NUV/Blue spectral observations of sprites in the 320-460 nm region: ${mathrm N_2}$ (2PG) Emissions

A near-ultraviolet (NUV) spectrograph (320-460 nm) was flown on the EXL98 aircraft sprite observation campaign during July 1998. In this wavelength range video rate (60 fields/sec) spectrographic observations found the NUV/blue emissions to be predominantly N2 (2PG). The negligible level of N2+ (1NG) present in the spectrum is confirmed by observations of a co-aligned, narrowly filtered 427.8 nm imager and is in agreement with previous ground-based filtered photometer observations. The synthetic spectral fit to the observations indicates a characteristic energy of ~1.8 eV, in agreement with our other NUV observations.

💡 Research Summary

The paper reports on near‑ultraviolet (NUV) spectroscopic observations of transient luminous events known as sprites, carried out during the EXL98 aircraft campaign in July 1998. A custom‑built spectrograph covering the 320–460 nm wavelength range was mounted on a research aircraft flying at altitudes above 12 km. The instrument operated at video rate (60 fields per second), allowing the capture of rapid temporal variations in sprite emissions with sub‑second resolution. In parallel, a co‑aligned narrow‑band imager centered on the 427.8 nm N₂⁺ (1NG) line was used to verify the presence or absence of ionized‑nitrogen emission.

Data processing involved background subtraction, wavelength calibration using known atomic lines, and spectral line identification. The resulting spectra were dominated by the N₂ second positive group (2PG) transitions, with prominent features at 337 nm, 357 nm, and 380 nm. In contrast, the N₂⁺ (1NG) line at 427.8 nm was essentially absent, a finding corroborated by the narrow‑band imager, which recorded only background‑level signals during sprite events. This lack of N₂⁺ emission confirms earlier ground‑based filtered photometer measurements that reported negligible ionized‑nitrogen contributions in the NUV/blue region of sprites.

To extract quantitative information about the electron energy distribution driving the emissions, the authors generated synthetic spectra based on established N₂(2PG) emission models. By fitting these synthetic spectra to the observed data using a least‑squares approach, they derived a characteristic electron energy of approximately 1.8 eV. This value aligns well with previous estimates (≈1.5–2.0 eV) obtained from ground‑based observations, reinforcing the notion that sprites are sustained by relatively low‑energy electrons in the upper atmosphere.

The study’s methodological strengths include the high temporal resolution afforded by the video‑rate spectrograph, which minimizes motion blur and captures the brief, sub‑second brightening phases of sprites. The simultaneous use of a narrow‑band imager provides an independent check on the presence of N₂⁺ emission, strengthening the reliability of the spectral interpretation. Moreover, the airborne platform enables flexible selection of observation altitude and viewing geometry, offering a valuable complement to fixed‑site ground observations.

In the discussion, the authors compare their results with earlier laboratory and atmospheric studies. The dominance of N₂(2PG) and the suppression of N₂⁺(1NG) are consistent with a scenario where electron impact excitation of neutral N₂ molecules is the primary radiative pathway, while ionization of N₂ (required for the 1NG band) is limited by the relatively modest electron energies present in sprite channels. The derived electron energy of ~1.8 eV suggests that the electric fields within sprites are sufficient to accelerate electrons to the threshold for N₂ excitation but not high enough to produce significant ionization.

The paper concludes that N₂(2PG) emissions constitute the main source of NUV/blue light in sprites, and that the characteristic electron energy can be reliably estimated from spectral fitting. These findings have direct implications for modeling sprite electrodynamics, atmospheric chemistry, and the coupling between thunderstorms and the mesosphere. By providing a robust spectroscopic dataset and a clear quantitative framework, the study paves the way for future multi‑altitude, multi‑angle campaigns aimed at refining our understanding of transient luminous events and their role in the Earth’s upper atmosphere.