Sferic clusters associated with early/fast VLF events

Characteristic subionospheric signal perturbations referred to as 'early/fa st' VLF events are found to consistently be associated with the simultaneou s occurrence of unusual clusters of radio atmospherics as observed in wideb and VLF data. ii burst of sferic activity occurs at or before the onset of the early/fast events and lasts from several hundred milliseconds to severa l seconds. The cluster energy attenuates with distance more rapidly than cl oud-to-ground (CG) lightning, suggesting that these clusters may be produce d by intracloud (IC) lightning, and that the charge movement within the clo ud may thus play a role in the causative mechanism of early/fast VLF events .