Negative Excursion of Surface Electric Fields During Gamma-Ray Glows in Winter Thunderstorms

During the 2020-2021 winter season, we detected 6 gamma-ray glows at Kanazawa University, Japan. Negative surface electric fields (E-fields; in the sign convention of atmospheric electricity) were observed by a field mill during all the glow cases. In five of the six cases, the peak E-field reached around -12 kV m(-1), and the E-field during the glow detection was the strongest in the interval including 3 hr before and after the detection time. Therefore, negative charges should have been dominant in the thunderclouds that produced the gamma-ray glows, and electrons were probably accelerated and multiplied by the E-fields between a predominantly negative charge layer and a localized positive charge layer below. In addition, we extracted 8 non-detection cases in the 2020-2021 winter season, in which surface E-fields were stronger than -12 kV m(-1). In 5 of the 8 cases, radar echoes were inadequately developed, suggesting insufficient charge accumulation. On the other hand, the remaining 3 cases had well-developed radar echoes, and there was no significant difference from the detection cases.