Modeling the relaxation of early VLF perturbations associated with transient luminous events

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Studies show that early VLF perturbations, characterized by abrupt signal onsets and long recoveries, occur often in relation with transient luminous events, especially sprites and sprite halos. Also, most of the early VLF events are attributed to forward scattering of subionospheric VLF transmissions incident upon horizontally elongated disturbances of elevated ionization in the upper D region. This concept is supported by the similarity of early VLF event recoveries to those of lightning‐induced electron precipitation events (LEPs), which are due to electron density enhancements in the upper D region caused by lightning and whistler‐induced precipitation of radiation belt electrons. Here, the simplified Glukhov‐Pasko‐Inan model, that has been developed for LEP investigations, is applied to simulate early VLF event recoveries observed simultaneously with sprite discharges in the D region. The present study shows that: (1) early VLF events with longer (shorter) recoveries are likely to come from higher altitudes of about 80–90 km (lower altitudes of about 70–80 km) and under conditions of lower (higher) electron density elevations relative to ambient values; (2) although negative ion and positive cluster ion production plays a role in electron density relaxation at lower heights, the electron–single ion dissociative recombination is likely the key process at upper D region heights that defines the relaxation of early VLF perturbations; and (3) tentative estimates of electron density increases responsible for early VLF events may reach values possibly higher than 104 electrons per cm3 in the upper D region ionosphere.