Phenomena related to lightning discharges in the atmosphere, in other words, lightning strikes and cloud discharges, have been considered mysterious phenomena that were objects of worship since ancient times. However, since the mid-18th century, when Benjamin Franklin demonstrated that "lightning discharge is an electrical phenomenon within clouds," and through the efforts of many related researchers, a general understanding of both lightning strikes and cloud discharges has been achieved. For instance, from the perspective of optical observation, the scenario of ground discharges has been proposed and introduced in many textbooks, thanks to the evolution from Boys cameras and streak cameras to high-speed cameras owing to advancements in electronic technology. This scenario includes the initiation of corona discharges, streamer propagation, stepped leader progression, return strokes, dart leaders (and sometimes dart-stepped leaders), and subsequent strokes. Similar confirmation of these phenomena is possible through observations of the electromagnetic waves emitted during lightning discharges, and aside from detailed points, this has become a common understanding within the atmospheric electricity community. Our group also prides itself on having contributed to the confirmation of discharge progression scenarios through observations with broadband interferometers in the VHF band.
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