ON THE SPATIAL RELATIONSHIP BETWEEN LIGHTNING DISCHARGES AND PROPAGATION PATHS OF PERTURBED SUBIONOSPHERIC VLF/LF SIGNALS

A study has been made of the spatial relationship between propagation paths of subionospheric VLF/LF signals exhibiting sudden amplitude perturbations (Trimpi events) and time correlated cloud-to-ground lightning flashes. On each of the 4 days examined the storm centers were located close to the signal path from the NAU transmitter (28.5-kHz) in Puerto Rico to Stanford (SU) and were at large distances from the propagation path of the 48.5-kHz transmitter signal from Nebraska to SU. Nevertheless, no Trimpi events were observed on the former path, while many were seen on the latter. Furthermore, the detected Trimpi perturbations of the 48.5-kHz signal received at Stanford were found to be associated with the lightning activity in the distant storm centers. Since the NAU-SU path lies entirely at L < 2 and the 48.5-SU path is located mostly at 2 < L < 3, the L dependent magnetospheric conditions which determine the level of lightning-induced electron precipitation are different along the two paths. Thus we postulate that the observed difference in Trimpi occurrence on the two paths was due to the different magnetospheric conditions. Hence the occurrence of Trimpi events over the geographical region corresponding to L < 3 may be more dominantly controlled by magnetospheric conditions than the source lightning distribution.