Sprites are transient luminous events (TLEs) observed at mesospheric altitudes from 40–90 km over thunderstorms. Sprites are typically associated with positive lightning strikes and believed to be caused by electrostatic fields in the lower ionosphere induced by the removal of charge in the lightning strike. Currently, sprite studies are conducted from satellites and from the ground by passive optics and spheric measurements. We will discuss two recent two achievements of the ground-based optical measurements.
Due to the transient nature of the sprite (< a few tens of milliseconds), ordinary video images have shown sprites time integrated over the their full, or greater part of, development. The study of the dynamics of sprite development is difficult. High-speed imaging with 50–1000 microsecond temporal resolution, to be shown, has revealed that the nature of the sprite development is similar to laboratory streamers. It may confirm the theoretical prediction that a sprite consists of a series of streamers.
Spectral measurements of sprites are very important in order to learn the energetics and the chemistry involved. The sprite blue emission is composed by the N₂ 2PG band (threshold energy 11.2 eV) and the N₂⁺ 1NG band (18.6 eV). The N₂⁺ Meinel band (16.5 eV) can be expected in sprite emission, but only one previous study shows the N₂⁺ Meinel band signature in sprite spectra. Our high-speed imaging spectrograph (3-ms temporal resolution) was deployed to establish the presence of the N₂⁺ Meinel band emission and to obtain the altitude resolved the sprite spectra in near infrared in summer 2005. Results from this measurement will be shown.