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

Subionospheric VLF signatures of lightning-associated ionospheric disturban ces observed at multiple sites in the central United States are used to inv estigate the occurrence properties and recovery signatures of characteristi c perturbations exhibiting rapid onset followed by slow recoveries. The two different types of events, so-called early/fast and lightning-induced elec tron precipitation (LEP) [Inan et al., 1996a], are distinguished by the lac k of presence of a few hundred millisecond delay between VLF event onsets a nd causative lightning discharges, respectively. Analysis of recovery signa tures of the two types of events indicate subtle but distinct differences i n 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 tw o classes of events are different, consistent with a recent theoretical sug gestion [Inan et al., 1996c]. The occurrence properties of early/fast event s observed by the Holographic Array for Ionospheric Lighting (HAIL) receive rs indicate that ionospheric disturbances act primarily as forward scattere rs.