D region ionization by lightning‐induced electromagnetic pulses

Abstract

The electromagnetic pulses (EMP) from tropospheric lightning produce transient luminous events (TLEs), known as elves, in the 80–90 km region above the lightning. The luminosity is evidence that the EMP carries sufficient electric field to excite optical emissions at these altitudes; however, it is still unknown whether the field is sufficient to ionize the atmosphere. The first multiwavelength, quantitative observatory on a free‐flying satellite dedicated to observing TLEs, the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) instrument on FORMOSAT‐2, formerly called ROCSAT‐2, confirmed that a significant number of lightning events are accompanied by elves. The instrument consists of a low light level imager and a set of multichannel photometers. In a few cases where the lightning occurred beyond the solid Earth limb, pure spectral measurements of the elves were obtained. Here we analyze such an event and show that the elves contained significant 391.4 nm emission of the N2+ ion. This is clear evidence that ionization takes place in elves. The ratio of cross sections for N2 ionization and the production of the upper state of 391.4 nm emission is not a constant for low‐energy electrons found in TLEs. We attempted to find this ratio by comparing our photometric measurements of the TLE produced emissions to theoretically derived emission intensities. The electron energy distribution and the ratios of the modeled N2 Lyman‐Birge‐Hopfield (LBH) and the N2+ first negative to the second positive were computed as a function of the reduced electric field. From these ratios it was possible to obtain the reduced electric field from the ratios. We estimated that the reduced electric field, which characterizes the local electron energy distribution, was >200 Td. We also made comparisons of the theoretically derived intensities to our measurements of the N2+ first positive and Lyman‐Birge‐Hopfield (LBH) band emission in the elves. On the basis of the ratio between the N2+ first negative emission and the time‐integrated ionization production, we estimate that the elves produced an average electron density of 210 electrons cm−3 over a large (165 km diameter) circular region having an assumed 10 km altitude extent. These observations indicate that thunderstorms are a significant source of ionization in the low‐ to midlatitude nighttime D region.