Recovery signatures and occurrence properties of lightning‐associated subionospheric VLF perturbations

Abstract

Subionospheric VLF signatures of lightning‐associated ionospheric disturbances observed at multiple sites in the central United States are used to investigate the occurrence properties and recovery signatures of characteristic perturbations exhibiting rapid onset followed by slow recoveries. The two different types of events, so‐called early/fast and lightning‐induced electron precipitation (LEP) [Inan et al., 1996a], are distinguished by the lack of presence of a few hundred millisecond delay between VLF event onsets and causative lightning discharges, respectively. Analysis of recovery signatures of the two types of events indicate subtle but distinct differences in the recovery rates. A majority of early/fast events were found to exhibit a more rapid initial recovery to preevent levels during the first 20 s of recovery, when compared to LEP events. This experimental evidence indicates that the physical nature of the ionospheric disturbance involved in the two classes of events are different, consistent with a recent theoretical suggestion [Inan et al., 1996c]. The occurrence properties of early/fast events observed by the Holographic Array for Ionospheric Lighting (HAIL) receivers indicate that ionospheric disturbances act primarily as forward scatterers.