Characteristic of subionospheric VLF perturbations associated with winter lightning around Japan

The VLF signature of ionospheric perturbations (Trimpis) associated with winter lightning around Japan has been investigated during a three‐month observation (December 2000 to February 2001). The absence of observed optical phenomena (mesospheric optical emissions such as sprites) in the Hokuriku area (Sea of Japan side) indicates that the period was not extremely active in lightning as compared with the previous winter [Hobara et al., 2001]. Based on VLF subionospheric observation at Moshiri (Hokkaido) and Maizuru (near Kyoto) for two VLF transmitter signals (NWC (Australia) and NPM (Hawaii)), we have found the following; (1) Lightning discharges are found not only on the Japan‐Sea side, but also on the Pacific side. Although more lightning was observed on the Pacific side of Japan, the Trimpi occurrence rate is more enhanced over the Sea of Japan. (2) Trimpis tend to occur only when the lightning activity is enhanced. (3) Trimpi occurrence rate shows no particular diurnal variation. (4) There is no significant difference in the Trimpi occurrence rate between + and − CG discharges. (5) The intensity of scattered signal (with respect to the unperturbed wave) is, in most case, ranging from −15 to −25 dB. These characteristics are discussed in terms of meteorological and lightning conditions.