Elves as the Optical Signature of Lightning Precursor Discharges in the Lower Ionosphere

Narrow bipolar events (NBEs), initial breakdown pulses (IBPs) and energetic in-cloud pulses (EIPs) are compact intracloud discharges, with spatial extent of 100s of meters, that usually precede lightning flashes. There is growing evidence that these discharges are associated with elves [Neubert et al, Science, 367, 183, 2020; Nature, 589, 371, 2021; Østgaard et al., JGR, 126, e2020JD033921, 2021]; a type of transient luminous event observed at altitude ~80-95 km as laterally expanding rings of light. Elves are produced when electromagnetic pulses from high peak current lightning discharges interact with the lower ionosphere [Fukunishi et al., GRL, 23, 2157,1996; Inan et al., GRL, 24, 583, 1997]. In particular, Marshall et al. [GRL, 42, 6112, 2015] show that compact intracloud discharges with peak currents of 200 kA can reproduce the optical signatures of elves. Observations have also shown elves associated with discharge events with peak currents as low as ~50 kA [Børge-Ingeland et al., JGR, 129, e2023JD039849, 2024]. Although estimates of source peak current of compact intracloud discharges are available, the exact current waveforms have not been directly measured. We recently calculated the source current waveforms of NBEs from their wideband electric field measurements [Pervez et al., GRL, 51, e2023GL107789, 2024]. In this work we apply the electromangetic fields from such current waveforms to the lower ionosphere at elve altitudes, and use a plasma fluid model to study the dynamics of the charged species. In particular, we quantify optical emissions resulting from excitation processes due to collisions between the electrons energized by the discharge electromagnetic field and the neutral atmospheric species.