Spectroscopic study of sprites

Abstract

Optical emissions from sprites--large electric discharges in the mesosphere caused by intense lightning strokes--have been studied for decades. Studies have identified that sprite emissions are primarily composed of molecular band emissions of nitrogen and notably identified the near ultraviolet and blue emission from the N₂⁺ First Negative system, which provided direct evidence of ionization in sprites. This implies that further evidence of the ionization may be provided by the visible and near infrared emission from the N₂⁺ Meinel system, which is more accessible from ground-based platforms, though anticipated strong quenching in the mesosphere and below have made the presence of the emission somewhat controversial. To investigate the presence of the Meinel emission along the vertical extent of sprites, we made ground-based spectral observations in 2005. The observed spectra were mainly composed of the N₂ First Positive system, and no or little indication of the Meinel bands were found. This study suggests that the quenching is indeed severe at sprite altitude, and it is difficult to study the ionization process in sprites via the Meinel emission. In addition, the data allowed us to investigate details of the First Positive emission from sprites. The observed First Positive spectra showed that the vibrational distribution of the upper state varies along the vertical extent of sprites, which is in agreement with previous reports, and furthermore this study indicates that the variation is associated with altitude, implying that collisional energy transfer processes play roles in exciting the First Positive emission, particularly at lower altitudes. Recent high-speed imaging observations have revealed the very dynamic nature of sprites: they develop within a few to 10 ms in forms of streamers and columnar glows. The underlying electron energies in these features have been inferred from their emissions in previous measurements, but they lacked either sufficient temporal or spatial resolution. To investigate the underlying electron energies, we made airborne spectral observations in 2009 with a slitless spectrograph, which provided temporal and spatial resolution improved over the previous measurements. The observed spectra clearly showed that the streamers consistently had a higher fraction of blue emission compared to the glows, indicating that the more energetic nature of the streamers. From the fractional blue emissions, the local electric fields were inferred to be 0.7 to 1.5Ek in the streamers and 0.3 to 0.6Ek in the glows, where Ek is the conventional breakdown electric field. The results support the interpretation of sprites as scaled analogs of streamer discharges observed in laboratory experiments.