were made and installed at the points which are separated several handred meter apart from earch other. Of the 65 magnetic links records, 60% were negative in polarity, 28% were positive and the remainig 12% were unknown. The maximum crest current was over 300 KA in a positive flash, and the larger crest currents were found in positive flashes. The measured current oscillogram shows that a winter lightning often has a very long duration. Moreover two kinds of bipolar strokes were frequently recorded in the winter season. Nine of the 60 flashes showed bipolar strokes. The vertical potential grdient at earth's surface and the wether conditions for the occurrence of winter thunde … More rstorms were studied. The observed signals of the electrostatic field were very variable in both magnitude and polarity, and often exceeded more than 20 kv/m. The surface gradient frequently changed to the opposite polarity in a few minutes. Several thunderstorms developed and attacked the region periodically over a long period of time during one day or more. Many photographs taken by sitll cameras and video cameras show that most of the flashes have upward branching. All nine of the bipolar oscillograms were similar to one or the other of two types. The first type initially has a low negative current and then a large positive current. The second type, on the other hand, begins with a large positive current, with a negative current subsequently superimposed on it, then change to a negative current. Bipolar stokes generally occur when lightning currents are relatively small. This would suggest that the bipolar current is gradually passing through the same channel from some regions of different polarity in the thunderclouds. Moreover it is significant that the return stroke current of bipolar lightning was superimposed with the opposite current pulses. Less