RESPONSE OF THE D-REGION IONOSPHERE TO FIREBALLS USING VLF/LF TRANSMITTER SIGNALS

We investigate the variations in the D-region ionosphere during a fireball occurred in Hokkaido, Japan, at 11:55:55 UT on 18 October 2018, using very low frequency (VLF, 3-30 kHz) / low frequency (LF, 30- 300 kHz) transmitter signals. The fireball was observed at height from 118 km down to 25 km with a velocity of 32 km/s based on optical network observations. The VLF/LF propagation paths used in this study were JJY (JJY40kHz, 40 kHz)-Rikubetsu (RKB, Japan), JJI (22.2 kHz)-RKB, and JJY (JJY60kHz, 60 kHz)-RKB paths. Periodic variations with periods of 100-200 s were observed on all paths for about 5 minutes from 12:01 UT after the fireball occurrence. The theoretical arrival time of acoustic waves excited from the end point of the fireball roughly matched with the onset timing of the VLF/LF variations. Based on time difference of the VLF/LF variations among each path, location of the variations in the D-region ionosphere was estimated to be about 70 km south from the end point.