Abstract
AbstractWinter thunderstorms in Japan have been recognized as an ideal target to observe high‐energy atmospheric phenomena thanks to low‐charge‐center cloud structures. During four winter seasons in Japan (from 2016 October to 2020 March), seven downward terrestrial gamma‐ray flashes (TGFs) were detected by gamma‐ray and broadband low‐frequency (LF: 0.8–500 kHz) monitors. All the detected TGFs took place at the initial stage of lightning flashes. Based on the LF observation, the seven downward TGFs in the present study can be classified into two types: energetic‐bipolar and small‐bipolar types. Three of them are energetic‐bipolar events, coincident with a high peak‐current LF pulse that originates from a negative return stroke with a peak current larger than 100 kA. The others are small‐bipolar events, followed by a negative bipolar LF pulse with a moderate peak current. Three of the four small‐bipolar events are multi‐pulse TGFs, while all of the energetic‐bipolar events in this study are single‐pulse TGFs.
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